Role of stellar physics in regulating the critical steps for life
Manasvi Lingam Abraham Loeb










package@fontpackage@font
title



    αβΒδΔϵηγΓικλΛ∇ωΩ∂ϕΦψψρσΣτþθΘΥεζ∂ℝ
        
        
        𝒜ℬ𝒞𝒟ℰℱ𝒢ℋℐ𝒥𝒦ℒℳ𝒩𝒪𝒫𝒬ℛ𝒮𝒯𝒰𝒱𝒲𝒳𝒴𝒵
         
        
         #1#2q^#1_ ,#2#1#2a^#1_ ,#2#̣1#2δ^#1_ ,#2#1#2q̇^#1_ ,#2Institute for Theory and Computation, Harvard University, 60 Garden St, Cambridge MA 02138, USARole of stellar physics in regulating the critical steps for life
            Abraham Loeb Electronic address: 
            
        =================================================================
        
        
        We use the critical step model to study the major transitions in evolution on Earth. We find that a total of five steps represents the most plausible estimate, in agreement with previous studies, and use the fossil record to identify the potential candidates. We apply the model to Earth-analogs around stars of different masses by incorporating the constraints on habitability set by stellar physics including the habitable zone lifetime, availability of ultraviolet radiation for prebiotic chemistry, and atmospheric escape. The critical step model suggests that the habitability of Earth-analogs around M-dwarfs is significantly suppressed. The total number of stars with planets containing detectable biosignatures of microbial life is expected to be highest for K-dwarfs. In contrast, we find that the corresponding value for intelligent life (technosignatures) should be highest for solar-mass stars. Thus, our work may assist in the identification of suitable targets in the search for biosignatures and technosignatures.
        
        
        § INTRODUCTION
        
        In less than a decade, our understanding of exoplanets has improved dramatically thanks to the Kepler mission, which was launched in 2009 <cit.>. The fields of exoplanetary science and astrobiology also received two major boosts over the last couple of years as a result of two remarkable discoveries. The first was the discovery of the potentially habitable planet Proxima b around Proxima Centauri, the star nearest to our Solar system <cit.>. The second was the discovery of at least seven Earth-sized planets orbiting the star TRAPPIST-1 at a distance of about 12 pc <cit.>, some of which may be capable of hosting liquid water on their surfaces. Looking ahead, there are a wide range of space- and ground-based telescopes that will become operational within the next 10-15 years with the express purpose of hunting for myriad exoplanetary biosignatures <cit.>.
        
        The Search for Extraterrestrial Intelligence (SETI) has also received an impetus in this period on both the theoretical and observational fronts <cit.>. Theoretically, many innovative technosignatures have been proposed for identifying artifacts of extraterrestrial species, both extant and extinct <cit.>. From the observational standpoint, the recently established Breakthrough Listen project <cit.> has injected new funding and rejuvenated SETI,[<https://breakthroughinitiatives.org/initiative/1>] after the unfortunate demise of federal funding in 1993. 
        
        Thus, from the perspective of searching for biosignatures and technosignatures, it is therefore necessary to understand what are the constraints imposed on planetary habitability by the host star. This will help in facilitating the optimal selection of suitable target stars and planets, where the prospects for life may be maximized <cit.>. In this paper, we will therefore use a model originally developed by <cit.>, where evolution is treated as a succession of critical steps, to assess the likelihood of primitive (microbial) and intelligent (technological) life, and the implications for detecting biosignatures and technosignatures. A brief description of the methodology is provided in Sec. <ref>, followed by an extended discussion of the critical step model in the context of Earth's evolutionary history in Sec. <ref>. Next, we assess the likelihood of these critical steps being successfully attained on other exoplanets in Sec. <ref>. We conclude with a summary of our major points in Sec. <ref>.
        
        
        
        § METHODOLOGY
        
        We begin with a brief summary of the mathematical preliminaries. A detailed derivation of these results can be found in <cit.>, <cit.> and <cit.>. In the critical-step model, the basic assumption is that there are n critical (i.e. “hard”) steps in all. Each step is stochastic in nature, and has an  associated probability of occurrence (denoted by λ_i with i = 1,…,n), and the condition λ_i t_H ≪ 1 must be satisfied ∀  i. Here, t_H denotes the total period of habitability, and its value for the Earth and other exoplanets will be addressed later.
        
        The central quantity of interest is the probability density function (PDF) for the case where the r-th step takes place at time t, and the remaining n-r steps take place after t. Denoting this quantity by P_r,n(t), the PDF can be expressed as
        
            P_r,n(t) = n !/(n-r)! (r-1)!t^r-1(t_H - t)^n-r/t_H^n.
        
        Hence, the mean time taken for the r-th step, represented by t̅_r,n, is
        
            t̅_r,n = ∫_0^t_H t P_r,n(t) dt = (r/n+1)t_H,
        
        and hence it follows that the average spacing (Δ t_n) between two consecutive steps is approximately equal,[This important fact - along with the idea that this methodology could be used to assess the accuracy of models describing the major evolutionary transitions on Earth - was first emphasized by Robin Hanson in his unpublished manuscript on evolutionary transitions: <http://mason.gmu.edu/ rhanson/hardstep.pdf>] with
        
            Δ t_n = t_H/n+1.
        
        The cumulative probability 𝒫_r,n(t) that the r-th step occurs at a time ≤ t is given by
        
            𝒫_r,n(t) = n !/(n-r)! (r-1)! B(t/t_H ; r , n-r+1),
        
        where B is the incomplete beta function. For the limiting case r=n, (<ref>) reduces to 𝒫_n,n(t) = (t/t_H)^n. 
        
        
        
        § CRITICAL STEPS AND MAJOR TRANSITIONS ON EARTH
        
        We briefly discuss the use of critical steps model as a heuristic for understanding the major breakthroughs in the evolutionary history of the Earth <cit.>.
        
        Before proceeding further, we wish to emphasize that sequences of evolutionary transitions encountered henceforth that are not in strong agreement with the theoretical model presented in Sec. <ref> may nevertheless be “correct”, since the real issue could stem from the mathematical framework employed herein (based on critical steps). In other words, the existence of alternative paradigms that do not envision evolution as a series of random critical steps remains a distinct possibility. Furthermore, it should be recognized that the classification of evolutionary steps into “easy” or “hard” (i.e. critical) categories is both abstract and binary in nature.
        
        For example, in the “long fuse” model <cit.>, a series of likely steps (each with a non-negligible timescale) unfold, culminating in slow, but near-inevitable, evolution - as per this framework, the emergence of any particular evolutionary innovation essentially becomes a matter of time. Hence, in this particular scenario, low-mass stars would be ideally suited for the evolution of intelligent life because of their longer main-sequence lifetimes. A second scenario is the “many paths” model in which the probability of a given major evolutionary transition is enhanced due to the fact that exist a large number of trajectories that can culminate in the desired final outcome <cit.>.
        
        
        
         §.§ How many critical steps were present?
        
        Although this question has been explored recently by means of the critical steps approach <cit.>, there are some major points of divergence in our analysis, as discussed below.
        
        One notable difference is that we assume the Earth was habitable approximately 4.5 Ga (Gyr ago), as opposed to previous treatments which specified the earliest point of habitability as 4 Ga. The primary reason for the choice of 4 Ga was motivated by the fact that the Late Heavy Bombardment (LHB) - a phase during which the Earth was subjected to cataclysmic bombardment by a high number of impactors <cit.> - was detrimental to habitability. However, there are several lines of evidence that now suggest that the LHB may not have been a significant impediment to habitability:
        
          *  There is some evidence indicating that the cratering record may also be explained via a sustained declining bombardment, instead of the intense LHB <cit.>. If this hypothesis is correct, the prospects for habitability are improved, and the Earth may have been geologically habitable as early as ≈ 4.5 Ga <cit.>. For instance, if the bombardment was relatively moderate, it could even have served as a valuable energy source for prebiotic chemistry <cit.>, leading to the synthesis of biomolecules such as amino acids, peptides and nucleobases <cit.>.
            
          *  Even if the LHB were present, numerical models which computed the extent of crustal melting indicate that hyperthermophiles may have survived in near-surface and subsurface environments <cit.>; see also <cit.>.
            
          *  Yet another possibility is that life-bearing ejecta spawned during the LHB can return to the Earth, and thereby reseed it over short (∼ 10^3 yr) timescales <cit.>, effectively ensuring that habitability was almost continuously prevalent during the Hadean-Archean eons.
        
        
        Thus, we start our habitability “clock” at 4.5 Ga. Several studies have attempted to assess the end of Earth's habitability in the future due to the increasing solar luminosity and the onset of the greenhouse effect. While early models yielded a value of 0.5 Gyr in the future <cit.>, more recent analyses have pushed forward this boundary to ≈ 1-2 Gyr in the future <cit.>. While the 2 Gyr limit is conceivably valid for extremophiles, the limits for more complex organisms (including humans) could be closer to 1 Gyr <cit.>. In addition, there may be other astrophysical risks posed to habitability over multi-Gyr timescales <cit.>. Hence, we will assume that the Earth becomes uninhabitable ≈ 1 Gyr in the future, but we will address the 2 Gyr case later in Sec. <ref>.
        
        As per the preceding discussion, t_H ≈ 4.5 + 1 ≈ 5.5 Gyr. Let us suppose that the evolution of technological intelligence (i.e. Homo sapiens) represents the n-th step, and use the fact that the mean timescale for our emergence was t̅_n,n≈ 4.5 Gyr. From (<ref>) and the above values, it follows that n ≈ 4.5. Thus, it seems plausible that a 4- or 5-step model may represent the best fit. This result is in good agreement with earlier studies that arrived at the conclusion n = 5<cit.>, and we will adopt this value henceforth. Classical frameworks for understanding the course of evolution on Earth also seemingly indicate that the total number of critical steps was quite small <cit.>, namely n ≲ 10, and could have been 5 to 6 in number <cit.>.
        
        
        
         §.§ What were the five critical steps?
        
        In order to determine the five critical steps, there are two routes that are open to us. The first approach assumes that these steps correspond to the major evolutionary transitions identified in the seminal work of <cit.>, wherein each step involves noteworthy changes in the storage and transmission of information. Such paradigms have been extensively invoked and utilized by several authors <cit.>. This strategy for identifying the critical steps was employed by <cit.>, who observed that the temporal constraints on the first three transitions (origin of replicating molecules, chromosomes, and the genetic code) indicate that not all of them are likely to be critical steps; instead, if the origin of prokaryotic cells is considered as a single critical step, the n = 5 model can be formulated accordingly.
         (1A) Origin of (Prokaryotic) Life: Of all the potential critical steps, dating the origin of life (abiogenesis) is the most difficult owing to the near-absence of sedimentary rocks and the action of processes like diagenesis and metamorphism <cit.>. We will adopt a conservative approach, and adopt the value of 3.7 Ga for the earliest robust evidence of life. There are two independent lines of evidence that support this date. The first is the recent discovery of stromatolite-like structures in the Isua Supracrustal Belt (ISB) by <cit.>. The second stems from the low δ^13C values in graphite globules from the ISB <cit.>, which is conventionally indicative of biological activity. The oldest microfossils, which arguably display evidence of cell structure (e.g. lumen and walls), were discovered in the Pilbara Craton and date from 3.4-3.5 Ga <cit.>. Here, it should be noted that even older claims for life do exist - the potentially biogenic carbon in a 4.1 Ga Jack Hills zircon <cit.> and putative microfossils > 3.8 Ga in the Nuvvuagittuq belt <cit.> are two such examples - but they are not unambiguous. As per our discussion, the timescale for abiogenesis on Earth (t_0) after the onset of habitability is t_0 ≈ 0.8 Gyr. From (<ref>), the cumulative probability is found to be 𝒫_1A = 0.54.
         (2A) Origin of Eukaryotes: The origin of the crown eukaryotes is believed to have occurred through endosymbiosis <cit.> between an archaeon <cit.> - probably a member of the Lokiarchaeota <cit.>, recently classified as belonging to the Asgard superphylum <cit.> - and a proto-mitochondrion <cit.> that was closely related to α-Proteobacteria <cit.>. This event was apparently a very important one from the standpoint of bioenergetics and the eventual increase in biological complexity <cit.>; see, however, <cit.> and <cit.> for dissenting viewpoints. The oldest fossils that appear to be unambiguously eukaryotic in origin are the vesicles from the Changzhougou Formation and date from approximately 1.65 Ga <cit.>. There are several other ostensibly eukaryotic microfossils that have been dated to 1.4-1.6 Ga, and possibly as old as 1.8 Ga <cit.>. Phylogenetic molecular clock models have yielded ages for the Last Eukaryotic Common Ancestor (LECA) ranging between 1 and 2 Ga, although recent studies are closer to the latter value <cit.>. Although earlier claims for eukaryotic microfossils exist, for e.g. in the 2.1 Ga Francevillian B Formation <cit.>, the 2.7 Ga shales from the Pilbara Craton <cit.>, the Transvaal Supergroup sediments from 2.5-2.7 Ga <cit.>, and the 2.7-2.8 Ga lacustrine deposits of South Africa <cit.>,[It should be noted that some analyses based on molecular clocks have also concluded that eukaryogenesis took place ≳ 2 Ga <cit.>.] we shall adopt the circumspect timing of 1.8 Ga for the origin of eukaryotes. The corresponding timescale of 2.7 Gyr leads us to the cumulative probability 𝒫_2A = 0.80.
         (3A) Origin of Plastids: In the original list of major evolutionary transitions <cit.>, sexual reproduction was present in place of plastids. An important reason for this alteration was because there exists sufficiently compelling evidence that LECA was a complex organism that was capable of sexual reproduction <cit.>; in other words, the origin of sexual reproduction was possibly coincident with eukaryogenesis <cit.>, although there is no a priori reason to believe that this apparent coincidence will always be valid on other inhabited exoplanets.
        
        The importance of plastids stems from the fact that they enable eukaryotic photosynthesis. Eukaryotes acquired this ability by means of endosymbiosis with a cyanobacterium <cit.>, thereby giving rise to the “primary” plastids in algae and plants <cit.>. This endosymbiosis is believed to have occurred around 1.5-1.75 Ga <cit.>, and these estimates appear to be consistent with the recent discovery of multicellular rhodophytes from 1.6 Ga <cit.>. However, recent evidence based on molecular clock analyses favors the origin of the Archaeplastida (that possess plastids) by 1.9 Ga <cit.>. We choose to err on the side of caution and use 1.5 Ga as the origin of the primary plastids. Upon calculating the cumulative probability using (<ref>), we find 𝒫_3A = 0.58.
         (4A) Origin of Complex Multicellularity: In this context, the rise of “complex multicellularity” refers to the emergence of plants, fungi and animals <cit.>. An important point worth noting here is that each of these clades could have originated at a different time. The earliest evidence for metazoan fossils has been argued to be at least 0.64 Ga <cit.>, but it cannot be regarded as wholly conclusive. Molecular clocks indicate that the last common ancestor of animals lived around 0.8 Ga or earlier <cit.>. The molecular clock evidence for plants suggests that their origins may extend as far back as ≈ 0.7-0.9 Ga <cit.>, although these methods are subject to much variability; the direct fossil evidence for plants is much more recent <cit.>. Lastly, the use of molecular clocks to determine the origin of fungi has led to the estimate of ≈ 0.76-1.06 Ga <cit.>. Thus, taken collectively, it seems plausible that the origin of complex multicellularity was about 0.8 Ga <cit.>, although the discovery of Bangiomorpha pubescens, whose age has been estimated to be ≲ 1.2 Ga <cit.>, could be construed as evidence for an earlier divergence time. This hypothesis gains further credibility in light of the distinctive increase in eukaryotic diversity documented in the fossil record at 0.8 Ga <cit.>. The cumulative probability for this step is 𝒫_4A = 0.47.
         (5A) Origin of Humans: More accurately, the revised version, <cit.> refers to the origin of “Societies with natural language”, thus emphasizing the role of language. Since anatomically and behaviorally modern humans evolved only ∼ 10^5 yr ago <cit.>, the timescale for the evolution of H. sapiens (or even genus Homo) since the onset of habitability is 4.5 Gyr. Hence, the cumulative probability is estimated to be 𝒫_5A = 0.37 by making use of (<ref>).
        
        
        Next, we shall outline the second strategy for deducing the five critical transitions. In order to do so, let us recall that the spacing between each critical step is roughly equal. From (<ref>), we find that Δ t_n ≈ 0.9 Ga. Thus, if we can identify five “important” transitions, i.e. the breakthroughs that occurred only once or a handful of times, during Earth's geobiological and evolutionary history that have a spacing of ≈ 0.9 Ga, they might potentially constitute the critical steps leading to technological intelligence. We will present our five transitions below, and offer reasons as to why they may happen to be the critical steps.
         (1B) Origin of Prokaryotic Life: Our choice of (1B) is the same as (1A). The issue of whether abiogenesis is an “easy” or a “hard” phenomenon remains currently unresolved <cit.>, but resolving this question will have important implications for gauging the likelihood of extraterrestrial life <cit.>. However, in the spirit of most conventional analyses, we will suppose that abiogenesis does constitute one of the critical steps. In this case, the cumulative probability turns out to be 𝒫_1B = 𝒫_1A = 0.54.
         (2B) Origin of Oxygenic Photosynthesis: The evolution of oxygenic photosynthesis, due to the origin of prokaryotes akin to modern cyanobacteria <cit.>, had a profound impact on the Earth's biosphere <cit.>. On metabolic grounds, there are strong reasons to posit the emergence of oxygenic photosynthesis as a major transition in its own right <cit.>. The many advantages due to oxygenic photosynthesis have been succinctly summarized by <cit.>. The addition of oxygen to the atmosphere led to the formation of the ozone layer, caused an increase in the diversity of minerals, enabled the creation of new ecological niches, and, above all, aerobic metabolism releases about an order of magnitude more energy compared to anaerobic metabolism <cit.>. The origin of oxygenic photoautotrophs remains very poorly constrained <cit.> with chronologies ranging between 3.8 Ga to 1.9 Ga, with the former estimate arising from indirect evidence of environmental oxidation based on U–Th–Pb isotopic ratios <cit.> and the latter representing the oldest direct evidence from microfossils <cit.>. If we naively take the mean of these two values, we obtain ≈ 2.7 Ga. There are several lines of evidence, although not all of them constitute robust biomarkers <cit.>, which appear to indicate that oxygenic photosynthesis evolved approximately 2.7 Ga or later <cit.>, i.e. a few 100 Myr prior to the onset of the Great Oxygenation Event (GOE). With the choice of t = 1.8 Gyr (which corresponds to 2.7 Ga) for oxygenic photosynthesis, we obtain a cumulative probability of 𝒫_2B = 0.53 after using (<ref>).
        
        As noted earlier, the origin of oxygenic photosynthesis has been subject to much controversy and uncertainty. Hence, it is quite conceivable that the GOE served as a critical step in the origin of complex (eukaryotic) life, and the attainment of sufficient oxygen levels could serve as an evolutionary bottleneck on exoplanets <cit.>. The GOE was a highly significant event that led to a considerable enhancement of Earth's atmospheric oxygen levels to ∼ 1% of the present-day value around 2.4 to 2.1 Ga <cit.> and potentially even higher afterwards <cit.> - see, however, <cit.> and <cit.> - thereby shaping Earth's subsequent evolutionary history <cit.>. If we choose the onset of the GOE as our critical step, we find that 𝒫_2B = 0.63.
         (3B) Origin of Eukaryotes: We have already remarked previously as to why eukaryogenesis represented such an important step. The origin of eukaryotes, entailing the putative endosymbiosis of mitochondria (followed by the acquisition of plastids and other organelles), has gained near-universal acceptance as a seminal innovation from the standpoints of phagocytosis, gene expression, bioenergetics, organismal complexity and cellular evolution <cit.>, and is conventionally classified as a major evolutionary transition <cit.>. The difference is that it constitutes the third step in our hypothesis, whereas it served as the second step in the original 5-step model. The cumulative probability in this case is 𝒫_3B = 0.48 since we have used the fact that eukaryogenesis occurred 1.8 Ga based on our preceding discussion in step (2A).
         (4B) Origin of Complex Multicellularity: Our choice of (4B) is the same as (4A). This is primarily motivated by the fact that the origin of these organisms (especially plants and animals) have led to a radical transformation of Earth's biosphere. More specifically, Earth's energy balance, biomass productivity, biogeochemical cycles, ecological niches and macroevolutionary processes have been shaped by the emergence of complex multicellular organisms <cit.>. Hence, in this case, we obtain the same cumulative probability, i.e. 𝒫_4B = 𝒫_4A = 0.47.
        
        An alternative possibility is to consider the Neoproterozoic Oxygenation Event (NOE) as the critical step. The NOE is akin to the GOE since it also entailed a rise in the atmospheric oxygen (to near-modern levels), but its exact timing and causes are unclear. In particular, it remains ambiguous as to whether the NOE served as a cause or a consequence of the origin of animals <cit.>. The timing is also very variable, with evidence from selenium isotopes apparently not ruling out the onset of the NOE as early as 0.75 Ga <cit.> while iron- and iodine-based proxies seem to demonstrate significant oxygenation only as recently as ∼ 0.4 Ga <cit.>. If we take the mean of these two quantities, the NOE would have taken place 0.55 Ga and this estimate is roughly consistent with recent analyses that have yielded values of ∼ 0.5-0.6 Ga <cit.>. If we assume the NOE to be a critical step instead, and use the value of t = 3.95 Gyr (i.e. 0.55 Ga), we obtain 𝒫_4B = 0.57.
        
        At this stage, the following issue merits a clarification. We have argued earlier that the diversification of metazoans commenced at 0.8 Ga <cit.>, while the NOE has been assigned a timing of 0.55 Ga. Hence, this raises the question as to how animal evolution took place in the presence of low oxygen levels. This discrepancy can be explained if the oxygen requirements for early animals (akin to modern demosponges) were sufficiently low <cit.>, or if the earliest metazoans were anaerobic altogether such as some species belonging to the phylum Loricifera<cit.>. Alternatively, factors other than oxygen - the most notable among them being, arguably, the availability of bioessential nutrients such as phosphorus <cit.> - may have been responsible for regulating the advent of animals.
         (5B) Origin of Humans (Technological Intelligence): Our fifth step is essentially the same as that of the previous model (5A) on account of the following reasons. In addition to the distinctive ability to construct and utilize sophisticated tools (giving rise to complex information- and technology-driven networks), other attributes such as cumulative cultural transmission, social learning, mental time travel, large-scale social cooperation, mind reading, recursion and syntactical-grammatical language are also often cited as being unique to humans <cit.>.[On the other hand, it should also be appreciated that several “human” characteristics such as culture, intelligence, morality, foresight and consciousness have been, to varying degrees of controversy, associated with other species <cit.>.] Lastly, humans have also caused major (perhaps irrevocable) large-scale shifts in the functioning of Earth's biosphere <cit.> to the extent that the Earth's current geological epoch, the Anthropocene, has been primarily shaped by us <cit.>.[It is conceivable that the emergence of technological intelligence on other exoplanets may be accompanied by an equivalent Anthropocene epoch <cit.>.] The cumulative probability for this step is given by 𝒫_5B = 𝒫_5A = 0.37.
        <cit.> introduced a parameter to estimate the goodness of fit:
        
            δ = 1/n[∑_r=1^n (𝒫_r,n - 0.5)^2]^1/2,
        
        and a lower value of δ corresponds to a better fit. If each of the cumulative probabilities (𝒫_r,n) were close to either 0 or 1, it would mean that the events are clustered towards the beginning or the end, thereby constituting a poor fit. For the 5-step model (1A-5A), we find δ_A = 0.068. In contrast, if we use the 5-step model (1B-5B), we find δ_B = 0.029; even if use the GOE and the NOE in place of the steps (2B) and (4B) respectively, we find δ_B = 0.04. Thus, we find that the second 5-step model (1B-5B) is approximately twice more accurate than the first model (1A-5A).
        
        
        
        
         §.§ A six-step model
        <cit.> concluded that a 5- or 6-step model represented the best fit for the total number of critical steps on our planet. Apart from the two 5-step models delineated earlier, we note that another candidate is the “energy expansions” paradigm proposed by <cit.> that also involves 5 steps in total. Here, we will outline a 6-step model based on the “megatrajectories” paradigm introduced by <cit.> and assess whether it constitutes a good fit for the critical step framework.
        
        
          * From the Origin of Life to the Last Common Ancestor (LCA) of Extant Life: As with the steps (1A) and (1B), we note that there is insufficient evidence to properly date the age when abiogenesis occurred and when the LCA lived. However, as we have argued in Sec. <ref>, the earliest definitive evidence for life appears to be around 3.7 Ga. In this scenario, with t_0 = 0.8 Gyr and t_H = 5.5 Gyr, we use (<ref>) to obtain 𝒫_1,6 = 0.61.
            
          * The Metabolic Diversification of Bacteria and Archaea: The first evidence for methanogens is arguably from hydrothermal precipitates dated 3.5 Ga <cit.>, although molecular clock analyses lead to the even earlier date of at least 3.8 Ga <cit.>. The earliest iron- and sulfate-reducing microbes also potentially appear in the fossil record at approximately the same time <cit.>. There is also some evidence suggesting that methanotrophy or the Wood-Ljungdahl pathway was operational at 3.4 Ga <cit.>. The record for nitrogen fixation implies that it was present by 3.2 Ga <cit.>, or perhaps even earlier <cit.>. Thus, taken collectively there is considerable evidence indicating that metabolic diversification had occurred by 3.4-3.5 Ga <cit.>. We will therefore adopt t = 1.1 Gyr (i.e. 3.4 Ga), which results in 𝒫_2,6 = 0.34.
            
          * Evolution  of  the  Eukaryotic Cell: This megatrajectory is essentially the same as steps (2A) and (3B). Using the timing identified therein, we find 𝒫_3,6 = 0.64. 
            
          * Multicellularity: It is well-known that multicellularity has evolved repeatedly over Earth's history, and has been therefore characterized as a “minor” major transition <cit.>. On the other hand, organisms that fall under the bracket of “complex multicellularity” belong to only six clades <cit.>. If the latter serves as the actual critical step, we have already discussed its timing in steps (4A) and (4B) and we end up with 𝒫_4,6 = 0.23.
            
          * Invasion of the Land: Although the first land-dwelling organisms appeared in the Precambrian <cit.>, the Paleozoic radiation of the land plants (embryophytes) facilitated a major ecological expansion. The earliest fossil evidence dates from the mid Ordovician <cit.>, although it is conceivable that land plants may have originated in the Cambrian <cit.>. Consequently, the fossil record is in good agreement with molecular clock evidence that dates land plants to 0.45-0.55 Ga <cit.>.[However, there are other molecular clock studies that favor a Proterozoic origin of land plants instead <cit.>.] Thus, by choosing t ≈ 4 Gyr, we find 𝒫_5,6 = 0.48.
            
          * Intelligence and Technology: This megatrajectory is essentially the same as steps (5A) and (5B). The corresponding cumulative probability is 𝒫_6,6 = 0.30.
        
        By using (<ref>), we compute the goodness of fit for this 6-step model. We find that δ = 0.069, which is virtually identical to δ_A (although lower than δ_B by a factor of about 2). Hence, this demonstrates that the megatrajectories considered here are a fairly good fit insofar our model is concerned; the resultant value of δ is lower than the 5- or 6-step model analyzed in <cit.>.
        
        
        
         §.§ The ramifications of an extended habitability interval
        
        As noted in Sec. <ref>, recent theoretical studies indicate that the Earth may remain habitable (modulo anthropogenic change) to 2 Gyr in the future. With this revised estimate, the value of t_H now becomes 6.5 Gyr. As before, let us assume that humans represent the n-th step. By calculating the value of n using (<ref>), we find n ≈ 2.25. Hence, this estimate suggests that a 2-step model (or possibly a 3-step one) has the greatest likelihood of being valid; <cit.> also reached a similar conclusion.  
        
        We are confronted with the question as to what was the first critical step. The spacing between the critical steps must be approximately 2.2 Gyr as seen from (<ref>). Since the advent of humans at 4.5 Gyr (i.e. 0 Ga) constitutes the second step, the timing of the first critical step must have been approximately 2.2 Ga. As noted in Sec. <ref>, the timing of the GOE (between 2.1 to 2.4 Ga) falls within this range. The GOE had profound consequences for Earth's subsequent evolutionary history, and therefore represents a strong contender for the first critical step. Other notable candidates that lie approximately within the same time frame include the evolution of (i) oxygenic photosynthesis and (ii) eukaryotes. In the 2-step model, the origin of life (abiogenesis) is not likely to have been a critical step; in this regard, the 2-step model is akin to the original 1-step model proposed by <cit.>.
        
        However, we can ask ourselves the following question: if the origin of life was a critical step, how many steps were there in total? If we assume that abiogenesis was the first step and that the mean time for this step was equal to the abiogenesis timescale of 0.8 Gyr, from (<ref>) we find n ≈ 7.1. In contrast, if we had assumed that t_H = 5.5 Gyr and repeat the calculation, we arrive at n ≈ 5.9. This leads us to the following conclusions:
        
          *  For the case where habitability ends 1 Gyr in the future, a 6-step model would be favored, although the 5-step model may also be plausible <cit.>. The choice of n=5 is consistent with previous analyses and the discussion in Sec. <ref>.
            
          *  When the habitability boundary extends to 2 Gyr in the future, a 7-step model would represent a good fit. Let us assume that t̅_r,7≈ 4.5 Gyr, i.e. that humans are the r-th critical step. From (<ref>), we obtain r ≈ 5.5, implying that the evolution of humans could have been either the fifth or sixth critical step. In other words, there are still 1 or 2 critical steps ahead in the future, which we will discuss shortly hereafter.
        
        
        As noted above, there is a possibility that humans are not the n-th critical step, but merely the r-th one (with r ≤ n). In Secs. <ref> and <ref>, we have seen that there are compelling reasons to believe that humanity was the fifth critical step. Therefore, with r = 5 and assuming t̅_5,n≈ 4.5 Gyr, we can estimate the value of n using (<ref>).
        
          *  If t_H = 5.5 Gyr, and using the above values, we find n = 5.1. In other words, when Earth's habitability ends about 1 Gyr in the future, the 5-step model is relatively favored and the evolution of humans is the last critical step.
            
          *  Using the above parameters in conjunction with t_H = 6.5 Gyr leads us to n ≈ 6.2. Hence, if the Earth becomes uninhabitable 2 Gyr in the future, the 6-step model seems the most likely. In this case, since humans are the fifth critical step, there is one critical step that is yet to occur.
        
        
        Based on our discussion thus far, two broad inferences can be drawn. First, assuming that the habitability window ends 1 Gyr in the future, the critical step model with n=5 is likely to be valid and humans represent the final critical step. In contrast, if the habitability window is extended to 2 Gyr in the future, we suggest that the 6-step model could be the best fit and that the rise of humans represents the fifth critical step. In other words, there is still one step in the future which is unaccounted for. Naturally, it is not possible to identify this critical step prior to its occurrence. 
        
        One possibility is the emergence of superintelligence <cit.>, especially in light of recent advancements (and concerns) in Artificial Intelligence (AI) - the paradigm shifts presumably necessary for the genesis of “human-like” AI have been discussed in detail by <cit.>. However, the major issue from the standpoint of the critical step model is that the timescale between the first appearance of H. sapiens and the dawn of AI superintelligence is currently predicted to be very low, i.e. on the order of 10^5 to 10^6 yrs, compared to the characteristic separation between successive critical steps (∼ 10^9 yrs); consequently, it remains unclear as to whether superintelligence can be regarded as a genuine critical step. This discrepancy might be resolved if the origin of superintelligence entails a much longer time than currently anticipated. 
        
        An underlying assumption pertaining to the above discussion is that we have automatically presupposed that the biological “complexity”<cit.> increases monotonically with time. The pitfalls of subscribing to implicit teleological arguments, certain theories of orthogenesis, and the “March of Progress” are many and varied <cit.>,[Yet, many of the critical step models discussed in the literature take it for granted that the evolution of humans constitutes the last critical step regardless of the duration of the habitable period of the Earth.] and therefore it does not automatically follow that the sixth step alluded to earlier will lead to species of greater complexity. Hence, it does not seem implausible that the contrary could occur, especially if the environmental conditions ∼ 1 Gyr in the future are less clement than today.
        
        
        
        § CRITICAL STEPS ON EXOPLANETS
        
        We will now study some of the salient features of multi-step models on exoplanets, and discuss the resulting implications. A similar topic was studied recently (using the Bayesian framework) by <cit.> recently, but we incorporate additional constraints on habitability imposed by stellar physics in our treatment.
        
        
        
         §.§ The Habitable Zone of Earth-Analogs
        
        
        
        The habitable zone (HZ) is defined as the region around the host star where liquid water can exist on the planet's surface. The HZ is dependent on both planetary and stellar parameters, and evolves dynamically over time <cit.>. The HZ is typically computed for “Earth-analogs”, i.e. planets whose basic physical, chemical and geological parameters are similar to that of Earth. In our subsequent discussion, we will implicitly deal with Earth-analogs in the HZ of their host stars.[Thus, we shall not focus on habitable worlds outside the HZ, which are expected to be much more commonplace compared to those within the HZ <cit.>.]
        
        Clearly, the upper bound on the habitability of a planet is the stellar lifetime. However, the maximum duration that the planet remains habitable is less than the stellar lifetime for a simple reason: the stellar luminosity increases over time, and the planet will eventually enter a runaway greenhouse phase and become uninhabitable (like Venus). Thus, the duration of habitability is essentially specified by the temporal extent of the continuously habitable zone (denoted here by CHZ). By using the knowledge about the inner and outer boundaries of the HZ in conjunction with stellar evolution models, it is feasible to estimate the total duration of time (t_HZ) that an Earth-analog will remain inside the HZ as a function of the stellar mass M_⋆. This effort was undertaken by <cit.>, and by making use of Fig. 11 and Table 5 in that paper, we introduce the scaling:
        
            t_HZ∼ 0.55 t_⊙ (M_⋆/M_⊙)^-2     M_⋆ > M_⊙, 
                   t_HZ∼ 0.55 t_⊙ (M_⋆/M_⊙)^-1      0.5 M_⊙ < M_⋆ < M_⊙, 
                   t_HZ∼ 0.46 t_⊙ (M_⋆/M_⊙)^-1.25   M_⋆ < 0.5 M_⊙,
        
        where t_⊙∼ 10 Gyr and M_⊙ is the solar mass. Our choice of normalization constant differs from <cit.> since we have adopted the more conservative habitability duration of 5.5 Gyr for the Earth-Sun system. By inspecting (<ref>), it is apparent that low-mass stars are characterized by CHZs that last for a longer duration of time, which is along expected lines since they have longer main-sequence lifetimes <cit.>.
        
        Now, let us consider the highly simplified model wherein we suppose that the timescale for abiogenesis is the same on all exoplanets, and that the duration of habitability is given by t_HZ. Since abiogenesis is taken to be the first critical step, from (<ref>) with r=1 we find
        
            n_⋆ = t_HZ/t_⊕(n_⊕ + 1) - 1,
        
        where t_⊕ = 5.5 Gyr is the habitability duration of the Earth and n_⊕ is the number of critical steps on Earth, while n_⋆ represents the corresponding number of steps for Earth-analog orbiting a star of mass M_⋆. We will henceforth use n_⊕ = 5 as this value has been advocated by several authors. Moreover, as we have seen from Sec. <ref>, there are reasons to believe that n_⊕ = 5 constitutes a fairly good fit. Thus, from (<ref>), n_⋆ can be estimated as a function of M_⋆, and this plot is shown in Fig. <ref>. The value of n_⋆ decreases when M_⋆ is increased, and shortly after 1.7 M_⊙ the value of n_⋆ drops below unity.
        
        
        
         §.§ Constraints on habitability imposed by stellar physics
        
        The preceding analysis implicitly assumed that the only timescale for habitability was t_HZ. In reality, there are a number of factors governed by stellar physics that influence habitability <cit.>. In particular, there has been a growing appreciation of the role of space weather in governing habitability, i.e. for e.g. the role of stellar flares <cit.>, coronal mass ejections <cit.>, stellar energetic particles <cit.> and stellar winds <cit.> to name a few.
        
        Since the presence of an atmosphere is necessary for maintaining liquid water on the surface of a planet, its complete depletion would lead to the termination of habitability insofar surficial life is concerned. For Earth-analogs that are closer to their low-mass host stars, they are subjected to intense stellar winds that can deplete their atmospheres over short timescales. The significance and magnitude of stellar wind erosion has been thoroughly documented in our Solar system <cit.>. The approximate timescale t_SW associated with total atmospheric escape due to stellar wind erosion <cit.> is
        
            t_SW∼  100 t_⊙ (M_⋆/M_⊙)^4.76,
        
        for an Earth-analog assuming that the star's rotation rate is similar to the Sun. The analytic model displays consistency with the trends and values discerned from numerical simulations <cit.>. A few points should be noted regarding the above formula. First, incorporating the effects of stellar flares and coronal mass ejections <cit.> is expected to decrease this timescale, perhaps by more than one order of magnitude. Second, this formula was derived under the assumption of a constant atmospheric escape rate. In reality, the escape rate may decrease by two orders of magnitude as the age of the star increases <cit.>. As a result, the higher escape rates in the past would further decrease the value of t_SW. Lastly, we observe that (<ref>) is applicable to unmagnetized planets. In contrast, if one considers strongly magnetized planets, the atmospheric escape rate could decrease by a factor of ≲ 10<cit.>, but the converse is also possible <cit.>. In general, the escape rate is anticipated to be a non-monotonic function of the magnetic field <cit.>.
        
        Thus, as per the preceding discussion, the actual duration of habitability should be defined as t_H = min{t_HZ,t_SW}. In other words, if t_SW < t_HZ, the planet loses its atmosphere before it exits the CHZ and vice-versa. From (<ref>) and (<ref>), we find
        
            t_H ∼ 0.55 t_⊙ (M_⋆/M_⊙)^-2     M_⋆ > M_⊙, 
                   t_H ∼ 0.55 t_⊙ (M_⋆/M_⊙)^-1      0.5 M_⊙ < M_⋆ < M_⊙, 
                   t_H ∼ 0.46 t_⊙ (M_⋆/M_⊙)^-1.25   0.41 M_⊙ < M_⋆ < 0.5 M_⊙, 
                   t_H ∼ 100 t_⊙ (M_⋆/M_⊙)^4.76      M_⋆ < 0.41 M_⊙.
        
        
        Our next stellar constraint stems from the availability of biologically active ultraviolet (UV) radiation. The importance of UV radiation partly stems from the fact that it constitutes the most dominant energy source for prebiotic synthesis on Earth <cit.>. Although theories of abiogenesis are many and varied <cit.>, there is a strong case that can be made for UV radiation as the driver of prebiotic chemistry <cit.>, especially with regards to the RNA world <cit.>, on account of the following reasons:
        
          *  Laboratory experiments have shown that UV light provides a selective advantage to RNA-like molecules due to the presence of nitrogenous bases <cit.>, and may therefore play an important role in facilitating their oligomerization <cit.>. 
            
          *  The observed tendency in myriad origin-of-life experiments to form complex organic mixtures incapable of Darwinian evolution is referred to as the “asphalt problem”. Recent experiments have shown that this issue might be bypassed in suitable geological environments (e.g. intermountain valleys), and that UV radiation can potentially facilitate the synthesis of nucleosides, nucleotides, and perhaps RNA <cit.>.
            
          *  The synthesis of important biomolecules without excessive human intervention and under conditions that are presumed to resemble Hadean-Archean environments has proven to be challenging. However, there have been several breakthroughs in recent times that are reliant on UV light <cit.>. More specifically, the biologically relevant compounds produced include: (i)  pyrimidine ribonucleotides and β-ribonucleosides <cit.>, (ii) building blocks of sugars, such as glycolaldehyde and glyceraldehyde <cit.>, (iii) precursors of nucleic acids, amino acids, lipids and carbohydrates <cit.>, and (iv) iron-sulfur clusters <cit.>.
            
          *  RNA nucleotides have been shown to be stable when radiated by UV photons, and this has been argued to be evidence that they could have originated in the high-UV environments of Hadean-Archean Earth <cit.>. 
        
        Another major theory for the origin of life posits that it occurred in submarine hydrothermal vents <cit.>. This theory does have many advantages of its own <cit.>, and recent evidence suggesting that the LCA was thermophilic in nature is consistent with hydrothermal vents being the sites of abiogenesis <cit.>. However, it cannot be said at this stage that the LCA was definitively a thermophile, since other studies point to a mesophilic origin <cit.>. A recent study by <cit.> assessed seven factors ostensibly necessary for life's origination, and concluded that submarine hydrothermal vents may potentially face difficulties in fulfilling all of these criteria. 
        
        It should be noted that UV radiation is expected to have other biological ramifications as well, both positive and negative. One of the downsides associated with high doses of UV radiation is that it can inhibit photosynthesis and cause damage to vital biomolecules (e.g., DNA) on Earth <cit.>. Yet, there are potential benefits stemming from UV radiation due to its proficiency in stimulating mutagenesis and thereby serving as a selection agent <cit.>. In particular, it is conceivable that UV radiation could have facilitated the emergence of evolutionary innovations such as sexual reproduction <cit.> and enhanced the rates of molecular evolution and speciation <cit.>. It is therefore important to appreciate the possibility that there may be a number of advantages arising from UV radiation.
        
        Hence, in our subsequent discussion, we will posit that the origin of life on Earth-analogs was driven by UV radiation. In this scenario, the rate of prebiotic chemical reactions is assumed to be constrained by the available bioactive UV flux at the surface <cit.>. The latter can be estimated solely as a function of M_⋆, thereby leading us to the abiogenesis timescale t_⋆<cit.>:
        
            t_⋆∼ t_0 (M_⋆/M_⊙)^-3     M_⋆≲ M_⊙, 
                   t_⋆∼ t_0 (M_⋆/M_⊙)^-1     M_⋆≳ M_⊙,
        
        where t_0 = 0.08 t_⊙ = 0.8 Gyr. Next, we consider the ratio μ≡ t_H/t_⋆ because of its significance. If μ < 1, then the duration of habitability is lower than the timescale for abiogenesis, thus implying that such Earth-analogs are not likely to host life. We have plotted μ as a function of M_⋆ in Fig. <ref>. A couple of conclusions can be drawn from this figure. For M_⋆≲ 0.4 M_⊙, we find that μ < 1 indicating that planets in the HZ of these stars have a lower chance of hosting life. Second, we find that the curve flattens out when M_⋆≳ M_⊙ but it does attain a slight peak at M_⋆ = M_⊙. Although this maximum is attained exactly at M_⊙ due to the ansatzen used in this paper, the peak of the curve has a high likelihood of being in the vicinity of M_⋆, thereby suggesting that Sun-like stars may represent the most appropriate targets in the search for life <cit.>.
        
        In making use of (<ref>), we have operated under the implicit assumption that stellar flares do not alter our results significantly. In actuality, stellar flares can deliver transient and elevated doses of UV radiation that are anticipated to have both positive and negative outcomes <cit.>. In particular, a recent analysis by <cit.> concluded that the background UV fluxes of stars with T_⋆ < 4400 K, with T_⋆ denoting the effective stellar temperature, may not suffice for enabling UV-mediated prebiotic pathways to function efficiently. In some instances, however, stellar flares deliver sufficient UV photons to permit these reactions to occur. When the occurrence rate of flares (Ṅ_f) exceeds the following threshold, UV-mediated prebiotic pathways could become functional.
        
            Ṅ_f ≳ 3.36 × 10^2 day^-1 (E_f/10^34 erg)^-1(R_⋆/R_⊙)^2 (T_⋆/T_⊙)^4,
        
        where E_f is the flare energy and R_⋆ is the stellar radius <cit.>. Based on the data from the TESS mission, it has been estimated that 62 stars out of a sample of 632 flaring M-dwarfs fulfill the above criterion <cit.>. Thus, it might be reasonable to assume that the majority of low-mass stars do not flare frequently enough to compensate for the paucity of UV photons.
        
        
        
        We turn our attention now to the 5-step model introduced in Sec. <ref>. Since we have argued that abiogenesis was the first critical step, using (<ref>) along with t_H = 5.5 Gyr for the Earth leads us to t̅_1,5≈ 0.92 Gyr. Thus, we see that the timescale specified for the origin of life on Earth (t_0 = 0.8 Gyr) obeys t_0 ≈t̅_1,5. As noted earlier, this is not surprising since the mean time taken for the r-th critical step in a viable model is comparable to its actual timescale <cit.>. If we assume that this condition is also valid on other potentially habitable planets, we have t̅_1,n_⋆≈ t_⋆. Using this relation in conjunction with (<ref>), (<ref>) and (<ref>), the value of n_⋆ is found to be
        
            n_⋆ = (t_H/t_⊕)(t_⋆/t_0)^-1(n_⊕ + 1) - 1.
        
        Fig. <ref> depicts the dependence of n_⋆ on the stellar mass. From this plot, we see that n > 0 occurs only for M ≳ 0.4 M_⊙ and this result is in agreement with Fig. <ref>, since planets orbiting such stars have a habitability duration that is shorter than the abiogenesis timescale. Second, we observe a clear peak at M_⋆ = M_⊙, and this behavior is also observed in some of the subsequent figures. This result is consistent with our earlier discussion: although the peak arises due to the scaling relations employed herein, there is still a strong possibility that the maximum number of critical steps occur when M_⋆≈ M_⊙. It lends further credibility to the notion that G-type stars are the optimal targets in the search for life-bearing planets. Our results are qualitatively consistent with the Bayesian analysis by <cit.>, who concluded that: (i) the likelihood of life around M-dwarfs must be selectively suppressed, (ii) G-type stars are the most suitable targets for SETI (Search for Extraterrestrial Intelligence) observations, and (iii) the number of critical steps leading to intelligence is not likely to exceed five.
        
        An important point to recognize here is that although the value of n ≈ 5 occurs in the vicinity of M_⋆≈ M_⊙, this does not altogether preclude stars outside this range from hosting planets with technologically sophisticated species. This is because the total number of critical steps leading to the emergence of life and intelligence on other planets is unknown, and there are no reasons to suppose the total number of critical steps will always be the same.[In light of the undoubted evolutionary and ecological significance of the breakthroughs discussed in Sec. <ref>, it may be tempting to conclude that they are sufficiently general, and argue that the convergent evolution of humanoids is “inevitable”  if all these transitions are successful <cit.>. However, in spite of the impressive and rapidly increasing list of convergent mechanisms and organs <cit.>, this standpoint appears to be overly anthropocentric.] On the other hand, once the number of critical steps drops below unity, it becomes rather unlikely that such stars (with M_⋆ < 0.5 M_⊙) would have planets where intelligence can arise.
        
        
        
        With these caveats in mind, we will, nevertheless, hypothesize that the critical steps discussed in Sec. <ref> (for Earth) are sufficiently general, and therefore applicable to other exoplanets. As we have seen, there are two constraints that were employed in our analysis: (I) the n steps must occur in the interval [0,t_H], and (II) the first step (r=1), namely abiogenesis, must occur at t_0. Hence, it follows that the remaining n-1 steps must unfold in remaining time interval. We introduce the expression for the PDF of the (n-1)-th step (in a sequence of n-1 steps) in the time t'-t_0:
        
            P_n-1,n-1(t'-t_0) = 𝒞(t'-t_0)^n-2,
        
        with 𝒞 being a constant, based on Sec. <ref>. By integrating this PDF over the interval [t_0,t_H], we obtain the probability, denoted by 𝒫_n(M_⋆), for all n steps to occur (since we have already imposed the constraint that the first step is attained at t_0). The constant of proportionality 𝒞 is calculated by demanding that 𝒫_n(M_⋆) = 1 when t_0 = 0 because of criterion (I). This yields
        
            𝒫_n(M_⋆) = (1 - t_0/t_H)^n-1,
        
        and the same formula can be obtained from (<ref>) with r → n-1, n → n-1 and t → t_H - t_0; see also <cit.>. Note that this formula is valid only when t_0 < t_H, which automatically excludes stars with M_⋆≲ 0.4 M_⊙. There are two important scenarios worth considering from the standpoint of detecting the fingerprints of life:
        
          *  The probability of technological intelligence: This requires n=5 based on the above assumptions. In this case, it will be theoretically possible to detect signs of intelligent life by searching for technosignatures because they are more distinctive. 
            
          *  The probability of detectable primitive life: From the standpoint of microbial life, most of the well-known biosignatures like oxygen and ozone are not detectable until they have attained a certain level <cit.>. Hence, although Earth had life throughout most of its history, the low concentrations of oxygen and ozone until the GOE would have led to a “false negative”<cit.>. Based on our discussion in Sec. <ref>, the evolution of oxygenic photosynthesis and the GOE correspond to n=2 (or n=3).
        
        We have plotted (<ref>) as a function of the stellar mass in Fig. <ref>. It is seen that the peak is at M_⋆ = M_⊙, and that the curves rise sharply at M_⋆≈ 0.5 M_⊙. The figure indicates that an Earth-analog around a G-type star would have the highest probability of achieving the critical steps necessary for detectable primitive or intelligent life. 
        
        We must however point out an important caveat here. For M_⋆ = M_⊙ and n = 5, we obtain the probability 𝒫_n(M_⊙) ≈ 0.5 upon making use of (<ref>). In other words, this result should imply that the likelihood of attaining the fifth critical step (intelligence), even with the constraints (I) and (II), is about 50%. Naturally, this value appears to be very high, but it must recognized that we have merely calculated the mathematical probability. In reality, there will be a vast number of other criteria - for example, the presence of oceans (and continents), sufficiently high concentrations of bioessential elements, the existence of plate tectonics, the maintenance of a stable climate over Gyr timescales <cit.> - that must be simultaneously satisfied in order for each critical step to occur.[Although the total number of necessary and sufficient conditions that must have been fulfilled for all of the major transitions in Earth's history to occur was probably very high, especially if evolutionary contingency played a noteworthy role <cit.>, we cannot say for certain whether every one of these evolutionary steps obeys the Anna Karenina principle, i.e. the premise that the absence (or breakdown) of even a single factor is capable of dooming a particular process to failure <cit.>.] Hence, it is more instructive to view (<ref>) as an upper bound, and use it to assess the relative chances of life-bearing planets existing around stars of differing masses.
        
        
        
        Hitherto, we have only discussed the prospects for life on an Earth-analog around a given star. However, it should be recognized that the total number of stars also varies depending of their mass, i.e. low-mass stars are more numerous than high-mass ones <cit.>. Thus, we can calculate the relative number of stars ζ_⋆ = 𝒩_⋆/𝒩_⊙ with detectable primitive or intelligent life, with the total number of stars 𝒩_⋆ defined as follows:
        
            𝒩_⋆ = 𝒫_n(M_⋆) d N_⋆/d(ln M_⋆),
        
        where dN_⋆/d(ln M_⋆) represents the number of stars per logarithmic mass interval, and is calculated from the stellar initial mass function (IMF); here, we will use the IMF proposed by <cit.>. Note that 𝒫_n(M_⋆) is given by (<ref>) and can be viewed as a measure of the probability of planets with life (and have passed through the n critical steps) per star.[We have not included an additional factor for the number of planets in the HZ of the host star because this quantity appears to be mostly independent of the stellar mass <cit.>.] 
        
        We have plotted ζ_⋆ as a function of M_⋆ in Fig. <ref>. Let us begin by observing that ζ_⋆ = 1 when M_⋆ = M_⊙ by definition. For n = 5, we find that the peak occurs at M_⋆ = M_⊙, implying that solar-mass stars in our Galaxy are the most numerous in terms of planets with intelligent life. On the other hand, for n = 2, the peak is seen at M_⋆≈ 0.6 M_⊙ (and at M_⋆≈ 0.77 M_⊙ when n = 3) indicating that K-type stars are potentially the most numerous in terms of having primitive, but detectable, life. For the values of n considered herein, we find that ζ_⋆ is almost constant in the range 0.5 M_⊙ < M_⋆ < 1.5 M_⊙ suggesting that these stars are the best targets in the search for life. 
        
        The underlying reason for K-type stars being potentially more numerous in terms of hosting planets with detectable microbial biospheres stems from the fact that there are two distinct factors in (<ref>). Hence, even though the probability per K-type star is comparatively lower than Sun-like stars - see Fig. <ref> - the second factor (relative number of K-type stars) compensates for the first, namely the probability 𝒫_n(M_⋆), in (<ref>). On the other hand, when it comes to hosting planets with technological intelligence, the first factor in (<ref>) dominates over the second, thereby ensuring that ζ_⋆ peaks for solar-type stars in Fig. <ref>.
        
        
        
        
        § CONCLUSION
        
        We began by outlining a simple mathematical model predicated on the notion that evolution is effectively modeled as a series of independent “hard” steps. One of the primary objectives was to study the ramifications of this model for the timing and likelihood of primitive and intelligent life on Earth and Earth-like exoplanets around other stars. 
        
        We began our analysis by focusing on the Earth and studying the total number of critical steps (n) that are likely on Earth based on the latest developments in geobiology. We found that the result depended on the time at which the Earth becomes uninhabitable in the future. For the more conservative estimate of 1 Gyr, we found that n=5 probably represents the best fit, in agreement with previous studies. Unlike the standard 5-step model <cit.> based on the classic paradigm of major evolutionary transitions <cit.>, we proposed that the following five steps could have represented vital breakthroughs in the history of life on Earth: (i) abiogenesis, (ii) oxygenic photosynthesis, (iii) eukaryogenesis (endosymbiosis leading to the acquisition of mitochondria and subsequently plastids), (iv) complex multicellularity (e.g. animals and plants), and (v) genus Homo (H. sapiens in particular). On the other hand, if the Earth's habitability comes to an end 2 Gyr in the future, we suggested that a 6-step model might represent the best fit, wherein the emergence of humans constituted the fifth critical step with one major transition yet to occur in the future.
        
        Subsequently, we applied this model to study the prospects for life on Earth-analogs orbiting stars of different masses. Our analysis took into account constraints based on: (i) the duration of the continuously habitable zone, (ii) atmospheric escape due to stellar wind erosion, and (iii) availability of bioactive UV flux to promote abiogenesis. We found that the timescale for abiogenesis is longer the duration of habitability for M_⋆ < 0.4 M_⊙, strongly suggesting that such stars are not likely to host life-bearing planets. The prospects for primitive or intelligent life are highest for a generic Earth-analog around a solar-mass star based on this analysis.
        
        Next, we computed the total number of stars (relative to the solar value) that could give rise to detectable signatures of primitive and intelligent life. With regards to the former, we found that the number peaks in the range 0.6-0.8 M_⊙, implying that certain K- and G-type stars should potentially be accorded the highest priority in the hunt for biosignatures. Our analysis and conclusions are in agreement with previous studies of this subject <cit.>. On the other hand, the total number of stars with intelligent life exhibited a peak near M_⋆≈ M_⊙, thereby implying that Sun-like stars represent the best targets for SETI. This could also serve to explain why technological intelligence like our own finds itself in the vicinity of a solar-mass star, despite the fact that low-mass stars are more numerous and long-lived <cit.>.
        
        Naturally, there are a number of caveats that must be borne in mind with regards to the above conclusions. It is by no means clear as to whether evolution truly proceeds through a series of “hard” steps, and that the number and nature of these steps will be similar on other exoplanets. Our analysis has dealt solely with the stellar mass, although other stellar parameters (e.g. activity, rotation, metallicity) play an important role. Moreover, by focusing exclusively on Earth-analogs, we have not taken the intricate non-equilibrium biogeochemical factors that have shaped Earth's evolutionary history into consideration. Our discussion also ignored the possible transfer of life between stars. Such transfer may involve lithopanspermia <cit.>, directed panspermia <cit.> or interstellar travel and habitation by technologically advanced species <cit.>. If such transfer events are common enough, which might be the case in some environments <cit.>, they could blur the quantitative conclusions of this paper because of diffusion processes and Galactic differential rotation <cit.>. 
        
        In spite of these limitations, it seems plausible that the critical step framework can be used to assess the relative merits of different models of the major evolutionary transitions on Earth. Furthermore, it also provides a useful formalism for gauging the relative likelihood of life on Earth-like planets orbiting different stars given the sparse data available at the current stage. Lastly, it offers testable predictions in the future, and, in principle, can therefore be falsified.
        
        
        
        § ACKNOWLEDGMENTS
        
        ML is grateful to Andrew Knoll for the illuminating and thought-provoking conversations. This work was supported in part by the Breakthrough Prize Foundation for the Starshot Initiative, Harvard University's Faculty of Arts and Sciences, and the Institute for Theory and Computation (ITC) at Harvard University.
        
        
        [Abramov and Mojzsis2009]AM09Abramov, O. and Mojzsis, S. J. (2009).
        Microbial habitability of the Hadean Earth during the late heavy
          bombardment.
         Nature, 459(7245):419–422.
        
        [Adams and Laughlin1997]AL97Adams, F. C. and Laughlin, G. (1997).
        A dying universe: the long-term fate and evolutionof astrophysical
          objects.
         Rev. Mod. Phys., 69(2):337–372.
        
        [Airapetian et al.2017]AG17Airapetian, V. S., Glocer, A., Khazanov, G. V., Loyd, R. O. P.,
          France, K., Sojka, J., Danchi, W. C., and Liemohn, M. W. (2017).
        How Hospitable Are Space Weather Affected Habitable Zones? The Role
          of Ion Escape.
         Astrophys. J. Lett., 836(1):L3.
        
        [Akanuma et al.2013]ANY13Akanuma, S., Nakajima, Y., Yokobori, S., Kimura, M., Nemoto, N.,
          Mase, T., Miyazono, K., Tanokura, M., and Yamagishi, A. (2013).
        Experimental evidence for the thermophilicity of ancestral life.
         Proc. Natl. Acad. Sci. USA, 110(27):11067–11072.
        
        [Albani et al.2010]ABC10Albani, A. E., Bengtson, S., Canfield, D. E., Bekker, A.,
          Macchiarelli, R., Mazurier, A., Hammarlund, E. U., Boulvais, P.,
          Dupuy, J.-J., Fontaine, C., Fürsich, F. T., Gauthier-Lafaye, F.,
          Janvier, P., Javaux, E., Ossa, F. O., Pierson-Wickmann, A.-C.,
          Riboulleau, A., Sardini, P., Vachard, D., Whitehouse, M., and
          Meunier, A. (2010).
        Large colonial organisms with coordinated growth in oxygenated
          environments 2.1Gyr ago.
         Nature, 466(7302):100–104.
        
        [Allen and Martin2007]AM07Allen, J. F. and Martin, W. (2007).
        Out of thin air.
         Nature, 445(7128):610–612.
        
        [Anglada-Escudé et al.2016]AAB16Anglada-Escudé, G., Amado, P. J., Barnes, J., Berdiñas, Z. M.,
          Butler, R. P., Coleman, G. A. L., de La Cueva, I., Dreizler, S.,
          Endl, M., Giesers, B., Jeffers, S. V., Jenkins, J. S., Jones,
          H. R. A., Kiraga, M., Kürster, M., López-González, M. J.,
          Marvin, C. J., Morales, N., Morin, J., Nelson, R. P., Ortiz, J. L.,
          Ofir, A., Paardekooper, S.-J., Reiners, A., Rodríguez, E.,
          Rodríguez-López, C., Sarmiento, L. F., Strachan, J. P.,
          Tsapras, Y., Tuomi, M., and Zechmeister, M. (2016).
        A terrestrial planet candidate in a temperate orbit around Proxima
          Centauri.
         Nature, 536(7617):437–440.
        
        [Archibald2009]ArchArchibald, J. M. (2009).
        The Puzzle of Plastid Evolution.
         Curr. Biol., 19(2):R81–R88.
        
        [Archibald2015]Ar15Archibald, J. M. (2015).
        Endosymbiosis and Eukaryotic Cell Evolution.
         Curr. Biol., 25(19):R911–R921.
        
        [Arndt and Nisbet2012]AN12Arndt, N. T. and Nisbet, E. G. (2012).
        Processes on the Young Earth and the Habitats of Early Life.
         Annu. Rev. Earth Planet. Sci., 40:521–549.
        
        [Arrhenius1908]Arr08Arrhenius, S. (1908).
        Worlds in the Making: The Evolution of the Universe.
         Harper & Brothers.
        
        [Bada and Lazcano2002]BL02Bada, J. L. and Lazcano, A. (2002).
        Some Like It Hot, But Not the First Biomolecules.
         Science, 296(5575):1982–1983.
        
        [Bailer-Jones2009]BJ09Bailer-Jones, C. A. L. (2009).
        The evidence for and against astronomical impacts on climate change
          and mass extinctions: a review.
         Int. J. Astrobiol., 8(3):213–219.
        
        [Bains and Schulze-Makuch2015]BSM15Bains, W. and Schulze-Makuch, D. (2015).
        Mechanisms of Evolutionary Innovation Point to Genetic Control Logic
          as the Key Difference Between Prokaryotes and Eukaryotes.
         J. Mol. Evol., 81(1-2):34–53.
        
        [Bains and Schulze-Makuch2016]BaSM16Bains, W. and Schulze-Makuch, D. (2016).
        The Cosmic Zoo: The (Near) Inevitability of the Evolution of
          Complex, Macroscopic Life.
         Life, 6(3):25.
        
        [Barks et al.2010]BBG10Barks, H. L., Buckley, R., Grieves, G. A., Di Mauro, E., Hud, N. V.,
          and Orlando, T. M. (2010).
        Guanine, Adenine, and Hypoxanthine Production in UV-Irradiated
          Formamide Solutions: Relaxation of the Requirements for Prebiotic Purine
          Nucleobase Formation.
         ChemBioChem, 11(9):1240–1243.
        
        [Barnosky et al.2012]BH12Barnosky, A. D., Hadly, E. A., Bascompte, J., Berlow, E. L., Brown,
          J. H., Fortelius, M., Getz, W. M., Harte, J., Hastings, A.,
          Marquet, P. A., Martinez, N. D., Mooers, A., Roopnarine, P.,
          Vermeij, G., Williams, J. W., Gillespie, R., Kitzes, J., Marshall,
          C., Matzke, N., Mindell, D. P., Revilla, E., and Smith, A. B. (2012).
        Approaching a state shift in Earth's biosphere.
         Nature, 486(7401):52–58.
        
        [Barnosky et al.2011]BMT11Barnosky, A. D., Matzke, N., Tomiya, S., Wogan, G. O. U., Swartz, B.,
          Quental, T. B., Marshall, C., McGuire, J. L., Lindsey, E. L.,
          Maguire, K. C., Mersey, B., and Ferrer, E. A. (2011).
        Has the Earth's sixth mass extinction already arrived? Nature, 471(7336):51–57.
        
        [Baross and Hoffman1985]BH85Baross, J. A. and Hoffman, S. E. (1985).
        Submarine hydrothermal vents and associated gradient environments as
          sites for the origin and evolution of life.
         Orig. Life Evol. Biosph., 15(4):327–345.
        
        [Barrow and Tipler1986]BT85Barrow, J. D. and Tipler, F. J. (1986).
        The Anthropic Cosmological Principle.
         Oxford: Clarendon Press.
        
        [Bastian et al.2010]BCM10Bastian, N., Covey, K. R., and Meyer, M. R. (2010).
        A Universal Stellar Initial Mass Function? A Critical Look at
          Variations.
         Annu. Rev. Astron. Astrophys., 48:339–389.
        
        [Batalha2014]Bat14Batalha, N. M. (2014).
        Exploring exoplanet populations with NASA's Kepler Mission.
         Proc. Natl. Acad. Sci. USA, 111(35):12647–12654.
        
        [Battistuzzi et al.2004]BFH04Battistuzzi, F. U., Feijao, A., and Hedges, S. B. (2004).
        A genomic timescale of prokaryote evolution: insights into the
          origin of methanogenesis, phototrophy, and the colonization of land.
         BMC Evol. Biol., 4:44.
        
        [Beckstead et al.2016]Beck16Beckstead, A. A., Zhang, Y., de Vries, M. S., and Kohler, B. (2016).
        Life in the light: nucleic acid photoproperties as a legacy of
          chemical evolution.
         Phys. Chem. Chem. Phys., 18(35):24228–24238.
        
        [Bekoff and Pierce2009]BP09Bekoff, M. and Pierce, J. (2009).
        Wild Justice: The Moral Lives of Animals.
         The Univ. of Chicago Press.
        
        [Belbruno et al.2012]BMM12Belbruno, E., Moro-Martín, A., Malhotra, R., and Savransky, D.
          (2012).
        Chaotic Exchange of Solid Material Between Planetary Systems:
          Implications for Lithopanspermia.
         Astrobiology, 12(8):754–774.
        
        [Bell et al.2015]BBH15Bell, E. A., Boehnke, P., Harrison, T. M., and Mao, W. L. (2015).
        Potentially biogenic carbon preserved in a 4.1 billion-year-old
          zircon.
         Proc. Natl. Acad. Sci. USA, 112(47):14518–14521.
        
        [Bengtson et al.2017]BSBWBengtson, S., Sallstedt, T., Belivanova, V., and Whitehouse, M. (2017).
        Three-dimensional preservation of cellular and subcellular
          structures suggests 1.6 billion-year-old crown-group red algae.
         PLOS Biol., 15(3):e2000735.
        
        [Benner et al.2012]BKC12Benner, S. A., Kim, H.-J., and Carrigan, M. A. (2012).
        Asphalt, Water, and the Prebiotic Synthesis of Ribose,
          Ribonucleosides, and RNA.
         Acc. Chem. Res., 45(12):2025–2034.
        
        [Berwick and Chomsky2016]BC16Berwick, R. C. and Chomsky, N. (2016).
        Why Only Us: Language and Evolution.
         The MIT Press.
        
        [Blackman and Tarduno2018]BT18Blackman, E. G. and Tarduno, J. A. (2018).
        Mass, energy, and momentum capture from stellar winds by magnetized
          and unmagnetized planets: implications for atmospheric erosion and
          habitability.
         Mon. Not. R. Astron. Soc., 481(4):5146–5155.
        
        [Blättler et al.2018]BCP18Blättler, C. L., Claire, M. W., Prave, A. R., Kirsimäe, K.,
          Higgins, J. A., Medvedev, P. V., Romashkin, A. E., Rychanchik, D. V.,
          Zerkle, A. L., Paiste, K., Kreitsmann, T., Millar, I. L., Hayles,
          J. A., Bao, H., Turchyn, A. V., Warke, M. R., and Lepland, A. (2018).
        Two-billion-year-old evaporites capture Earth’s great oxidation.
         Science, 360(6386):320–323.
        
        [Bogonovich2011]Bog11Bogonovich, M. (2011).
        Intelligence's likelihood and evolutionary time frame.
         Int. J. Astrobiol., 10(2):113–122.
        
        [Bonfio et al.2017]BVS17Bonfio, C., Valer, L., Scintilla, S., Shah, S., Evans, D. J., Jin,
          L., Szostak, J. W., Sasselov, D. D., Sutherland, J. D., and Mansy,
          S. S. (2017).
        UV-light-driven prebiotic synthesis of iron–sulfur clusters.
         Nat. Chem., 9:1229–1234.
        
        [Bontognali et al.2012]BSA12Bontognali, T. R. R., Sessions, A. L., Allwood, A. C., Fischer, W. W.,
          Grotzinger, J. P., Summons, R. E., and Eiler, J. M. (2012).
        From the Cover: Sulfur isotopes of organic matter preserved in
          3.45-billion-year-old stromatolites reveal microbial metabolism.
         Proc. Natl. Acad. Sci. USA, 109(38):15146–15151.
        
        [Booth and Doolittle2015]BD15Booth, A. and Doolittle, W. F. (2015).
        Eukaryogenesis, how special really? Proc. Natl. Acad. Sci., 112(33):10278–10285.
        
        [Borucki2016]Bor16Borucki, W. J. (2016).
        KEPLER Mission: development and overview.
         Rep. Prog. Phys., 79(3):036901.
        
        [Borucki et al.2010]Bor10Borucki, W. J., Koch, D., Basri, G., Batalha, N., Brown, T.,
          Caldwell, D., Caldwell, J., Christensen-Dalsgaard, J., Cochran,
          W. D., DeVore, E., Dunham, E. W., Dupree, A. K., Gautier, T. N.,
          Geary, J. C., Gilliland, R., Gould, A., Howell, S. B., Jenkins,
          J. M., Kondo, Y., Latham, D. W., Marcy, G. W., Meibom, S.,
          Kjeldsen, H., Lissauer, J. J., Monet, D. G., Morrison, D.,
          Sasselov, D., Tarter, J., Boss, A., Brownlee, D., Owen, T.,
          Buzasi, D., Charbonneau, D., Doyle, L., Fortney, J., Ford, E. B.,
          Holman, M. J., Seager, S., Steffen, J. H., Welsh, W. F., Rowe, J.,
          Anderson, H., Buchhave, L., Ciardi, D., Walkowicz, L., Sherry, W.,
          Horch, E., Isaacson, H., Everett, M. E., Fischer, D., Torres, G.,
          Johnson, J. A., Endl, M., MacQueen, P., Bryson, S. T., Dotson, J.,
          Haas, M., Kolodziejczak, J., Van Cleve, J., Chandrasekaran, H.,
          Twicken, J. D., Quintana, E. V., Clarke, B. D., Allen, C., Li, J.,
          Wu, H., Tenenbaum, P., Verner, E., Bruhweiler, F., Barnes, J., and
          Prsa, A. (2010).
        Kepler Planet-Detection Mission: Introduction and First Results.
         Science, 327(5968):977–980.
        
        [Bostrom2014]Bos14Bostrom, N. (2014).
        Superintelligence: Paths, Dangers, Strategies.
         Oxford Univ. Press.
        
        [Bottke and Norman2017]BN17Bottke, W. F. and Norman, M. D. (2017).
        The Late Heavy Bombardment.
         Annu. Rev. Earth Planet. Sci., 45:619–647.
        
        [Boyd et al.2011]BRH11Boyd, R., Richerson, P. J., and Henrich, J. (2011).
        The cultural niche: Why social learning is essential for human
          adaptation.
         Proc. Natl. Acad. Sci. USA, 108(Supplement 2):10918–10925.
        
        [Bradbury et al.2011]BCD11Bradbury, R. J., Cirkovic, M. M., and Dvorsky, G. (2011).
        Dysonian Approach to SETI: A Fruitful Middle Ground? J. Br. Interplanet. Soc., 64:156–165.
        
        [Brain et al.2016]BBMNSBrain, D. A., Bagenal, F., Ma, Y.-J., Nilsson, H., and Stenberg
          Wieser, G. (2016).
        Atmospheric escape from unmagnetized bodies.
         J. Geophys. Res. E, 121(12):2364–2385.
        
        [Brasier et al.2015]BAS15Brasier, M. D., Antcliffe, J., Saunders, M., and Wacey, D. (2015).
        Changing the picture of Earth's earliest fossils (3.5-1.9 Ga) with
          new approaches and new discoveries.
         Proc. Natl. Acad. Sci. USA, 112(16):4859–4864.
        
        [Brocks et al.1999]BLB99Brocks, J. J., Logan, G. A., Buick, R., and Summons, R. E. (1999).
        Archean Molecular Fossils and the Early Rise of Eukaryotes.
         Science, 285(5430):1033–1036.
        
        [Buccino et al.2007]BLM07Buccino, A. P., Lemarchand, G. A., and Mauas, P. J. D. (2007).
        UV habitable zones around M stars.
         Icarus, 192(2):582–587.
        
        [Buick2008]Bui08Buick, R. (2008).
        When did oxygenic photosynthesis evolve? Philos. Trans. Royal Soc. B, 363(1504):2731–2743.
        
        [Burchell2004]Bur04Burchell, M. J. (2004).
        Panspermia today.
         Int. J. Astrobiol., 3(2):73–80.
        
        [Butterfield2000]Butt00Butterfield, N. J. (2000).
        Bangiomorpha pubescens n. gen., n. sp.: implications for the
          evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic
          radiation of eukaryotes.
         Paleobiology, 26(3):386–404.
        
        [Butterfield2007]Butt07Butterfield, N. J. (2007).
        Macroevolution and Macroecology through deep time.
         Palaeontology, 50(1):41–55.
        
        [Butterfield2011]Butt11Butterfield, N. J. (2011).
        Animals and the invention of the Phanerozoic Earth system.
         Trends Ecol. Evol., 26(2):81–87.
        
        [Butterfield2015]Butt15Butterfield, N. J. (2015).
        Early evolution of the Eukaryota.
         Palaeontology, 58(1):5–17.
        
        [Cabrol2016]Cab16Cabrol, N. A. (2016).
        Alien Mindscapes–A Perspective on the Search for Extraterrestrial
          Intelligence.
         Astrobiology, 16(9):661–676.
        
        [Cadet et al.2005]CSD05Cadet, J., Sage, E., and Douki, T. (2005).
        Ultraviolet radiation-mediated damage to cellular DNA.
         Mutat. Res., 571(1-2):3–17.
        
        [Calcott and Sterelny2011]CS11Calcott, B. and Sterelny, K. (2011).
        The Major Transitions in Evolution Revisited.
         The MIT Press.
        
        [Caldeira and Kasting1992]CK92Caldeira, K. and Kasting, J. F. (1992).
        The life span of the biosphere revisited.
         Nature, 360(6406):721–723.
        
        [Cantine and Fournier2018]CF17Cantine, M. D. and Fournier, G. P. (2018).
        Environmental Adaptation from the Origin of Life to the Last
          Universal Common Ancestor.
         Orig. Life Evol. Biosph., 48(1):35–54.
        
        [Carroll2001]Carr01Carroll, S. B. (2001).
        Chance and necessity: the evolution of morphological complexity and
          diversity.
         Nature, 409(6823):1102–1109.
        
        [Carter1983]Cart83Carter, B. (1983).
        The Anthropic Principle and its Implications for Biological
          Evolution.
         Philos. Trans. Royal Soc. A, 310(1512):347–363.
        
        [Carter2008]Cart08Carter, B. (2008).
        Five- or six-step scenario for evolution? Int. J. Astrobiol., 7(2):177–182.
        
        [Catling et al.2005]CG05Catling, D. C., Glein, C. R., Zahnle, K. J., and McKay, C. P. (2005).
        Why O_2 Is Required by Complex Life on Habitable Planets and the
          Concept of Planetary “Oxygenation Time”.
         Astrobiology, 5(3):415–438.
        
        [Chen et al.2018]CFL18Chen, H., Forbes, J. C., and Loeb, A. (2018).
        Habitable Evaporated Cores and the Occurrence of Panspermia Near the
          Galactic Center.
         Astrophys. J. Lett., 855(1):L1.
        
        [Chen and Kipping2018]CK18Chen, J. and Kipping, D. (2018).
        On the Rate of Abiogenesis from a Bayesian Informatics Perspective.
         Astrobiology, 18(12):1574–1584.
        
        [Chen et al.2015]CLV15Chen, X., Ling, H.-F., Vance, D., Shields-Zhou, G. A., Zhu, M.,
          Poulton, S. W., Och, L. M., Jiang, S.-Y., Li, D., Cremonese, L.,
          and Archer, C. (2015).
        Rise to modern levels of ocean oxygenation coincided with the
          Cambrian radiation of animals.
         Nat. Commun., 6:7142.
        
        [Chyba and Sagan1992]CS92Chyba, C. and Sagan, C. (1992).
        Endogenous production, exogenous delivery and impact-shock synthesis
          of organic molecules: an inventory for the origins of life.
         Nature, 355(6356):125–132.
        
        [Clarke et al.2011]CWD11Clarke, J. T., Warnock, R., and Donoghue, P. C. J. (2011).
        Establishing a time‐scale for plant evolution.
         New Phytol., 192(1):266–301.
        
        [Cockell et al.2016]Cock16Cockell, C. S., Bush, T., Bryce, C., Direito, S., Fox-Powell, M.,
          Harrison, J. P., Lammer, H., Landenmark, H., Martin-Torres, J.,
          Nicholson, N., Noack, L., O'Malley-James, J., Payler, S. J.,
          Rushby, A., Samuels, T., Schwendner, P., Wadsworth, J., and
          Zorzano, M. P. (2016).
        Habitability: A Review.
         Astrobiology, 16(1):89–117.
        
        [Corballis2011]Cor11Corballis, M. C. (2011).
        The Recursive Mind: The Origins of Human Language, Thought, and
          Civilization.
         Princeton Univ. Press.
        
        [Cranmer2017]Cran17Cranmer, S. R. (2017).
        Mass-loss Rates from Coronal Mass Ejections: A Predictive
          Theoretical Model for Solar-type Stars.
         Astrophys. J., 840(2):114.
        
        [Crawford1990]Craw90Crawford, I. A. (1990).
        Interstellar Travel: A Review for Astronomers.
         Quart. J. Roy. Astron. Soc., 31:377–400.
        
        [Crespo-Hernández et al.2004]CCHKCrespo-Hernández, C. E., Cohen, B., Hare, P. M., and Kohler, B.
          (2004).
        Ultrafast Excited-State Dynamics in Nucleic Acids.
         Chem. Rev., 104(4):1977–2020.
        
        [Crick and Orgel1973]CO73Crick, F. H. C. and Orgel, L. E. (1973).
        Directed panspermia.
         Icarus, 19(3):341–346.
        
        [Crowe et al.2013]CDB13Crowe, S. A., Døssing, L. N., Beukes, N. J., Bau, M., Kruger,
          S. J., Frei, R., and Canfield, D. E. (2013).
        Atmospheric oxygenation three billion years ago.
         Nature, 501(7468):535–538.
        
        [Cunningham et al.2017]CLBD17Cunningham, J. A., Liu, A. G., Bengtson, S., and Donoghue, P. C. J.
          (2017).
        The origin of animals: Can molecular clocks and the fossil record be
          reconciled? BioEssays, 39(1):1–12.
        
        [Cuntz and Guinan2016]CG16Cuntz, M. and Guinan, E. F. (2016).
        About Exobiology: The Case for Dwarf K Stars.
         Astrophys. J., 827(1):79.
        
        [Dacks et al.2016]DFB16Dacks, J. B., Field, M. C., Buick, R., Eme, L., Gribaldo, S.,
          Roger, A. J., Brochier-Armanet, C., and Devos, D. P. (2016).
        The changing view of eukaryogenesis–fossils, cells, lineages and
          how they all come together.
         J Cell Sci, 129(20):3695–3703.
        
        [Danovaro et al.2010]DD10Danovaro, R., Dell'Anno, A., Pusceddu, A., Gambi, C., Heiner, I., and
          Kristensen, R. M. (2010).
        The first metazoa living in permanently anoxic conditions.
         BMC Biol., 8:30.
        
        [Danovaro et al.2016]DG16Danovaro, R., Gambi, C., Dell'Anno, A., Corinaldesi, C., Pusceddu,
          A., Neves, R. C., and Kristensen, R. M. (2016).
        The challenge of proving the existence of metazoan life in
          permanently anoxic deep-sea sediments.
         BMC Biol., 14:43.
        
        [Dartnell2011]Dart11Dartnell, L. R. (2011).
        Ionizing Radiation and Life.
         Astrobiology, 11(6):551–582.
        
        [Davies2003]Dav03Davies, P. C. W. (2003).
        Does Life's Rapid Appearance Imply a Martian Origin? Astrobiology, 3(4):673–679.
        
        [de Duve2005]deDude Duve, C. (2005).
        Singularities: Landmarks on the Pathways of Life.
         Cambridge Univ. Press.
        
        [De Waal2016]DW16De Waal, F. (2016).
        Are We Smart Enough to Know How Smart Animals Are? W. W. Norton & Company.
        
        [Deacon1998]Dea98Deacon, T. W. (1998).
        The Symbolic Species: The Co-evolution of Language and the
          Brain.
         W. W. Norton & Company.
        
        [Deamer and Damer2017]DD17Deamer, D. and Damer, B. (2017).
        Can Life Begin on Enceladus? A Perspective from Hydrothermal
          Chemistry.
         Astrobiology, 17(9):834–839.
        
        [Deamer and Weber2010]DW10Deamer, D. and Weber, A. L. (2010).
        Bioenergetics and Life's Origins.
         Cold Spring Harb. Perspect. Biol., 2(2):a004929.
        
        [Dennett2017]DCD17Dennett, D. C. (2017).
        From Bacteria to Bach and Back: The Evolution of Minds.
         W. W. Norton & Company.
        
        [Di Stefano and Ray2016]DSR16Di Stefano, R. and Ray, A. (2016).
        Globular Clusters as Cradles of Life and Advanced Civilizations.
         Astrophys. J., 827(1):54.
        
        [Diamond1997]Dia97Diamond, J. (1997).
        Guns, Germs, and Steel: The Fates of Human Societies.
         W. W. Norton & Company.
        
        [Dibrova et al.2012]DCG12Dibrova, D. V., Chudetsky, M. Y., Galperin, M. Y., Koonin, E. V., and
          Mulkidjanian, A. Y. (2012).
        The Role of Energy in the Emergence of Biology from Chemistry.
         Orig. Life Evol. Biosph., 42(5):459–468.
        
        [Djokic et al.2017]DVKCDjokic, T., van Kranendonk, M. J., Campbell, K. A., Walter, M. R., and
          Ward, C. R. (2017).
        Earliest signs of life on land preserved in ca. 3.5 Ga hot spring
          deposits.
         Nat. Commun., 8:15263.
        
        [Dodd et al.2017]DPG17Dodd, M. S., Papineau, D., Grenne, T., Slack, J. F., Rittner, M.,
          Pirajno, F., O'Neil, J., and Little, C. T. S. (2017).
        Evidence for early life in Earth's oldest hydrothermal vent
          precipitates.
         Nature, 543(7643):60–64.
        
        [Dole1964]DoleDole, S. H. (1964).
        Habitable planets for man.
         Blaisdell Pub. Co.
        
        [Dong et al.2017a]DHL17Dong, C., Huang, Z., Lingam, M., Tóth, G., Gombosi, T., and
          Bhattacharjee, A. (2017a).
        The Dehydration of Water Worlds via Atmospheric Losses.
         Astrophys. J. Lett., 847(1):L4.
        
        [Dong et al.2018a]DJL18Dong, C., Jin, M., Lingam, M., Airapetian, V. S., Ma, Y., and van
          der Holst, B. (2018a).
        Atmospheric escape from the TRAPPIST-1 planets and implications for
          habitability.
         Proc. Natl. Acad. Sci. USA, 115(2):260–265.
        
        [Dong et al.2018b]DLM18Dong, C., Lee, Y., Ma, Y., Lingam, M., Bougher, S., Luhmann, J.,
          Curry, S., Toth, G., Nagy, A., Tenishev, V., Fang, X., Mitchell,
          D., Brain, D., and Jakosky, B. (2018b).
        Modeling Martian Atmospheric Losses over Time: Implications for
          Exoplanetary Climate Evolution and Habitability.
         Astrophys. J. Lett., 859(1):L14.
        
        [Dong et al.2017b]DLMCDong, C., Lingam, M., Ma, Y., and Cohen, O. (2017b).
        Is Proxima Centauri b Habitable? A Study of Atmospheric Loss.
         Astrophys. J. Lett., 837(2):L26.
        
        [Douzery et al.2004]DSB04Douzery, E. J. P., Snell, E. A., Bapteste, E., Delsuc, F., and
          Philippe, H. (2004).
        The timing of eukaryotic evolution: Does a relaxed molecular clock
          reconcile proteins and fossils? Proc. Natl. Acad. Sci. USA, 101(43):15386–15391.
        
        [Drake et al.2013]DCY13Drake, J. J., Cohen, O., Yashiro, S., and Gopalswamy, N. (2013).
        Implications of Mass and Energy Loss due to Coronal Mass Ejections
          on Magnetically Active Stars.
         Astrophys. J., 764(2):170.
        
        [Eigenbrode and Freeman2006]EF06Eigenbrode, J. L. and Freeman, K. H. (2006).
        Late Archean rise of aerobic microbial ecosystems.
         Proc. Natl. Acad. Sci. USA, 103(43):15759–15764.
        
        [Ellis et al.2013]EK13Ellis, E. C., Kaplan, J. O., Fuller, D. Q., Vavrus, S., Klein
          Goldewijk, K., and Verburg, P. H. (2013).
        Used planet: A global history.
         Proc. Natl. Acad. Sci. USA, 110(20):7978–7985.
        
        [Embley and Martin2006]EM06Embley, T. M. and Martin, W. (2006).
        Eukaryotic evolution, changes and challenges.
         Nature, 440(7084):623–630.
        
        [Eme et al.2014]ES14Eme, L., Sharpe, S. C., Brown, M. W., and Roger, A. J. (2014).
        On the Age of Eukaryotes: Evaluating Evidence from Fossils and
          Molecular Clocks.
         Cold Spring Harb. Perspect. Biol., 6(8):a016139.
        
        [Eme et al.2017]ESL17Eme, L., Spang, A., Lombard, J., Stairs, C. W., and Ettema, T. J. G.
          (2017).
        Archaea and the origin of eukaryotes.
         Nat. Rev. Microbiol., 15:711–723.
        
        [Erwin et al.2011]EF11Erwin, D. H., Laflamme, M., Tweedt, S. M., Sperling, E. A., Pisani,
          D., and Peterson, K. J. (2011).
        The Cambrian Conundrum: Early Divergence and Later Ecological
          Success in the Early History of Animals.
         Science, 334(6059):1091–1097.
        
        [Evans and Gaston2005]EG05Evans, K. L. and Gaston, K. J. (2005).
        Can the evolutionary‐rates hypothesis explain species‐energy
          relationships? Funct. Ecol., 19(6):899–915.
        
        [Falcón et al.2010]FMC10Falcón, L. I., Magallón, S., and Castillo, A. (2010).
        Dating the cyanobacterial ancestor of the chloroplast.
         The ISME Journal, 4(6):777–783.
        
        [Falkowski et al.2004]FKK04Falkowski, P. G., Katz, M. E., Knoll, A. H., Quigg, A., Raven, J. A.,
          Schofield, O., and Taylor, F. J. R. (2004).
        The Evolution of Modern Eukaryotic Phytoplankton.
         Science, 305(5682):354–360.
        
        [Farquhar et al.2011]FZB11Farquhar, J., Zerkle, A. L., and Bekker, A. (2011).
        Geological constraints on the origin of oxygenic photosynthesis.
         Photosynth. Res., 107(1):11–36.
        
        [Fischer et al.2016a]FHJ16Fischer, W. W., Hemp, J., and Johnson, J. E. (2016a).
        Evolution of Oxygenic Photosynthesis.
         Annu. Rev. Earth Planet. Sci., 44:647–683.
        
        [Fischer et al.2016b]FHV16Fischer, W. W., Hemp, J., and Valentine, J. S. (2016b).
        How did life survive Earth's great oxygenation? Curr. Opin. Chem. Biol., 31:166–178.
        
        [Flannery et al.2018]FAS18Flannery, D. T., Allwood, A. C., Summons, R. E., Williford, K. H.,
          Abbey, W., Matys, E. D., and Ferralis, N. (2018).
        Spatially-resolved isotopic study of carbon trapped in ∼3.43 Ga
          Strelley Pool Formation stromatolites.
         Geochim. Cosmochim. Acta, 223:21–35.
        
        [Franck et al.2000]FBVBFranck, S., Block, A., von Bloh, W., Bounama, C., Schellnhuber,
          H. J., and Svirezhev, Y. (2000).
        Reduction of biosphere life span as a consequence of geodynamics.
         Tellus B, 52(1):94–107.
        
        [Franck et al.2006]FBV06Franck, S., Bounama, C., and von Bloh, W. (2006).
        Causes and timing of future biosphere extinctions.
         Biogeosciences, 3:85–92.
        
        [Frank et al.2018]FCAKFrank, A., Carroll-Nellenback, J., Alberti, M., and Kleidon, A. (2018).
        The Anthropocene Generalized: Evolution of Exo-Civilizations and
          Their Planetary Feedback.
         Astrobiology, 18(5):503–518.
        
        [Frank et al.2017]FKA17Frank, A., Kleidon, A., and Alberti, M. (2017).
        Earth as a Hybrid Planet: The Anthropocene in an Evolutionary
          Astrobiological Context.
         Anthropocene, 19:13–21.
        
        [Frank and Sullivan2014]FS14Frank, A. and Sullivan, W. (2014).
        Sustainability and the astrobiological perspective: Framing human
          futures in a planetary context.
         Anthropocene, 5:32–41.
        
        [Frei et al.2016]FCB16Frei, R., Crowe, S. A., Bau, M., Polat, A., Fowle, D. A., and
          Døssing, L. N. (2016).
        Oxidative elemental cycling under the low O_2 Eoarchean
          atmosphere.
         Sci. Rep., 6:21058.
        
        [French et al.2015]FHH15French, K. L., Hallmann, C., Hope, J. M., Schoon, P. L., Zumberge,
          J. A., Hoshino, Y., Peters, C. A., George, S. C., Love, G. D.,
          Brocks, J. J., Buick, R., and Summons, R. E. (2015).
        Reappraisal of hydrocarbon biomarkers in Archean rocks.
         Proc. Natl. Acad. Sci. USA, 112(19):5915–5920.
        
        [Fujii et al.2018]FADFujii, Y., Angerhausen, D., Deitrick, R., Domagal-Goldman, S.,
          Grenfell, J. L., Hori, Y., Kane, S. R., Pallé, E., Rauer, H.,
          Siegler, N., Stapelfeldt, K., and Stevenson, K. B. (2018).
        Exoplanet Biosignatures: Observational Prospects.
         Astrobiology, 18(6):739–778.
        
        [Furukawa et al.2015]FNS15Furukawa, Y., Nakazawa, H., Sekine, T., Kobayashi, T., and Kakegawa,
          T. (2015).
        Nucleobase and amino acid formation through impacts of meteorites on
          the early ocean.
         Earth Planet. Sci. Lett., 429:216–222.
        
        [Garraffo et al.2016]GDC16Garraffo, C., Drake, J. J., and Cohen, O. (2016).
        The Space Weather of Proxima Centauri b.
         Astrophys. J. Lett., 833(1):L4.
        
        [Garraffo et al.2017]GDC17Garraffo, C., Drake, J. J., Cohen, O., Alvarado-Gómez, J. D., and
          Moschou, S. P. (2017).
        The Threatening Magnetic and Plasma Environment of the TRAPPIST-1
          Planets.
         Astrophys. J. Lett., 843(2):L33.
        
        [Gensel2008]Gen08Gensel, P. G. (2008).
        The Earliest Land Plants.
         Annu. Rev. Ecol. Evol. Syst., 39:459–477.
        
        [Gilbert1986]Gil86Gilbert, W. (1986).
        Origin of life: The RNA world.
         Nature, 319:618.
        
        [Gillon et al.2016]GJL16Gillon, M., Jehin, E., Lederer, S. M., Delrez, L., de Wit, J.,
          Burdanov, A., Van Grootel, V., Burgasser, A. J., Triaud, A. H. M. J.,
          Opitom, C., Demory, B.-O., Sahu, D. K., Bardalez Gagliuffi, D.,
          Magain, P., and Queloz, D. (2016).
        Temperate Earth-sized planets transiting a nearby ultracool dwarf
          star.
         Nature, 533(7602):221–224.
        
        [Gillon et al.2017]GTD17Gillon, M., Triaud, A. H. M. J., Demory, B.-O., Jehin, E., Agol, E.,
          Deck, K. M., Lederer, S. M., de Wit, J., Burdanov, A., Ingalls,
          J. G., Bolmont, E., Leconte, J., Raymond, S. N., Selsis, F.,
          Turbet, M., Barkaoui, K., Burgasser, A., Burleigh, M. R., Carey,
          S. J., Chaushev, A., Copperwheat, C. M., Delrez, L., Fernandes,
          C. S., Holdsworth, D. L., Kotze, E. J., Van Grootel, V., Almleaky,
          Y., Benkhaldoun, Z., Magain, P., and Queloz, D. (2017).
        Seven temperate terrestrial planets around the nearby ultracool
          dwarf star TRAPPIST-1.
         Nature, 542(7642):456–460.
        
        [Ginsburg et al.2018]GLL18Ginsburg, I., Lingam, M., and Loeb, A. (2018).
        Galactic Panspermia.
         Astrophys. J. Lett., 868(1):L12.
        
        [Gladman et al.2005]GDL05Gladman, B., Dones, L., Levison, H. F., and Burns, J. A. (2005).
        Impact Seeding and Reseeding in the Inner Solar System.
         Astrobiology, 5(4):483–496.
        
        [Gold et al.2017]GCF17Gold, D. A., Caron, A., Fournier, G. P., and Summons, R. E. (2017).
        Paleoproterozoic sterol biosynthesis and the rise of oxygen.
         Nature, 543(7645):420–423.
        
        [Goldblatt and Watson2012]GW12Goldblatt, C. and Watson, A. J. (2012).
        The runaway greenhouse: implications for future climate change,
          geoengineering and planetary atmospheres.
         Philos. Trans. Royal Soc. A, 370:4197–4216.
        
        [Gomes et al.2005]GLT05Gomes, R., Levison, H. F., Tsiganis, K., and Morbidelli, A. (2005).
        Origin of the cataclysmic Late Heavy Bombardment period of the
          terrestrial planets.
         Nature, 435(7041):466–469.
        
        [Gould et al.2008]GWM08Gould, S. B., Waller, R. F., and McFadden, G. I. (2008).
        Plastid Evolution.
         Annu. Rev. Plant Biol, 59:491–517.
        
        [Gould1996]Gou96Gould, S. J. (1996).
        Full House.
         Harmony Books.
        
        [Gould2002]Gould02Gould, S. J. (2002).
        The Structure of Evolutionary Theory.
         Harvard Univ. Press.
        
        [Gray et al.1999]GBL99Gray, M. W., Burger, G., and Lang, B. F. (1999).
        Mitochondrial Evolution.
         Science, 283(5407):1476–1481.
        
        [Griffin2001]Gri01Griffin, D. R. (2001).
        Animal Minds: Beyond Cognition to Consciousness.
         The Univ. of Chicago Press.
        
        [Grimm and Marchi2018]GM18Grimm, R. E. and Marchi, S. (2018).
        Direct thermal effects of the Hadean bombardment did not limit early
          subsurface habitability.
         Earth Planet. Sci. Lett., 485:1–8.
        
        [Grosberg and Strathmann2007]GS07Grosberg, R. K. and Strathmann, R. R. (2007).
        The Evolution of Multicellularity: A Minor Major Transition? Annu. Rev. Ecol. Evol. Syst., 38:621–654.
        
        [Gumsley et al.2017]GCBS17Gumsley, A. P., Chamberlain, K. R., Bleeker, W., Söderlund, U., de
          Kock, M. O., Larsson, E. R., and Bekker, A. (2017).
        Timing and tempo of the Great Oxidation Event.
         Proc. Natl. Acad. Sci. USA, 114(8):1811–1816.
        
        [Gunell et al.2018]GMN18Gunell, H., Maggiolo, R., Nilsson, H., Stenberg Wieser, G., Slapak,
          R., Lindkvist, J., Hamrin, M., and De Keyser, J. (2018).
        Why an intrinsic magnetic field does not protect a planet against
          atmospheric escape.
         Astron. Astrophys., 614:L3.
        
        [Günther et al.2019]GZS19Günther, M. N., Zhan, Z., Seager, S., Rimmer, P. B., Ranjan, S.,
          Stassun, K. G., Oelkers, R. J., Daylan, T., Newton, E., Gillen, E.,
          Rappaport, S., Ricker, G. R., Latham, D. W., Winn, J. N., Jenkins,
          J. M., Glidden, A., Fausnaugh, M., Levine, A. M., Dittmann, J. A.,
          Quinn, S. N., Krishnamurthy, A., and Ting, E. B. (2019).
        Stellar Flares from the First Tess Data Release: Exploring a New
          Sample of M-dwarfs.
         Astrophys. J.[Gustavsson et al.2010]GIM10Gustavsson, T., Improta, R., and Markovitsi, D. (2010).
        DNA/RNA: Building Blocks of Life Under UV Irradiation.
         J. Phys. Chem. Lett., 1(13):2025–2030.
        
        [Han and Runnegar1992]HR92Han, T.-M. and Runnegar, B. (1992).
        Megascopic Eukaryotic Algae from the 2.1-Billion-Year-Old Negaunee
          Iron-Formation, Michigan.
         Science, 257(5067):232–235.
        
        [Haqq-Misra et al.2018]HMK18Haqq-Misra, J., Kopparapu, R. K., and Wolf, E. T. (2018).
        Why do we find ourselves around a yellow star instead of a red
          star? Int. J. Astrobiol., 17:77–86.
        
        [Harrison2009]Har09Harrison, T. M. (2009).
        The Hadean Crust: Evidence from >4Ga Zircons.
         Annu. Rev. Earth Planet. Sci., 37:479–505.
        
        [Hauser et al.2014]HYB14Hauser, M. D., Yang, C., Berwick, R. C., Tattersall, I., Ryan, M. J.,
          Watumull, J., Chomsky, N., and Lewontin, R. C. (2014).
        The mystery of language evolution.
         Front. Psychol., 5:401.
        
        [Heckman et al.2001]HGE01Heckman, D. S., Geiser, D. M., Eidell, B. R., Stauffer, R. L.,
          Kardos, N. L., and Hedges, S. B. (2001).
        Molecular Evidence for the Early Colonization of Land by Fungi and
          Plants.
         Science, 293(5532):1129–1133.
        
        [Hedges et al.2004]HBVSHedges, S. B., Blair, J. E., Venturi, M. L., and Shoe, J. L. (2004).
        A molecular timescale of eukaryote evolution and the rise of complex
          multicellular life.
         BMC Evol. Biol., 4:2.
        
        [Hedges and Kumar2009]HK09Hedges, S. B. and Kumar, S. (2009).
        The Timetree of Life.
         Oxford Univ. Press.
        
        [Heller and Armstrong2014]HA14Heller, R. and Armstrong, J. (2014).
        Superhabitable Worlds.
         Astrobiology, 14(1):50–66.
        
        [Henrich2016]Hen16Henrich, J. (2016).
        The Secret of Our Success.
         Princeton Univ. Press.
        
        [Heyes and Frith2014]HF14Heyes, C. M. and Frith, C. D. (2014).
        The cultural evolution of mind reading.
         Science, 344(6190):1243091.
        
        [Higgs and Lehman2015]HL15Higgs, P. G. and Lehman, N. (2015).
        The RNA World: molecular cooperation at the origins of life.
         Nat. Rev. Genet., 16:7–17.
        
        [Hohmann-Marriott and Blankenship2011]HB11Hohmann-Marriott, M. F. and Blankenship, R. E. (2011).
        Evolution of Photosynthesis.
         Annu. Rev. Plant Biol., 62:515–548.
        
        [Holland2006]Holl06Holland, H. D. (2006).
        The oxygenation of the atmosphere and oceans.
         Philos. Trans. Royal Soc. B, 361(1470):903–915.
        
        [Horner and Jones2010]HJ10Horner, J. and Jones, B. W. (2010).
        Determining habitability: which exo-Earths should we search for
          life? Int. J. Astrobiol., 9(4):273–291.
        
        [Howard et al.2018]HTC18Howard, W. S., Tilley, M. A., Corbett, H., Youngblood, A., Loyd,
          R. O. P., Ratzloff, J. K., Law, N. M., Fors, O., del Ser, D.,
          Shkolnik, E. L., Ziegler, C., Goeke, E. E., Pietraallo, A. D., and
          Haislip, J. (2018).
        The First Naked-eye Superflare Detected from Proxima Centauri.
         Astrophys. J. Lett., 860(2):L30.
        
        [Huang1959]Huang59Huang, S.-S. (1959).
        Occurrence of Life in the Universe.
         Am. Sci., 47:397–402.
        
        [Isaacson et al.2017]IS17Isaacson, H., Siemion, A. P. V., Marcy, G. W., Lebofsky, M., Price,
          D. C., MacMahon, D., Croft, S., DeBoer, D., Hickish, J., Werthimer,
          D., Sheikh, S., Hellbourg, G., and Enriquez, J. E. (2017).
        The Breakthrough Listen Search for Intelligent Life: Target
          Selection of Nearby Stars and Galaxies.
         Publ. Astron. Soc. Pac., 129(975):054501.
        
        [Islam and Powner2017]IP17Islam, S. and Powner, M. W. (2017).
        Prebiotic Systems Chemistry: Complexity Overcoming Clutter.
         Chem, 2(4):470–501.
        
        [Jablonka and Lamb2006]JL06Jablonka, E. and Lamb, M. J. (2006).
        The evolution of information in the major transitions.
         J. Theor. Biol., 239(2):236–246.
        
        [Jakosky et al.2017]Jak17Jakosky, B. M., Slipski, M., Benna, M., Mahaffy, P., Elrod, M.,
          Yelle, R., Stone, S., and Alsaeed, N. (2017).
        Mars' atmospheric history derived from upper-atmosphere measurements
          of ^38Ar/^36Ar.
         Science, 355(6332):1408–1410.
        
        [Javaux and Dehant2010]JD10Javaux, E. J. and Dehant, V. (2010).
        Habitability: from stars to cells.
         Astron. Astrophys. Rev, 18(3):383–416.
        
        [Javaux and Knoll2017]JK17Javaux, E. J. and Knoll, A. H. (2017).
        Micropaleontology of the lower Mesoproterozoic Roper Group,
          Australia, and implications for early eukaryotic evolution.
         J. Paleontol, 91(2):199–229.
        
        [Joyce2002]Joy02Joyce, G. F. (2002).
        The antiquity of RNA-based evolution.
         Nature, 418(6894):214–221.
        
        [Judson2017]Jud17Judson, O. P. (2017).
        The energy expansions of evolution.
         Nat. Ecol. Evol., 1:0138.
        
        [Kaltenegger2017]Kal17Kaltenegger, L. (2017).
        How to Characterize Habitable Worlds and Signs of Life.
         Annu. Rev. Astron. Astrophys., 55:433–485.
        
        [Kasting2010]Kas10Kasting, J. (2010).
        How to Find a Habitable Planet.
         Princeton Univ. Press.
        
        [Kasting and Catling2003]KC03Kasting, J. F. and Catling, D. (2003).
        Evolution of a Habitable Planet.
         Annu. Rev. Astron. Astrophys., 41:429–463.
        
        [Kasting et al.1993]KWR93Kasting, J. F., Whitmire, D. P., and Reynolds, R. T. (1993).
        Habitable Zones around Main Sequence Stars.
         Icarus, 101(1):108–128.
        
        [Kay et al.2016]KOK16Kay, C., Opher, M., and Kornbleuth, M. (2016).
        Probability of CME Impact on Exoplanets Orbiting M Dwarfs and
          Solar-like Stars.
         Astrophys. J., 826(2):195.
        
        [Kaźmierczak et al.2016]KK16Kaźmierczak, J., Kremer, B., Altermann, W., and Franchi, I. (2016).
        Tubular microfossils from ∼2.8 to 2.7 Ga-old lacustrine
          deposits of South Africa: A sign for early origin of eukaryotes? Precambrian Res., 286:180–194.
        
        [Keeling2010]Keel10Keeling, P. J. (2010).
        The endosymbiotic origin, diversification and fate of plastids.
         Philos. Trans. Royal Soc. B, 365(1541):729–748.
        
        [Khodachenko et al.2007]KRL07Khodachenko, M. L., Ribas, I., Lammer, H., Grießmeier, J.-M.,
          Leitner, M., Selsis, F., Eiroa, C., Hanslmeier, A., Biernat, H. K.,
          Farrugia, C. J., and Rucker, H. O. (2007).
        Coronal Mass Ejection (CME) Activity of Low Mass M Stars as An
          Important Factor for The Habitability of Terrestrial Exoplanets. I. CME
          Impact on Expected Magnetospheres of Earth-Like Exoplanets in Close-In
          Habitable Zones.
         Astrobiology, 7(1):167–184.
        
        [Kite et al.2018]KGO18Kite, E. S., Gaidos, E., and Onstott, T. C. (2018).
        Valuing Life-Detection Missions.
         Astrobiology, 18(7):834–840.
        
        [Klein1995]Kle95Klein, R. G. (1995).
        Anatomy, behavior, and modern human origins.
         J. World Prehistory, 9(2):167–198.
        
        [Knoll1985]Kno85Knoll, A. H. (1985).
        The precambrian evolution of terrestrial life.
         In Papagiannis, M. D., editor,  The Search for Extraterrestrial
          Life: Recent Developments, volume 112 of  IAU Symposium, pages
          201–211.
        
        [Knoll2011]Kno11Knoll, A. H. (2011).
        The Multiple Origins of Complex Multicellularity.
         Annu. Rev. Earth Planet. Sci., 39:217–239.
        
        [Knoll2014]Knoll14Knoll, A. H. (2014).
        Paleobiological Perspectives on Early Eukaryotic Evolution.
         Cold Spring Harb. Perspect. Biol., 6(1):a016121.
        
        [Knoll2015a]Knoll15Knoll, A. H. (2015a).
        Life on a Young Planet: The First Three Billion Years of
          Evolution on Earth.
         Princeton Science Library. Princeton Univ. Press.
        
        [Knoll2015b]Kno15Knoll, A. H. (2015b).
        Paleobiological Perspectives on Early Microbial Evolution.
         Cold Spring Harb. Perspect. Biol., 7(7):a018093.
        
        [Knoll2017]Kno17Knoll, A. H. (2017).
        Food for early animal evolution.
         Nature, 548(7669):528–530.
        
        [Knoll and Bambach2000]KB00Knoll, A. H. and Bambach, R. K. (2000).
        Directionality in the history of life: diffusion from the left wall
          or repeated scaling of the right? Paleobiology, 26(sp4):1–14.
        
        [Knoll et al.2016]KBS16Knoll, A. H., Bergmann, K. D., and Strauss, J. V. (2016).
        Life: the first two billion years.
         Phil. Trans. R. Soc. B, 371(1707):20150493.
        
        [Knoll et al.2006]KJHCKnoll, A. H., Javaux, E. J., Hewitt, D., and Cohen, P. (2006).
        Eukaryotic organisms in Proterozoic oceans.
         Philos. Trans. Royal Soc. B, 361(1470):1023–1038.
        
        [Knoll and Nowak2017]KN17Knoll, A. H. and Nowak, M. A. (2017).
        The timetable of evolution.
         Sci. Adv., 3(5):e1603076.
        
        [Knoll and Sperling2014]KS14Knoll, A. H. and Sperling, E. A. (2014).
        Oxygen and animals in Earth history.
         Proc. Natl. Acad. Sci. USA, 111(11):3907–3908.
        
        [Koch and Britton2008]KB08Koch, L. G. and Britton, S. L. (2008).
        Aerobic metabolism underlies complexity and capacity.
         J. Physiol., 586(1):83–95.
        
        [Koonin2007]Koo7Koonin, E. V. (2007).
        The Biological Big Bang model for the major transitions in
          evolution.
         Biol. Direct, 2:21.
        
        [Koonin2010]Koo10Koonin, E. V. (2010).
        The origin and early evolution of eukaryotes in the light of
          phylogenomics.
         Genome Biol., 11(5):209.
        
        [Kopparapu et al.2013]KRKKopparapu, R. K., Ramirez, R., Kasting, J. F., Eymet, V., Robinson,
          T. D., Mahadevan, S., Terrien, R. C., Domagal-Goldman, S., Meadows,
          V., and Deshpande, R. (2013).
        Habitable Zones around Main-sequence Stars: New Estimates.
         Astrophys. J., 765(2):131.
        
        [Kopparapu et al.2014]KRS14Kopparapu, R. K., Ramirez, R. M., SchottelKotte, J., Kasting, J. F.,
          Domagal-Goldman, S., and Eymet, V. (2014).
        Habitable Zones around Main-sequence Stars: Dependence on Planetary
          Mass.
         Astrophys. J. Lett., 787(2):L29.
        
        [Krissansen-Totton et al.2018]KT18Krissansen-Totton, J., Olson, S., and Catling, D. C. (2018).
        Disequilibrium biosignatures over Earth history and implications for
          detecting exoplanet life.
         Sci. Adv., 4(1):eaao5747.
        
        [Kroupa2001]Kro01Kroupa, P. (2001).
        On the variation of the initial mass function.
         Mon. Not. R. Astron. Soc., 322(2):231–246.
        
        [Laakso and Schrag2018]LS18Laakso, T. A. and Schrag, D. P. (2018).
        Limitations on Limitation.
         Global Biogeochem. Cycles.
        
        [Lake et al.2017]LUTGLake, B. M., Ullman, T. D., Tenenbaum, J. B., and Gershman, S. J.
          (2017).
        Building machines that learn and think like people.
         Behav. Brain Sci., 40:e253.
        
        [Laland2017]Lal17Laland, K. N. (2017).
        Darwin's Unfinished Symphony: How Culture Made the Human
          Mind.
         Princeton Univ. Press.
        
        [Laland and Galef2009]LaGa09Laland, K. N. and Galef, B. G. (2009).
        The Question of Animal Culture.
         Harvard Univ. Press.
        
        [Laland et al.2015]LUF15Laland, K. N., Uller, T., Feldman, M. W., Sterelny, K., Müller,
          G. B., Moczek, A., Jablonka, E., and Odling-Smee, J. (2015).
        The extended evolutionary synthesis: its structure, assumptions and
          predictions.
         Proc. Royal Soc. B, 282(1813):20151019.
        
        [Lamb et al.2009]LAC09Lamb, D. M., Awramik, S. M., Chapman, D. J., and Zhu, S. (2009).
        Evidence for eukaryotic diversification in the ∼1800
          million-year-old Changzhougou Formation, North China.
         Precambrian Res., 173(1-4):93–104.
        
        [Lammer et al.2009]Lam09Lammer, H., Bredehöft, J. H., Coustenis, A., Khodachenko, M. L.,
          Kaltenegger, L., Grasset, O., Prieur, D., Raulin, F., Ehrenfreund,
          P., Yamauchi, M., Wahlund, J.-E., Grießmeier, J.-M., Stangl, G.,
          Cockell, C. S., Kulikov, Y. N., Grenfell, J. L., and Rauer, H.
          (2009).
        What makes a planet habitable? Astron. Astrophys. Rev., 17(2):181–249.
        
        [Lane2009]La09Lane, N. (2009).
        Life Ascending: The Ten Great Inventions of Evolution.
         Profile Books.
        
        [Lane and Martin2010]LM10Lane, N. and Martin, W. (2010).
        The energetics of genome complexity.
         Nature, 467(7318):929–934.
        
        [Lee et al.2018]LDP18Lee, Y., Dong, C., Pawlowski, D., Thiemann, E., Tenishev, V.,
          Mahaffy, P., Benna, M., Combi, M., Bougher, S., and Eparvier, F.
          (2018).
        Effects of a Solar Flare on the Martian Hot O Corona and
          Photochemical Escape.
         Geophys. Res. Lett., 45(14):6814–6822.
        
        [Lenton and Watson2011]LW11Lenton, T. and Watson, A. (2011).
        Revolutions that Made the Earth.
         Oxford Univ. Press.
        
        [Lenton and von Bloh2001]LVB01Lenton, T. M. and von Bloh, W. (2001).
        Biotic feedback extends the life span of the biosphere.
         Geophys. Res. Lett., 28(9):1715–1718.
        
        [Lewis and McCourt2004]LM04Lewis, L. A. and McCourt, R. M. (2004).
        Green algae and the origin of land plants.
         Am. J. Bot., 91(10):1535–1556.
        
        [Lewis and Maslin2015]LM15Lewis, S. L. and Maslin, M. A. (2015).
        Defining the Anthropocene.
         Nature, 519(7542):171–180.
        
        [Lewontin2000]Lew00Lewontin, R. C. (2000).
        The Triple Helix: Gene, Organism, and Environment.
         Harvard Univ. Press.
        
        [Li et al.2013]LLSLi, H., Lu, S., Su, W., Xiang, Z., Zhou, H., and Zhang, Y. (2013).
        Recent advances in the study of the Mesoproterozoic geochronology in
          the North China Craton.
         J. Asian Earth Sci., 72:216–227.
        
        [Lineweaver and Davis2002]LD02Lineweaver, C. H. and Davis, T. M. (2002).
        Does the Rapid Appearance of Life on Earth Suggest that Life Is
          Common in the Universe? Astrobiology, 2(3):293–304.
        
        [Lingam2016a]ML16Lingam, M. (2016a).
        Analytical approaches to modelling panspermia - beyond the
          mean-field paradigm.
         Mon. Not. R. Astron. Soc., 455(3):2792–2803.
        
        [Lingam2016b]Ling16Lingam, M. (2016b).
        Interstellar Travel and Galactic Colonization: Insights from
          Percolation Theory and the Yule Process.
         Astrobiology, 16(6):418–426.
        
        [Lingam et al.2018]LDF18Lingam, M., Dong, C., Fang, X., Jakosky, B. M., and Loeb, A. (2018).
        The Propitious Role of Solar Energetic Particles in the Origin of
          Life.
         Astrophys. J., 853(1):10.
        
        [Lingam and Loeb2017a]ManLiLingam, M. and Loeb, A. (2017a).
        Enhanced interplanetary panspermia in the TRAPPIST-1 system.
         Proc. Natl. Acad. Sci. USA, 114(26):6689–6693.
        
        [Lingam and Loeb2017b]Man17Lingam, M. and Loeb, A. (2017b).
        Reduced Diversity of Life around Proxima Centauri and TRAPPIST-1.
         Astrophys. J. Lett., 846(2):L21.
        
        [Lingam and Loeb2017c]MLin17Lingam, M. and Loeb, A. (2017c).
        Risks for Life on Habitable Planets from Superflares of Their Host
          Stars.
         Astrophys. J., 848(1):41.
        
        [Lingam and Loeb2018a]Ling18Lingam, M. and Loeb, A. (2018a).
        Is Extraterrestrial Life Suppressed on Subsurface Ocean Worlds due
          to the Paucity of Bioessential Elements? Astron. J., 156(4):151.
        
        [Lingam and Loeb2018b]ManasviLingam, M. and Loeb, A. (2018b).
        Is life most likely around Sun-like stars? J. Cosmol. Astropart. Phys., 5:020.
        
        [Lingam and Loeb2018c]mlal18Lingam, M. and Loeb, A. (2018c).
        Limitations of Chemical Propulsion for Interstellar Escape from
          Habitable Zones Around Low-mass Stars.
         Res. Notes AAS, 2(3):154.
        
        [Lingam and Loeb2018d]LL18Lingam, M. and Loeb, A. (2018d).
        Optimal Target Stars in the Search for Life.
         Astrophys. J. Lett., 857(2):L17.
        
        [Lingam and Loeb2018e]LiLoLingam, M. and Loeb, A. (2018e).
        Physical constraints for the evolution of life on exoplanets.
         Rev. Mod. Phys. (arXiv:1810.02007).
        
        [Lingam and Loeb2018f]LiLo17Lingam, M. and Loeb, A. (2018f).
        Physical constraints on the likelihood of life on exoplanets.
         Int. J. Astrobiol., 17(2):116–126.
        
        [Lingam and Loeb2018g]LL17Lingam, M. and Loeb, A. (2018g).
        Subsurface Exolife.
         Int. J. Astrobiol. (arXiv:1711.09908).
        
        [Lingam and Loeb2019a]ManLo18Lingam, M. and Loeb, A. (2019a).
        Dependence of Biological Activity on the Surface Water Fraction of
          Planets.
         Astron. J., 157(1):25.
        
        [Lingam and Loeb2019b]mlal19Lingam, M. and Loeb, A. (2019b).
        Photosynthesis on habitable planets around low-mass stars.
         Mon. Not. R. Astron. Soc. (arXiv:1901.01270).
        
        [Lingam and Loeb2019c]Mali18Lingam, M. and Loeb, A. (2019c).
        Relative Likelihood of Success in the Search for Primitive versus
          Intelligent Extraterrestrial Life.
         Astrobiology, 19(1):28–39.
        
        [Loeb et al.2016]LBS16Loeb, A., Batista, R. A., and Sloan, D. (2016).
        Relative likelihood for life as a function of cosmic time.
         J. Cosmol. Astropart. Phys., 8:040.
        
        [López-García et al.2017]LEM17López-García, P., Eme, L., and Moreira, D. (2017).
        Symbiosis in eukaryotic evolution.
         J. Theor. Biol., 434:20–33.
        
        [López-García and
          Moreira2015]LoMo15López-García, P. and Moreira, D. (2015).
        Open questions on the origin of eukaryotes.
         Trends Ecol. Evol., 30(11):697–708.
        
        [Love et al.2009]LG09Love, G. D., Grosjean, E., Stalvies, C., Fike, D. A., Grotzinger,
          J. P., Bradley, A. S., Kelly, A. E., Bhatia, M., Meredith, W.,
          Snape, C. E., Bowring, S. A., Condon, D. J., and Summons, R. E.
          (2009).
        Fossil steroids record the appearance of Demospongiae during the
          Cryogenian period.
         Nature, 457(7230):718–721.
        
        [Lovelock and Whitfield1982]LW82Lovelock, J. E. and Whitfield, M. (1982).
        Life span of the biosphere.
         Nature, 296(5857):561–563.
        
        [Lu et al.2018]LRT18Lu, W., Ridgwell, A., Thomas, E., Hardisty, D. S., Luo, G., Algeo,
          T. J., Saltzman, M. R., Gill, B. C., Shen, Y., Ling, H.-F.,
          Edwards, C. T., Whalen, M. T., Zhou, X., Gutchess, K. M., Jin, L.,
          Rickaby, R. E. M., Jenkyns, H. C., Lyons, T. W., Lenton, T. M.,
          Kump, L. R., and Lu, Z. (2018).
        Late inception of a resiliently oxygenated upper ocean.
         Science, 361(6398):174–177.
        
        [Lubin2016]Lub16Lubin, P. (2016).
        A Roadmap to Interstellar Flight.
         J. Br. Interplanet. Soc., 69:40–72.
        
        [Lücking et al.2009]LHP09Lücking, R., Huhndorf, S., Pfister, D. H., Plata, E. R., and
          Lumbsch, H. T. (2009).
        Fungi evolved right on track.
         Mycologia, 101(6):810–822.
        
        [Lunine2013]Lun13Lunine, J. I. (2013).
        Earth: Evolution of a Habitable World.
         Cambridge Univ. Press.
        
        [Luo et al.2016]LO16Luo, G., Ono, S., Beukes, N. J., Wang, D. T., Xie, S., and Summons,
          R. E. (2016).
        Rapid oxygenation of Earths atmosphere 2.33 billion years ago.
         Sci. Adv., 2(5):e1600134.
        
        [Lynch and Marinov2015]LyM15Lynch, M. and Marinov, G. K. (2015).
        The bioenergetic costs of a gene.
         Proc. Natl. Acad. Sci. USA, 112(51):15690–15695.
        
        [Lyons et al.2014]LRP14Lyons, T. W., Reinhard, C. T., and Planavsky, N. J. (2014).
        The rise of oxygen in Earth's early ocean and atmosphere.
         Nature, 506(7488):307–315.
        
        [MacGregor et al.2018]MWW18MacGregor, M. A., Weinberger, A. J., Wilner, D. J., Kowalski, A. F.,
          and Cranmer, S. R. (2018).
        Detection of a Millimeter Flare from Proxima Centauri.
         Astrophys. J. Lett., 855(1):L2.
        
        [Magallón et al.2013]MHQ13Magallón, S., Hilu, K. W., and Quandt, D. (2013).
        Land plant evolutionary timeline: Gene effects are secondary to
          fossil constraints in relaxed clock estimation of age and substitution
          rates.
         Am. J. Bot., 100(3):556–573.
        
        [Maloof et al.2010]MR10Maloof, A. C., Rose, C. V., Beach, R., Samuels, B. M., Calmet, C. C.,
          Erwin, D. H., Poirier, G. R., Yao, N., and Simons, F. J. (2010).
        Possible animal-body fossils in pre-Marinoan limestones from South
          Australia.
         Nat. Geosci., 3(9):653–659.
        
        [Margulis1981]Mar81Margulis, L. (1981).
        Symbiosis in Cell Evolution: Life and Its Environment on the
          Early Earth.
         W. H. Freeman & Co.
        
        [Martin et al.2008]MB08Martin, W., Baross, J., Kelley, D., and Russell, M. J. (2008).
        Hydrothermal vents and the origin of life.
         Nat. Rev. Microbiol., 6:805–814.
        
        [Martin et al.2015]MGZ15Martin, W. F., Garg, S., and Zimorski, V. (2015).
        Endosymbiotic theories for eukaryote origin.
         Phil. Trans. R. Soc. B, 370(1678):20140330.
        
        [Martin et al.2017]MTM17Martin, W. F., Tielens, A. G. M., Mentel, M., Garg, S. G., and Gould,
          S. B. (2017).
        The Physiology of Phagocytosis in the Context of Mitochondrial
          Origin.
         Microbiol. Mol. Biol. Rev., 81(3):e00008–17.
        
        [Martins et al.2013]MP13Martins, Z., Price, M. C., Goldman, N., Sephton, M. A., and Burchell,
          M. J. (2013).
        Shock synthesis of amino acids from impacting cometary and icy
          planet surface analogues.
         Nat. Geosci., 6(12):1045–1049.
        
        [Maruyama et al.2013]MIG13Maruyama, S., Ikoma, M., Genda, H., Hirose, K., Yokoyama, T., and
          Santosh, M. (2013).
        The naked planet Earth: Most essential pre-requisite for the origin
          and evolution of life.
         Geosci. Front., 4(2):141–165.
        
        [Mautner1997]Mau97Mautner, M. N. (1997).
        Directed Panspermia. 3. strategies and Motivation for Seeding
          Star-Forming Clouds.
         J. Br. Interplanet. Soc., 50:93–102.
        
        [Mayr1985]Mayr85Mayr, E. (1985).
        The probability of extraterrestrial intelligent life.
         In Regis, E., editor,  Extraterrestrials: Science and Alien
          Intelligence, pages 23–30. Cambridge Univ. Press.
        
        [McCabe and Lucas2010]ML10McCabe, M. and Lucas, H. (2010).
        On the origin and evolution of life in the Galaxy.
         Int. J. Astrobiol., 9(4):217–226.
        
        [McCollom2007]McC07McCollom, T. M. (2007).
        Geochemical Constraints on Sources of Metabolic Energy for
          Chemolithoautotrophy in Ultramafic-Hosted Deep-Sea Hydrothermal Systems.
         Astrobiology, 7(6):933–950.
        
        [McCollom2013]McCo13McCollom, T. M. (2013).
        Miller-Urey and Beyond: What Have We Learned About Prebiotic Organic
          Synthesis Reactions in the Past 60 Years? Annu. Rev. Earth Planet. Sci., 41:207–229.
        
        [McCollom and Seewald2007]MCS07McCollom, T. M. and Seewald, J. S. (2007).
        Abiotic Synthesis of Organic Compounds in Deep-Sea Hydrothermal
          Environments.
         Chem. Rev., 107(2):382–401.
        
        [McGhee2011]McG11McGhee, G. R. (2011).
        Convergent Evolution: Limited Forms Most Beautiful.
         The Vienna Series in Theoretical Biology. The MIT Press.
        
        [McInerney et al.2014]MOP14McInerney, J. O., O'Connell, M. J., and Pisani, D. (2014).
        The hybrid nature of the Eukaryota and a consilient view of life on
          Earth.
         Nat. Rev. Microbiol., 12:449–455.
        
        [McShea and Brandon2010]MSB10McShea, D. W. and Brandon, R. N. (2010).
        Biology's First Law.
         The Univ. of Chicago Press.
        
        [Meadows2017]Mead17Meadows, V. S. (2017).
        Reflections on O_2 as a Biosignature in Exoplanetary
          Atmospheres.
         Astrobiology, 17(10):1022–1052.
        
        [Melott and Thomas2011]MT11Melott, A. L. and Thomas, B. C. (2011).
        Astrophysical Ionizing Radiation and Earth: A Brief Review and
          Census of Intermittent Intense Sources.
         Astrobiology, 11(4):343–361.
        
        [Miller and Lazcano1995]ML95Miller, S. L. and Lazcano, A. (1995).
        The origin of life–did it occur at high temperatures? J. Mol. Evol., 41(6):689–692.
        
        [Mills and Canfield2014]MC14Mills, D. B. and Canfield, D. E. (2014).
        Oxygen and animal evolution: Did a rise of atmospheric oxygen
          “trigger” the origin of animals? BioEssays, 36(12):1145–1155.
        
        [Mills et al.2014]MWJ14Mills, D. B., Ward, L. M., Jones, C., Sweeten, B., Forth, M.,
          Treusch, A. H., and Canfield, D. E. (2014).
        Oxygen requirements of the earliest animals.
         Proc. Natl. Acad. Sci. USA, 111(11):4168–4172.
        
        [Monod1971]Mon71Monod, J. (1971).
        Chance and Necessity: An Essay on the Natural Philosophy of
          Modern Biology.
         Alfred A. Knopf, New York.
        
        [Moore et al.2017]MJG17Moore, E. K., Jelen, B. I., Giovannelli, D., Raanan, H., and
          Falkowski, P. G. (2017).
        Metal availability and the expanding network of microbial
          metabolisms in the Archaean eon.
         Nat. Geosci., 10(9):629–636.
        
        [Morris et al.2018]MPC18Morris, J. L., Puttick, M. N., Clark, J. W., Edwards, D., Kenrick,
          P., Pressel, S., Wellman, C. H., Yang, Z., Schneider, H., and
          Donoghue, P. C. J. (2018).
        The timescale of early land plant evolution.
         Proc. Natl. Acad. Sci. USA, 115(10):E2274–E2283.
        
        [Morris2003]Mor03Morris, S. C. (2003).
        Life's Solution: Inevitable Humans in a Lonely Universe.
         Cambridge Univ. Press.
        
        [Mulkidjanian et al.2003]MCG03Mulkidjanian, A. Y., Cherepanov, D. A., and Galperin, M. Y. (2003).
        Survival of the fittest before the beginning of life: selection of
          the first oligonucleotide-like polymers by UV light.
         BMC Evol. Biol., 3:12.
        
        [Mulkidjanian et al.2006]MKM06Mulkidjanian, A. Y., Koonin, E. V., Makarova, K. S., Mekhedov, S. L.,
          Sorokin, A., Wolf, Y. I., Dufresne, A., Partensky, F., Burd, H.,
          Kaznadzey, D., Haselkorn, R., and Galperin, M. Y. (2006).
        The cyanobacterial genome core and the origin of photosynthesis.
         Proc. Natl. Acad. Sci. USA, 103(35):13126–13131.
        
        [Neveu et al.2013]NKB13Neveu, M., Kim, H.-J., and Benner, S. A. (2013).
        The “Strong” RNA World Hypothesis: Fifty Years Old.
         Astrobiology, 13(4):391–403.
        
        [Newman et al.2016]NNR16Newman, D. K., Neubauer, C., Ricci, J. N., Wu, C.-H., and Pearson, A.
          (2016).
        Cellular and Molecular Biological Approaches to Interpreting Ancient
          Biomarkers.
         Annu. Rev. Earth Planet. Sci., 44:493–522.
        
        [Newman and Sagan1981]NS81Newman, W. I. and Sagan, C. (1981).
        Galactic civilizations - Population dynamics and interstellar
          diffusion.
         Icarus, 46(3):293–327.
        
        [Noffke et al.2013]NCW13Noffke, N., Christian, D., Wacey, D., and Hazen, R. M. (2013).
        Microbially Induced Sedimentary Structures Recording an Ancient
          Ecosystem in the ca.3.48 Billion-Year-Old Dresser Formation, Pilbara, Western
          Australia.
         Astrobiology, 13(12):1103–1124.
        
        [Nutman et al.2016]NB16Nutman, A. P., Bennett, V. C., Friend, C. R. L., van Kranendonk, M. J.,
          and Chivas, A. R. (2016).
        Rapid emergence of life shown by discovery of 3,700-million-year-old
          microbial structures.
         Nature, 537(7621):535–538.
        
        [Och and Shields-Zhou2012]OS12Och, L. M. and Shields-Zhou, G. A. (2012).
        The Neoproterozoic oxygenation event: Environmental perturbations
          and biogeochemical cycling.
         Earth Sci. Rev., 110(1):26–57.
        
        [Ochoa de Alda et al.2014]OED14Ochoa de Alda, J. A. G., Esteban, R., Diago, M. L., and Houmard, J.
          (2014).
        The plastid ancestor originated among one of the major
          cyanobacterial lineages.
         Nat. Commun., 5:4937.
        
        [Odert et al.2017]OLH17Odert, P., Leitzinger, M., Hanslmeier, A., and Lammer, H. (2017).
        Stellar coronal mass ejections - I. Estimating occurrence
          frequencies and mass-loss rates.
         Mon. Not. R. Astron. Soc., 472(1):876–890.
        
        [Odling-Smee et al.2003]OLF03Odling-Smee, F. J., Laland, K. N., and Feldman, M. W. (2003).
        Niche Construction: The Neglected Process in Evolution.
         Number 37 in Monographs in Population Biology. Princeton Univ. Press.
        
        [Ohtomo et al.2014]OK14Ohtomo, Y., Kakegawa, T., Ishida, A., Nagase, T., and Rosing, M. T.
          (2014).
        Evidence for biogenic graphite in early Archaean Isua
          metasedimentary rocks.
         Nat. Geosci., 7(1):25–28.
        
        [O'Malley and Powell2016]OMP16O'Malley, M. A. and Powell, R. (2016).
        Major problems in evolutionary transitions: how a metabolic
          perspective can enrich our understanding of macroevolution.
         Biol. Philos., 31(2):159–189.
        
        [O'Malley-James and Kaltenegger2017]OMJ17O'Malley-James, J. T. and Kaltenegger, L. (2017).
        UV surface habitability of the TRAPPIST-1 system.
         Mon. Not. R. Astron. Soc. Lett., 469(1):L26–L30.
        
        [Orgel2004]Org04Orgel, L. E. (2004).
        Prebiotic Chemistry and the Origin of the RNA World.
         Crit. Rev. Biochem. Mol. Biol., 39(2):99–123.
        
        [Oró et al.1990]OML90Oró, J., Miller, S. L., and Lazcano, A. (1990).
        The Origin and Early Evolution of Life on Earth.
         Annu. Rev. Earth Planet. Sci., 18:317–356.
        
        [Parfrey et al.2011]PLK11Parfrey, L. W., Lahr, D. J. G., Knoll, A. H., and Katz, L. A. (2011).
        Estimating the timing of early eukaryotic diversification with
          multigene molecular clocks.
         Proc. Natl. Acad. Sci. USA, 108(33):13624–13629.
        
        [Pascal2012]Pas12Pascal, R. (2012).
        Suitable energetic conditions for dynamic chemical complexity and
          the living state.
         J. Sys. Chem., 3:3.
        
        [Patel et al.2015]Pat15Patel, B. H., Percivalle, C., Ritson, D. J., Duffy, C. D., and
          Sutherland, J. D. (2015).
        Common origins of RNA, protein and lipid precursors in a
          cyanosulfidic protometabolism.
         Nat. Chem., 7(4):301–307.
        
        [Patsourakos and Georgoulis2017]PG17Patsourakos, S. and Georgoulis, M. K. (2017).
        A Helicity-Based Method to Infer the CME Magnetic Field Magnitude in
          Sun and Geospace: Generalization and Extension to Sun-Like and M-Dwarf Stars
          and Implications for Exoplanet Habitability.
         Sol. Phys., 292(7):89.
        
        [Payne et al.2009]PBB09Payne, J. L., Boyer, A. G., Brown, J. H., Finnegan, S., Kowalewski,
          M., Krause, Jr., R. A., Lyons, S. K., McClain, C. R., McShea, D. W.,
          Novack-Gottshall, P. M., Smith, F. A., Stempien, J. A., and Wang,
          S. C. (2009).
        Two-phase increase in the maximum size of life over 3.5 billion
          years reflects biological innovation and environmental opportunity.
         Proc. Natl. Acad. Sci. USA, 106(1):24–27.
        
        [Penn et al.2008]PHP08Penn, D. C., Holyoak, K. J., and Povinelli, D. J. (2008).
        Darwin's mistake: Explaining the discontinuity between human and
          nonhuman minds.
         Behav. Brain Sci., 31(2):109–130.
        
        [Pittis and Gabaldón2016]PG16Pittis, A. A. and Gabaldón, T. (2016).
        Late acquisition of mitochondria by a host with chimaeric
          prokaryotic ancestry.
         Nature, 531(7592):101–104.
        
        [Planavsky et al.2014]PAH14Planavsky, N. J., Asael, D., Hofmann, A., Reinhard, C. T., Lalonde,
          S. V., Knudsen, A., Wang, X., Ossa Ossa, F., Pecoits, E., Smith,
          A. J. B., Beukes, N. J., Bekker, A., Johnson, T. M., Konhauser,
          K. O., Lyons, T. W., and Rouxel, O. J. (2014).
        Evidence for oxygenic photosynthesis half a billion years before the
          Great Oxidation Event.
         Nat. Geosci., 7(4):283–286.
        
        [Pogge von Strandmann et al.2015]PS15Pogge von Strandmann, P. A. E., Stüeken, E. E., Elliott, T.,
          Poulton, S. W., Dehler, C. M., Canfield, D. E., and Catling, D. C.
          (2015).
        Selenium isotope evidence for progressive oxidation of the
          Neoproterozoic biosphere.
         Nat. Commun., 6:10157.
        
        [Poole and Gribaldo2014]PG14Poole, A. M. and Gribaldo, S. (2014).
        Eukaryotic Origins: How and When Was the Mitochondrion Acquired? Cold Spring Harb. Perspect. Biol., 6(12):a015990.
        
        [Post and Palkovacs2009]PP09Post, D.-M. and Palkovacs, E.-P. (2009).
        Eco-evolutionary feedbacks in community and ecosystem ecology:
          interactions between the ecological theatre and the evolutionary play.
         Phil. Trans. R. Soc. B, 364(1523):1629–1640.
        
        [Powner et al.2009]PGS09Powner, M. W., Gerland, B., and Sutherland, J. D. (2009).
        Synthesis of activated pyrimidine ribonucleotides in prebiotically
          plausible conditions.
         Nature, 459(7244):239–242.
        
        [Price et al.2012]PCY12Price, D. C., Chan, C. X., Yoon, H. S., Yang, E. C., Qiu, H.,
          Weber, A. P. M., Schwacke, R., Gross, J., Blouin, N. A., Lane, C.,
          Reyes-Prieto, A., Durnford, D. G., Neilson, J. A. D., Lang, B. F.,
          Burger, G., Steiner, J. M., Löffelhardt, W., Meuser, J. E.,
          Posewitz, M. C., Ball, S., Arias, M. C., Henrissat, B., Coutinho,
          P. M., Rensing, S. A., Symeonidi, A., Doddapaneni, H., Green, B. R.,
          Rajah, V. D., Boore, J., and Bhattacharya, D. (2012).
        Cyanophora paradoxa Genome Elucidates Origin of Photosynthesis in
          Algae and Plants.
         Science, 335(6070):843–847.
        
        [Ranjan and Sasselov2016]RS16Ranjan, S. and Sasselov, D. D. (2016).
        Influence of the UV Environment on the Synthesis of Prebiotic
          Molecules.
         Astrobiology, 16(1):68–88.
        
        [Ranjan et al.2017]RWS17Ranjan, S., Wordsworth, R., and Sasselov, D. D. (2017).
        The Surface UV Environment on Planets Orbiting M-Dwarfs:
          Implications for Prebiotic Chemistry and the Need for Experimental
          Follow-up.
         Astrophys. J., 843(2):110.
        
        [Rapf and Vaida2016]RV16Rapf, R. J. and Vaida, V. (2016).
        Sunlight as an energetic driver in the synthesis of molecules
          necessary for life.
         Phys. Chem. Chem. Phys., 18(30):20067–20084.
        
        [Rasmussen et al.2008]RFB08Rasmussen, B., Fletcher, I. R., Brocks, J. J., and Kilburn, M. R.
          (2008).
        Reassessing the first appearance of eukaryotes and cyanobacteria.
         Nature, 455(7216):1101–1104.
        
        [Reinhard et al.2017a]RO17Reinhard, C. T., Olson, S. L., Schwieterman, E. W., and Lyons, T. W.
          (2017a).
        False Negatives for Remote Life Detection on Ocean-Bearing Planets:
          Lessons from the Early Earth.
         Astrobiology, 17(4):287–297.
        
        [Reinhard et al.2017b]RPGReinhard, C. T., Planavsky, N. J., Gill, B. C., Ozaki, K., Robbins,
          L. J., Lyons, T. W., Fischer, W. W., Wang, C., Cole, D. B., and
          Konhauser, K. O. (2017b).
        Evolution of the global phosphorus cycle.
         Nature, 541(7637):386–389.
        
        [Reyes-Prieto et al.2007]RWB07Reyes-Prieto, A., Weber, A. P. M., and Bhattacharya, D. (2007).
        The Origin and Establishment of the Plastid in Algae and Plants.
         Annu. Rev. Genet., 41:147–168.
        
        [Richerson and Boyd2008]RB08Richerson, P. J. and Boyd, R. (2008).
        Not By Genes Alone: How Culture Transformed Human Evolution.
         The Univ. of Chicago Press.
        
        [Richter and King2013]RK13Richter, D. J. and King, N. (2013).
        The Genomic and Cellular Foundations of Animal Origins.
         Annual Review of Genetics, 47:509–537.
        
        [Rimmer et al.2018]RXT18Rimmer, P. B., Xu, J., Thompson, S. J., Gillen, E., Sutherland,
          J. D., and Queloz, D. (2018).
        The origin of RNA precursors on exoplanets.
         Sci. Adv., 4(8):eaar3302.
        
        [Rios and Tor2013]RT13Rios, A. C. and Tor, Y. (2013).
        On the Origin of the Canonical Nucleobases: An Assessment of
          Selection Pressures across Chemical and Early Biological Evolution.
         Isr. J. Chem., 53(6-7):469–483.
        
        [Ritson and Sutherland2012]RS12Ritson, D. and Sutherland, J. D. (2012).
        Prebiotic synthesis of simple sugars by photoredox systems
          chemistry.
         Nat. Chem., 4(11):895–899.
        
        [Ritson et al.2018]RBL18Ritson, D. J., Battilocchio, C., Ley, S. V., and Sutherland, J. D.
          (2018).
        Mimicking the surface and prebiotic chemistry of early Earth using
          flow chemistry.
         Nat. Commun., 9:1821.
        
        [Ritson and Sutherland2013]RS13Ritson, D. J. and Sutherland, J. D. (2013).
        Synthesis of Aldehydic Ribonucleotide and Amino Acid Precursors by
          Photoredox Chemistry.
         Angew. Chem. Int. Ed., 52(22):5845–5847.
        
        [Roberts and Feeney2009]RF09Roberts, W. A. and Feeney, M. C. (2009).
        The comparative study of mental time travel.
         Trends Cogn. Sci., 13(6):271–277.
        
        [Rodríguez-Ezpeleta et al.2005]RBB05Rodríguez-Ezpeleta, N., Brinkmann, H., Burey, S. C., Roure, B.,
          Burger, G., Löffelhardt, W., Bohnert, H. J., Philippe, H., and
          Lang, B. F. (2005).
         Monophyly of primary photosynthetic eukaryotes: green plants, red
          algae, and glaucophytes.
         Curr. Biol., 15(14):1325–1330.
        
        [Roger et al.2017]RMK17Roger, A. J., Muñoz-Gómez, S. A., and Kamikawa, R. (2017).
        The Origin and Diversification of Mitochondria.
         Curr. Biol., 27(21):R1177–R1192.
        
        [Rokas2008]Rok08Rokas, A. (2008).
        The Origins of Multicellularity and the Early History of the Genetic
          Toolkit For Animal Development.
         Annu. Rev. Genet., 42:235–251.
        
        [Rosing1999]Ros99Rosing, M. T. (1999).
        13C-Depleted Carbon Microparticles in >3700-Ma Sea-Floor
          Sedimentary Rocks from West Greenland.
         Science, 283(5402):674–676.
        
        [Rosing and Frei2004]RF04Rosing, M. T. and Frei, R. (2004).
        U-rich Archaean sea-floor sediments from Greenland - indications of
          >3700 Ma oxygenic photosynthesis.
         Earth Planet. Sci. Lett., 217(3-4):237–244.
        
        [Roth2015]Roth15Roth, G. (2015).
        Convergent evolution of complex brains and high intelligence.
         Phil. Trans. R. Soc. B, 370(1684):20150049.
        
        [Roth and Dicke2005]RD05Roth, G. and Dicke, U. (2005).
        Evolution of the brain and intelligence.
         Trends Cogn. Sci., 9(5):250–257.
        
        [Rothschild1999]Roth99Rothschild, L. J. (1999).
        The Influence of UV Radiation on Protistan Evolution.
         J. Eukaryot. Microbiol., 46(5):548–555.
        
        [Rowlands2015]Row15Rowlands, M. (2015).
        Can Animals Be Moral? Oxford Univ. Press.
        
        [Ruiz-Mirazo et al.2014]RBD14Ruiz-Mirazo, K., Briones, C., and de la Escosura, A. (2014).
        Prebiotic Systems Chemistry: New Perspectives for the Origins of
          Life.
         Chem. Rev., 114(1):285–366.
        
        [Ruse1996]Ruse96Ruse, M. (1996).
        Monad to Man: The Concept of Progress in Evolutionary
          Biology.
         Harvard Univ. Press.
        
        [Rushby et al.2013]RCOWRushby, A. J., Claire, M. W., Osborn, H., and Watson, A. J. (2013).
        Habitable Zone Lifetimes of Exoplanets around Main Sequence Stars.
         Astrobiology, 13(9):833–849.
        
        [Russell et al.2014]RBB14Russell, M. J., Barge, L. M., Bhartia, R., Bocanegra, D., Bracher,
          P. J., Branscomb, E., Kidd, R., McGlynn, S., Meier, D. H.,
          Nitschke, W., Shibuya, T., Vance, S., White, L., and Kanik, I.
          (2014).
        The Drive to Life on Wet and Icy Worlds.
         Astrobiology, 14(4):308–343.
        
        [Sagan1973]Sag73Sagan, C. (1973).
        Ultraviolet selection pressure on the earliest organisms.
         J. Theor. Biol., 39(1):195–200.
        
        [Sagan and Khare1971]SK71Sagan, C. and Khare, B. N. (1971).
        Long-Wavelength Ultraviolet Photoproduction of Amino Acids on the
          Primitive Earth.
         Science, 173(3995):417–420.
        
        [Sagan1967]Sag67Sagan, L. (1967).
        On the origin of mitosing cells.
         J. Theor. Biol., 14(3):225–274.
        
        [Sakai et al.2018]SST18Sakai, S., Seki, K., Terada, N., Shinagawa, H., Tanaka, T., and
          Ebihara, Y. (2018).
        Effects of a Weak Intrinsic Magnetic Field on Atmospheric Escape
          From Mars.
         Geophys. Res. Lett., 45(18):9336–9343.
        
        [Sánchez-Baracaldo et al.2017]SRPK17Sánchez-Baracaldo, P., Raven, J. A., Pisani, D., and Knoll, A. H.
          (2017).
        Early photosynthetic eukaryotes inhabited low-salinity habitats.
         Proc. Natl. Acad. Sci. USA, 114(37):E7737–E7745.
        
        [Sanderson et al.2004]STWB04Sanderson, M. J., Thorne, J. L., Wikström, N., and Bremer, K.
          (2004).
        Molecular evidence on plant divergence times.
         Am. J. Bot., 91(10):1656–1665.
        
        [Satkoski et al.2015]SBL15Satkoski, A. M., Beukes, N. J., Li, W., Beard, B. L., and Johnson,
          C. M. (2015).
        A redox-stratified ocean 3.2 billion years ago.
         Earth Planet. Sci. Lett., 430:43–53.
        
        [Schirrmeister et al.2015]SGD15Schirrmeister, B. E., Gugger, M., and Donoghue, P. C. J. (2015).
        Cyanobacteria and the Great Oxidation Event: evidence from genes and
          fossils.
         Palaeontology, 58(5):769–785.
        
        [Schirrmeister et al.2016]SSW16Schirrmeister, B. E., Sanchez-Baracaldo, P., and Wacey, D. (2016).
        Cyanobacterial evolution during the Precambrian.
         Int. J. Astrobiol., 15(3):187–204.
        
        [Schopf1994]Sch94Schopf, J. W. (1994).
        Disparate rates, differing fates: tempo and mode of evolution
          changed from the Precambrian to the Phanerozoic.
         Proc. Natl. Acad. Sci. USA, 91(15):6735–6742.
        
        [Schopf et al.2018]SKS18Schopf, J. W., Kitajima, K., Spicuzza, M. J., Kudryavtsev, A. B., and
          Valley, J. W. (2018).
        SIMS analyses of the oldest known assemblage of microfossils
          document their taxon-correlated carbon isotope compositions.
         Proc. Natl. Acad. Sci. USA, 115(1):53–58.
        
        [Schwieterman et al.2018]SK18Schwieterman, E. W., Kiang, N. Y., Parenteau, M. N., Harman, C. E.,
          DasSarma, S., Fisher, T. M., Arney, G. N., Hartnett, H. E.,
          Reinhard, C. T., Olson, S. L., Meadows, V. S., Cockell, C. S.,
          Walker, S. I., Grenfell, J. L., Hegde, S., Rugheimer, S., Hu, R.,
          and Lyons, T. W. (2018).
        Exoplanet Biosignatures: A Review of Remotely Detectable Signs of
          Life.
         Astrobiology, 18(6):663–708.
        
        [Segura et al.2010]SWM10Segura, A., Walkowicz, L. M., Meadows, V., Kasting, J., and Hawley,
          S. (2010).
        The Effect of a Strong Stellar Flare on the Atmospheric Chemistry of
          an Earth-like Planet Orbiting an M Dwarf.
         Astrobiology, 10(7):751–771.
        
        [Serrano-Andres and Merchan2009]SM09Serrano-Andres, L. and Merchan, M. (2009).
        Are the five natural DNA/RNA base monomers a good choice from
          natural selection?: A photochemical perspective.
         J. Photochem. Photobiol. C, 10(1):21–32.
        
        [Shen et al.2001]SBC01Shen, Y., Buick, R., and Canfield, D. E. (2001).
        Isotopic evidence for microbial sulphate reduction in the early
          Archaean era.
         Nature, 410(6824):77–81.
        
        [Shih et al.2017]SHW17Shih, P. M., Hemp, J., Ward, L. M., Matzke, N. J., and Fischer, W. W.
          (2017).
        Crown group Oxyphotobacteria postdate the rise of oxygen.
         Geobiology, 15(1):19–29.
        
        [Shklovskii and Sagan1966]SS66Shklovskii, I. S. and Sagan, C. (1966).
        Intelligent Life in the Universe.
         Holden-Day, Inc.
        
        [Simpson1964]Sim64Simpson, G. G. (1964).
        The Nonprevalence of Humanoids.
         Science, 143(3608):769–775.
        
        [Simpson1967]Simp67Simpson, G. G. (1967).
        The Meaning of Evolution, volume 23.
         Yale Univ. Press.
        
        [Sloan et al.2017]SAL17Sloan, D., Alves Batista, R., and Loeb, A. (2017).
        The Resilience of Life to Astrophysical Events.
         Sci. Rep., 7:5419.
        
        [Smith and Szathmáry1995]JMS95Smith, J. M. and Szathmáry, E. (1995).
        The Major Transitions in Evolution.
         Oxford Univ. Press.
        
        [Smith and Szathmáry1999]MS99Smith, J. M. and Szathmáry, E. (1999).
        The Origins of Life: From the Birth of Life to the Origin of
          Language.
         Oxford Univ. Press.
        
        [Smith et al.2010]SBD10Smith, S. A., Beaulieu, J. M., and Donoghue, M. J. (2010).
        An uncorrelated relaxed-clock analysis suggests an earlier origin
          for flowering plants.
         Proc. Natl. Acad. Sci. USA, 107(13):5897–5902.
        
        [Sojo et al.2016]SHW16Sojo, V., Herschy, B., Whicher, A., Camprubí, E., and Lane, N.
          (2016).
        The Origin of Life in Alkaline Hydrothermal Vents.
         Astrobiology, 16(2):181–197.
        
        [Spang et al.2015]SSJ15Spang, A., Saw, J. H., Jørgensen, S. L., Zaremba-Niedzwiedzka, K.,
          Martijn, J., Lind, A. E., van Eijk, R., Schleper, C., Guy, L., and
          Ettema, T. J. G. (2015).
        Complex archaea that bridge the gap between prokaryotes and
          eukaryotes.
         Nature, 521(7551):173–179.
        
        [Speijer et al.2015]SLE15Speijer, D., Lukeš, J., and Eliáš, M. (2015).
        Sex is a ubiquitous, ancient, and inherent attribute of eukaryotic
          life.
         Proc. Natl. Acad. Sci. USA, 112(29):8827–8834.
        
        [Sperling et al.2013]SHK13Sperling, E. A., Halverson, G. P., Knoll, A. H., Macdonald, F. A., and
          Johnston, D. T. (2013).
        A basin redox transect at the dawn of animal life.
         Earth Planet. Sci. Lett., 371:143–155.
        
        [Sperling et al.2015]SW15Sperling, E. A., Wolock, C. J., Morgan, A. S., Gill, B. C., Kunzmann,
          M., Halverson, G. P., MacDonald, F. A., Knoll, A. H., and Johnston,
          D. T. (2015).
        Statistical analysis of iron geochemical data suggests limited late
          Proterozoic oxygenation.
         Nature, 523(7561):451–454.
        
        [Spiegel and Turner2012]ST12Spiegel, D. S. and Turner, E. L. (2012).
        Bayesian analysis of the astrobiological implications of life's
          early emergence on Earth.
         Proc. Natl. Acad. Sci. USA, 109(2):395–400.
        
        [Steffen et al.2015]SBD15Steffen, W., Broadgate, W., Deutsch, L., Gaffney, O., and Ludwig, C.
          (2015).
        The trajectory of the Anthropocene: The Great Acceleration.
         The Anthropocene Review, 2(1):81–98.
        
        [Steffen et al.2011]SG11Steffen, W., Grinevald, J., Crutzen, P., and McNeill, J. (2011).
        The Anthropocene: conceptual and historical perspectives.
         Philos. Trans. Royal Soc. A, 369(1938):842–867.
        
        [Sterelny2011]Ste11Sterelny, K. (2011).
        From hominins to humans: how sapiens became behaviourally modern.
         Philos. Trans. Royal Soc. B, 366(1566):809–822.
        
        [Stern2016]Ste16Stern, R. J. (2016).
        Is plate tectonics needed to evolve technological species on
          exoplanets? Geosci. Front., 7(4):573–580.
        
        [Stewart and Cohen1997]SC97Stewart, I. and Cohen, J. (1997).
        Figments of Reality: The Evolution of the Curious Mind.
         Cambridge Univ. Press.
        
        [Stüeken2016]Stu16Stüeken, E. E. (2016).
        Nitrogen in Ancient Mud: A Biosignature? Astrobiology, 16(9):730–735.
        
        [Stüeken et al.2015]SBG15Stüeken, E. E., Buick, R., Guy, B. M., and Koehler, M. C. (2015).
        Isotopic evidence for biological nitrogen fixation by
          molybdenum-nitrogenase from 3.2 Gyr.
         Nature, 520(7549):666–669.
        
        [Stüeken et al.2012]SCB12Stüeken, E. E., Catling, D. C., and Buick, R. (2012).
        Contributions to late Archaean sulphur cycling by life on land.
         Nat. Geosci., 5(10):722–725.
        
        [Suddendorf2013]Sud13Suddendorf, T. (2013).
        The Gap: The Science of What Separates Us from Other Animals.
         Basic Books.
        
        [Sutherland2016]Suth16Sutherland, J. D. (2016).
        The Origin of Life–Out of the Blue.
         Angew. Chem. Int. Ed., 55(1):104–121.
        
        [Sutherland2017]Suth17Sutherland, J. D. (2017).
        Studies on the origin of life – the end of the beginning.
         Nat. Rev. Chem., 1:0012.
        
        [Szathmáry2015]Sza15Szathmáry, E. (2015).
        Toward major evolutionary transitions theory 2.0.
         Proc. Natl. Acad. Sci. USA, 112(33):10104–10111.
        
        [Szathmáry and Smith1995]SS95Szathmáry, E. and Smith, J. M. (1995).
         The major evolutionary transitions.
         Nature, 374(6519):227–232.
        
        [Tattersall2009]Tat09Tattersall, I. (2009).
        Human origins: Out of Africa.
         Proc. Natl. Acad. Sci. USA, 106(38):16018–16021.
        
        [Teramura and Sullivan1994]TS94Teramura, A. H. and Sullivan, J. H. (1994).
        Effects of UV-B radiation on photosynthesis and growth of
          terrestrial plants.
         Photosynth. Res., 39(3):463–473.
        
        [Tian and Ida2015]TI15Tian, F. and Ida, S. (2015).
        Water contents of Earth-mass planets around M dwarfs.
         Nature Geoscience, 8(3):177–180.
        
        [Todd et al.2018]TFM18Todd, Z. R., Fahrenbach, A. C., Magnani, C. J., Ranjan, S.,
          Björkbom, A., Szostak, J. W., and Sasselov, D. D. (2018).
        Solvated-electron production using cyanocuprates is compatible with
          the UV-environment on a Hadean–Archaean Earth.
         Chem. Commun., 54(9):1121–1124.
        
        [Tomasello2008]Tom08Tomasello, M. (2008).
        Origins of Human Communication.
         The MIT Press.
        
        [Tomasello2014]Tom14Tomasello, M. (2014).
        A Natural History of Human Thinking.
         Harvard Univ. Press.
        
        [Tomasello2016]Tom16Tomasello, M. (2016).
        A Natural History of Human Morality.
         Harvard Univ. Press.
        
        [Ueno et al.2008]UO08Ueno, Y., Ono, S., Rumble, D., and Maruyama, S. (2008).
        Quadruple sulfur isotope analysis of ca. 3.5 Ga Dresser Formation:
          New evidence for microbial sulfate reduction in the early Archean.
         Geochim. Cosmochim. Acta, 72(23):5675–5691.
        
        [Ueno et al.2006]UYY06Ueno, Y., Yamada, K., Yoshida, N., Maruyama, S., and Isozaki, Y.
          (2006).
        Evidence from fluid inclusions for microbial methanogenesis in the
          early Archaean era.
         Nature, 440(7083):516–519.
        
        [Šponer et al.2016]SSG16Šponer, J. E., Szabla, R., Góra, R. W., Saitta, A. M.,
          Pietrucci, F., Saija, F., Di Mauro, E., Saladino, R., Ferus, M.,
          Civiš, S., and Šponer, J. (2016).
        Prebiotic synthesis of nucleic acids and their building blocks at
          the atomic level - merging models and mechanisms from advanced computations
          and experiments.
         Phys. Chem. Chem. Phys., 18(30):20047–20066.
        
        [Valley et al.2002]VP02Valley, J. W., Peck, W. H., King, E. M., and Wilde, S. A. (2002).
        A cool early Earth.
         Geology, 30(4):351–354.
        
        [Vida et al.2017]VKP17Vida, K., Kővári, Z., Pál, A., Oláh, K., and
          Kriskovics, L. (2017).
        Frequent Flaring in the TRAPPIST-1 System–Unsuited for Life? Astrophys. J., 841(2):124.
        
        [Vidotto et al.2013]VJM13Vidotto, A. A., Jardine, M., Morin, J., Donati, J.-F., Lang, P., and
          Russell, A. J. B. (2013).
        Effects of M dwarf magnetic fields on potentially habitable
          planets.
         Astron. Astrophys., 557:A67.
        
        [Wacey et al.2011a]WK11Wacey, D., Kilburn, M. R., Saunders, M., Cliff, J., and Brasier,
          M. D. (2011a).
        Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old
          rocks of Western Australia.
         Nat. Geosci., 4(10):698–702.
        
        [Wacey et al.2011b]WSB11Wacey, D., Saunders, M., Brasier, M. D., and Kilburn, M. R. (2011b).
        Earliest microbially mediated pyrite oxidation in ∼ 3.4
          billion-year-old sediments.
         Earth Planet. Sci. Lett., 301(1-2):393–402.
        
        [Wachowius et al.2017]WAH17Wachowius, F., Attwater, J., and Holliger, P. (2017).
        Nucleic acids: Function and potential for abiogenesis.
         Q. Rev. Biophys., 50:E4.
        
        [Wagner2011]Wag11Wagner, A. (2011).
        The Origins of Evolutionary Innovations.
         Oxford Univ. Press.
        
        [Waldbauer et al.2009]WSS09Waldbauer, J. R., Sherman, L. S., Sumner, D. Y., and Summons, R. E.
          (2009).
        Late Archean molecular fossils from the Transvaal Supergroup record
          the antiquity of microbial diversity and aerobiosis.
         Precambrian Res., 169(1-4):28–47.
        
        [Walker2017]IW17Walker, S. I. (2017).
        Origins of life: a problem for physics, a key issues review.
         Rep. Prog. Phys., 80(9):092601.
        
        [Waltham2017]Wal17Waltham, D. (2017).
        Star Masses and Star-Planet Distances for Earth-like Habitability.
         Astrobiology, 17(1):61–77.
        
        [Ward and Brownlee2000]WaB00Ward, P. and Brownlee, D. (2000).
        Rare Earth: Why Complex Life Is Uncommon in the Universe.
         Copernicus.
        
        [Waters et al.2016]WZ16Waters, C. N., Zalasiewicz, J., Summerhayes, C., Barnosky, A. D.,
          Poirier, C., Gałuszka, A., Cearreta, A., Edgeworth, M., Ellis,
          E. C., Ellis, M., Jeandel, C., Leinfelder, R., McNeill, J. R.,
          Richter, D. d., Steffen, W., Syvitski, J., Vidas, D., Wagreich, M.,
          Williams, M., Zhisheng, A., Grinevald, J., Odada, E., Oreskes, N.,
          and Wolfe, A. P. (2016).
        The Anthropocene is functionally and stratigraphically distinct from
          the Holocene.
         Science, 351(6269):aad2622.
        
        [Watson2008]Wat08Watson, A. J. (2008).
        Implications of an Anthropic Model of Evolution for Emergence of
          Complex Life and Intelligence.
         Astrobiology, 8(1):175–185.
        
        [Weiss et al.2016]Weiss16Weiss, M. C., Sousa, F. L., Mrnjavac, N., Neukirchen, S., Roettger,
          M., Nelson-Sathi, S., and Martin, W. F. (2016).
        The physiology and habitat of the last universal common ancestor.
         Nat. Microbiol., 1:16116.
        
        [Wellman and Strother2015]WS15Wellman, C. H. and Strother, P. K. (2015).
        The terrestrial biota prior to the origin of land plants
          (embryophytes): a review of the evidence.
         Palaeontology, 58(4):601–627.
        
        [Wells et al.2003]WAG03Wells, L. E., Armstrong, J. C., and Gonzalez, G. (2003).
        Reseeding of early earth by impacts of returning ejecta during the
          late heavy bombardment.
         Icarus, 162(1):38–46.
        
        [West et al.2015]WFG15West, S. A., Fisher, R. M., Gardner, A., and Toby Kiers, E. (2015).
        Major evolutionary transitions in individuality.
         Proc. Natl. Acad. Sci. USA, 112(33):10112–10119.
        
        [Whitehead and Rendell2015]WR15Whitehead, H. and Rendell, L. (2015).
        The Cultural Lives of Whales and Dolphins.
         The Univ. of Chicago Press.
        
        [Whiten and Erdal2012]WE12Whiten, A. and Erdal, D. (2012).
        The human socio-cognitive niche and its evolutionary origins.
         Philos. Trans. Royal Soc. B, 367(1599):2119–2129.
        
        [Whiten and van Schaik2007]WVS07Whiten, A. and van Schaik, C. P. (2007).
        The evolution of animal ‘cultures’ and social intelligence.
         Phil. Trans. R. Soc. B, 362(1480):603–620.
        
        [Wickramasinghe2010]Wick10Wickramasinghe, C. (2010).
        The astrobiological case for our cosmic ancestry.
         Int. J. Astrobiol., 9(2):119–129.
        
        [Wolf and Toon2015]WT15Wolf, E. T. and Toon, O. B. (2015).
        The evolution of habitable climates under the brightening Sun.
         J. Geophys. Res. D, 120(12):5775–5794.
        
        [Worden et al.2017]WD17Worden, S. P., Drew, J., Siemion, A., Werthimer, D., DeBoer, D.,
          Croft, S., MacMahon, D., Lebofsky, M., Isaacson, H., Hickish, J.,
          Price, D., Gajjar, V., and Wright, J. T. (2017).
        Breakthrough Listen - A new search for life in the universe.
         Acta Astronautica, 139:98–101.
        
        [Wray et al.1996]WLSWray, G. A., Levinton, J. S., and Shapiro, L. H. (1996).
        Molecular Evidence for Deep Precambrian Divergences Among Metazoan
          Phyla.
         Science, 274(5287):568–573.
        
        [Wright et al.2016]WCZ16Wright, J. T., Cartier, K. M. S., Zhao, M., Jontof-Hutter, D., and
          Ford, E. B. (2016).
        The Search for Extraterrestrial Civilizations with Large Energy
          Supplies. IV. The Signatures and Information Content of Transiting
          Megastructures.
         Astrophys. J., 816(1):17.
        
        [Wright et al.2014]WMS14Wright, J. T., Mullan, B., Sigurdsson, S., and Povich, M. S. (2014).
        The Ĝ Infrared Search for Extraterrestrial Civilizations with
          Large Energy Supplies. I. Background and Justification.
         Astrophys. J., 792:26.
        
        [Xu et al.2018]XRR18Xu, J., Ritson, D. J., Ranjan, S., Todd, Z. R., Sasselov, D. D., and
          Sutherland, J. D. (2018).
        Photochemical reductive homologation of hydrogen cyanide using
          sulfite and ferrocyanide.
         Chem. Commun., 54(44):5566–5569.
        
        [Xu et al.2017]XT17Xu, J., Tsanakopoulou, M., Magnani, C. J., Szabla, R., Šponer,
          J. E., Šponer, J., Góra, R. W., and Sutherland, J. D. (2017).
        A prebiotically plausible synthesis of pyrimidine
          β-ribonucleosides and their phosphate derivatives involving
          photoanomerization.
         Nat. Chem., 9:303–309.
        
        [Yoon et al.2004]YHCYoon, H. S., Hackett, J. D., Ciniglia, C., Pinto, G., and
          Bhattacharya, D. (2004).
        A Molecular Timeline for the Origin of Photosynthetic Eukaryotes.
         Mol. Biol. Evol., 21(5):809–818.
        
        [Yutin et al.2009]YWWKYutin, N., Wolf, M. Y., Wolf, Y. I., and Koonin, E. V. (2009).
        The origins of phagocytosis and eukaryogenesis.
         Biol. Direct., 4:9.
        
        [Zahnle et al.2007]ZA07Zahnle, K., Arndt, N., Cockell, C., Halliday, A., Nisbet, E.,
          Selsis, F., and Sleep, N. H. (2007).
        Emergence of a Habitable Planet.
         Space Sci. Rev., 129(1-3):35–78.
        
        [Zaremba-Niedzwiedzka et al.2017]ZNC17Zaremba-Niedzwiedzka, K., Caceres, E. F., Saw, J. H.,
          Bäckström, D., Juzokaite, L., Vancaester, E., Seitz, K. W.,
          Anantharaman, K., Starnawski, P., Kjeldsen, K. U., Stott, M. B.,
          Nunoura, T., Banfield, J. F., Schramm, A., Baker, B. J., Spang, A.,
          and Ettema, T. J. G. (2017).
        Asgard archaea illuminate the origin of eukaryotic cellular
          complexity.
         Nature, 541(7637):353–358.
