SCIENZA E RICERCA
The unsolved questions of evolutionary theory. How do mass extinctions work?
Amphibians are among the life forms most endangered by the current biodiversity crisis. Photo: Sonika Agarwal/Unsplash
Hundreds of millions of years ago, the Earth was inhabited by strange creatures. You could find radial organisms similar to the modern jellyfish, bodies showing a bilateral or rotational symmetry, weird leaf-shaped animals, perhaps permanent inhabitants of the deep sea. In the remote past, the planet was populated by a fauna that was completely different from the one we know today. It was the Ediacara fauna, whose fossil remains date back to 635-541 million years ago, well before the famous “Cambrian radiation”, which seems to be the ancestor of today’s diversity of life forms.
From the beginning of the Cambrian Period onwards, there is no trace of the Ediacara fauna in the fossil record. What might have happened to that astonishing variety of life forms remains a mystery still today. Based on what we can understand from the fossil record, it has been hypothesised that at some point the Ediacara fauna almost completely disappeared and was replaced by a new, totally different ensemble of organisms: the Cambrian fauna. Most likely, it was an extinction event – one of many that have shaped the history of life on Earth.
Since the first life forms made their appearance on the “blue planet”, life has continuously thrived. Yet, it has never been static. It has changed, evolved, and explored new paths and possibilities. But it has also undergone abrupt interruptions. Over the course of evolutionary history, five great extinction events – the so-called “Big Five” – have brought life of the brink of complete extinction and have profoundly altered its evolutionary path, marking the turnover of geological periods.
Then, what do we mean by mass extinctions? Some 99% of all the species that have ever existed on Earth have already gone extinct. Indeed, extinction is an essential mechanism of life – the dark side of evolution.
However, mass extinctions are different from background extinctions (that is, the number of life forms that goes extinct at the steady pace of 1-10 every million species per year). The distinctive features of mass extinctions are not only quantifiable for their extent, intensity, and rapidity; instead, there are qualitative differences, that make a mass extinction a pivotal moment, marking a before and after in the history of life and profoundly modifying the evolutionary paths of almost all life forms.
Watch the interview with Michael Benton and Massimo Bernardi. Filming: Sofia Belardinelli. Editing: Elisa Speronello
Mass extinctions: An incredible source of diversity
«Between a background extinction and a mass extinction there are all kinds of intermediate level extinctions», states Michael Benton, palaeontologist and Professor of Vertebrate Palaeontology at the University of Bristol. «During the course of its evolutionary history, Homo sapiens, our species, has caused the extinction of several species. For example, after the human arrival in the American continent the vast majority of the American great mammals went extinct; the colonization of remote islands triggered the extinction of various endemic species as well.
Many questions surrounding the phenomenon of mass extinctions are still unsolved. We have to date recognized five big mass extinction in the past, but there might be more. However, the key point is: who cares of how mass extinctions work? What is the lesson for us? Indeed, advancing our knowledge about the remote past of the history of life is proving very useful to understand and to address the current crisis – the Sixth mass extinction».
These events have indeed played a key role in shaping life as it is today. «Even Charles Darwin had already understood that extinctions had something very important to do with the evolution of life», points out Massimo Bernardi, palaeobiologist and Curator of Palaeontology at MUSE, the Science Museum of Trento. «A mass extinction is a major perturbation of ecological and evolutionary equilibria that does not have only deleterious consequences, clearing ecosystemic spaces and cutting biodiversity, as we would expect. Through extinction events, chance breaks in the regularities of the biological laws; and this element of contingency plays a central role in the macroevolutionary dynamics, as it opens novel paths of evolutionary exploration and boosts the emergence of new life forms.
Take the major anoxic events, global warming, ocean acidification occurred in the past. For all the living forms that had become well adapted to their ecological niche throughout million of years of biological evolution, these events were a catastrophe, leading to extinction; for others, on the contrary, these events represented a new opportunity, leading to new equilibria and new possibilities».
A short interruption in the continuity of evolution
Michael Benton has been studying extinction events for a long time, and he is convinced that conventional evolutionary laws are suspended during a mass extinction. «David Raup, one of the first palaeontologists to discuss this question, compared mass extinctions to a minefield. Imagine a trench in the World War I, where 100 soldiers are preparing to fight; of those hundred soldiers, half are strong, well fed, and well trained, while the other half are weak and unable to use a gun. But when the whistle blows they all climb out of the trench, run towards the enemy and the minefield just kills them all.
If, in this example, natural selection had been at work, the strongest and best equipped would have survived, while the weakest would have died. But this does not happen during a mass extinction event, because the drivers of the crisis are not predictable in terms of normal evolutionary challenges. Mass extinctions are triggered by uncommon and unpredictable events, to which no life form is prepared.
Many have tried to understand the reasons why some species rather than others survive to catastrophes such as volcanic eruptions or the impact of a meteorite. Why, at the end of the Cretaceous period, did small mammalian species prevail over the dinosaurs? Although we might identify traits that gave certain species a selective advantage, I think Raup was right that most species cannot adapt to new disrupting conditions. So, we have to admit that in most cases survival is almost exclusively a matter of luck».
Foto: Tanya Grypachevskaya/Unsplash
Homo sapiens, a geological force
Studying mass extinctions is not just a palaeontologist’s hobby. Figuring out how life recovers from a catastrophe, studying its resilience, is also essential to understand the events occurring in the present. «As in many other cases in science, new data and discoveries are radically changing our previous knowledge about the speed of ecosystem recovery after a catastrophic event», clarifies Bernardi. After each of the five major extinctions, life probably needed several million years to restore its previous level of ecological complexity; however, new technologies that allow more accurate dating of fossil records are providing increasing evidence that some groups and species may have recovered faster than others, and that not all the planetary ecosystems were affected in the same way».
«Despite the growing attention that the late Cretaceous extinction (which was triggered by a meteorite impact, an exogenous factor) has received in recent years, we now know that in most cases the main drivers of extinction events are endogenous phenomena, such as large volcanic eruptions», Benton adds. Such vast eruptions, which affect entire regions or even have planetary consequences, initiate complex processes that can threaten the very existence of the majority of life forms. «For example, think of the global warming caused by anomalous gas emissions, and ocean acidification. These two phenomena are also happening today before our eyes; but the difference is that this time, those processes have been triggered by the activities of only one species: us».
«This sudden alteration of ecological conditions are endangering the survival of countless species. We are at the beginning of the Sixth mass extinction, the first to be caused not by massive geological events, but by a young, invasive, and highly adaptable African species.
«Palaeontology is a historical science», Benton continues. «It cannot predict the future but, by understanding the mechanisms that occurred in the past, it can help us understand the present and prepare for what might happen in the future. Today, with technology, we are able to develop models that can make accurate projections based on the data from the past. Therefore, predicting the future is today a little less uncertain».
Looking at the past, we can draw at least one conclusion, that life will overcome the present crisis in one way or another. «Our biggest uncertainty concerns in what form life will survive», Bernardi admits. «As Georges Cuvier argued, great extinctions are true revolutions, because they completely alter the status quo, and nothing is the same after these shocks.
For example, given our resilience as a species, it is unlikely that we humans will become extinct; the unanswered question to date is what part of the current diversity of life will survive along with us, and what the world will look like in a few years. We may not be witnessing the “end of the world”, but this is certainly a momentous transition. For this reason, we need to focus on how to deal with this contingency».
SCIENZA E RICERCA Maswali yasiyo kutatuliwa ya nadharia ya mageuzi. Uharibifu wa Wingi Unafanyaje Kazi? Wanyama wanaoishi katika bahari na baharini ni miongoni mwa viumbe wanaoathiriwa zaidi na mgogoro wa sasa wa viumbe hai. Picha: Sonika Agarwal, Unspell: Mamia ya mamilioni ya miaka iliyopita, Dunia ilikuwa na viumbe wa ajabu. "Hii inaweza kuwa na "" radial "" viumbe, kama jellyfish ya kisasa, miili kuonyesha symmetry pande mbili au mzunguko, wanyama ajabu jani-umbo, labda wakazi wa kudumu wa bahari ya kina." Katika nyakati za kale, dunia ilikuwa na wanyama ambao walikuwa tofauti kabisa na wale tunaowajua leo. "Kama vile wanyama wa Ediacaran, mabaki ya visukuku ya wanyama hao yalitokea miaka milioni 635-541 iliyopita, muda mrefu kabla ya ""Cambrian radiation"" maarufu, ambayo inaonekana kuwa baba wa aina mbalimbali za maisha ya leo." Kuanzia mwanzo wa Kipindi cha Cambrian na kuendelea, hakuna alama ya wanyama wa Ediacara katika rekodi ya visukuku. Ni nini ambacho kingeweza kutokea kwa aina hiyo ya ajabu ya viumbe hai? Kwa kuzingatia kile tunachoweza kuelewa kutoka kwa rekodi ya visukuku, imedokezwa kwamba wakati fulani wanyama wa Ediacaran walipotea kabisa na kubadilishwa na kikundi kipya, tofauti kabisa cha viumbe: wanyama wa Cambrian. Yaelekea zaidi, ilikuwa tukio la kutoweka - mojawapo ya matukio mengi ambayo yamefanyiza historia ya uhai duniani. Tangu aina za kwanza za uhai zilipoonekana kwenye " sayari ya bluu", uhai umekuwa ukitawala kwa kuendelea. Hata hivyo, haijawahi kuwa imara. Imebadilika, imebadilika, na kuchunguza njia mpya na uwezekano. Lakini pia imevunjwa ghafula. Katika historia ya mageuzi, matukio matano makubwa ya kutoweka - yanayoitwa "Big Five" - yameleta uhai kwenye ukingo wa kutoweka kabisa na yamebadilisha sana njia yake ya mageuzi, ikiashiria mabadiliko ya vipindi vya kijiolojia. Basi, tunamaanisha nini kwa kuangamia kwa wingi? 99% ya spishi zote zilizowahi kuishi duniani zimetoweka. Kwa kweli, kutoweka ni utaratibu muhimu wa uhai - upande wa giza wa mageuzi. Hata hivyo, kutoweka kwa wingi ni tofauti na kutoweka kwa msingi (yaani, idadi ya aina za maisha ambazo zinatoweka kwa kasi ya 1-10 kwa kila spishi milioni kwa mwaka). Sifa za kutoweka kwa wingi hazina uwezo wa kupimwa kwa kiwango, nguvu, na kasi tu; badala yake, kuna tofauti za ubora ambazo hufanya kutoweka kwa wingi kuwa wakati muhimu, kuashiria kabla na baada katika historia ya maisha na kubadilisha sana njia za mageuzi ya karibu aina zote za maisha. Tazama mahojiano na Michael Benton na Massimo Bernardi. Uandishi wa filamu: Sofia Belardinelli. "Miongoni mwa ""kuangamia kwa masafa"" kuna aina zote za uangamiaji wa ngazi ya kati, anasema Michael Benton, paleontologist na Profesa wa Paleontolojia ya Vertebrate katika Chuo Kikuu cha Bristol." <unk> Katika historia yake ya mageuzi, Homo sapiens, spishi yetu, imesababisha kutoweka kwa spishi kadhaa. Kwa mfano, baada ya kuwasili kwa binadamu katika bara la Amerika, idadi kubwa ya mamalia wakubwa wa Amerika waliangamia; ukoloni wa visiwa vya mbali ulisababisha kutoweka kwa spishi mbalimbali za asili pia. Maswali mengi yanayohusu matukio ya kutoweka kwa jamii nyingi bado hayajasuluhishwa. Kwa sasa, kuna matukio matano makubwa ya kutoweka kwa jamii, lakini kuna mengi zaidi. Hata hivyo, jambo kuu ni hili: ni nani anayejali jinsi maangamizi makubwa ya viumbe yanavyofanya kazi? Tunajifunza nini kutokana na jambo hilo? Kwa kweli, kuendeleza maarifa yetu kuhusu historia ya maisha ya zamani ni muhimu sana kuelewa na kushughulikia mgogoro wa sasa - kutoweka kwa wingi wa sita. Kwa kweli, matukio hayo yamekuwa na matokeo muhimu katika kuunda maisha kama yalivyo leo. "Hata Charles Darwin alikuwa tayari ameelewa kwamba kutoweka kulikuwa na kitu muhimu sana kufanya na mageuzi ya maisha, ""anasema Massimo Bernardi, paleobiologist na Mkurugenzi wa Paleontology katika MUSE, Makumbusho ya Sayansi ya Trento." "Uharibifu mkubwa ni usumbufu mkubwa wa usawa wa ikolojia na mageuzi ambayo haina tu matokeo mabaya, kuondoa nafasi za ikolojia na kupunguza utofauti wa kibiolojia, kama tunavyotarajia. """ Kwa njia ya matukio ya kutoweka, bahati huvunja sheria za kibiolojia; na kipengele hiki cha uwezekano hucheza jukumu kuu katika nguvu za macroevolutionary, kwani hufungua njia mpya za uvumbuzi wa mageuzi na kuimarisha kuibuka kwa aina mpya za maisha. Kwa mfano, kuna matukio makubwa ya anoxic, joto la dunia, na acidification ya bahari. Kwa aina zote za viumbe vilivyojipatanisha vizuri na mazingira yao kwa mamilioni ya miaka ya mageuzi ya kibiolojia, matukio haya yalikuwa janga, yakisababisha kutoweka; kwa wengine, kinyume chake, matukio haya yaliwakilisha fursa mpya, ikisababisha usawa mpya na uwezekano mpya. Michael Benton amekuwa akisoma matukio ya kutoweka kwa muda mrefu, na anaamini kwamba sheria za kawaida za mageuzi zimesimamishwa wakati wa kutoweka kwa wingi. David Raup, mmoja wa wataalamu wa kale wa mimea wa kwanza kuzungumzia swali hili, alilinganisha maangamizi makubwa ya viumbe na uwanja wa mabomu ya ardhini. Hebu wazia katika vita vya kwanza vya dunia, askari 100 wanajitayarisha kupigana, nusu yao ni wenye nguvu, wamelishwa vizuri na wamefunzwa vizuri, na wengine ni dhaifu na hawawezi kutumia bunduki. Lakini wakati filimbi inapopiga wote wanaruka nje ya shimo, kukimbia kuelekea adui na uwanja wa mabomu ya ardhini tu kuwaua wote. Kama katika mfano huu, uteuzi wa asili ungekuwa unafanya kazi, wenye nguvu zaidi na wenye vifaa bora wangaliishi, wakati walio dhaifu zaidi wangekufa. Lakini hii haifanyi wakati wa tukio la kutoweka kwa wingi, kwa sababu madereva ya mgogoro hayawezi kutabiriwa katika suala la changamoto za kawaida za mageuzi. Maangamizi makubwa husababishwa na matukio yasiyo ya kawaida na yasiyotabirika, ambayo hakuna aina ya uhai iliyotayarishwa kukabiliana nayo. Watu wengi wamejaribu kuelewa sababu zinazofanya spishi fulani ziokoke katika misiba kama vile milipuko ya volkano au mlipuko wa meteoriti. Kwa nini, mwishoni mwa kipindi cha Cretaceous, spishi ndogo za mamalia zilishinda dinosaurs? Ingawa tunaweza kutambua sifa ambazo zilitoa aina fulani faida ya kuchagua, nadhani Raup alikuwa sahihi kwamba spishi nyingi haziwezi kukabiliana na hali mpya za kuvuruga. Kwa hiyo, ni lazima tukiri kwamba katika visa vingi kuokoka ni karibu tu suala la bahati. Picha: Tanya Grypachevskaya/Unsplash Homo sapiens, nguvu ya kijiolojia Kujifunza kutoweka kwa wingi si burudani tu ya mtaalamu wa vitu vya kale. Kujua jinsi maisha yanavyojiokoa baada ya msiba, kujifunza uwezo wake wa kukabiliana, pia ni muhimu kuelewa matukio yanayotokea sasa. "Kama ilivyo katika visa vingine vingi katika sayansi, data mpya na uvumbuzi ni kubadilisha kabisa maarifa yetu ya awali kuhusu kasi ya kufufua mazingira baada ya tukio la maafa", anaeleza Bernardi. Baada ya kila moja ya maangamizi makubwa matano, maisha yaweza kuhitaji mamilioni ya miaka ili kurudisha kiwango chake cha awali cha utata wa ikolojia; hata hivyo, teknolojia mpya zinazoruhusu tarehe sahihi zaidi ya rekodi za visukuku zinatoa ushahidi unaozidi kuwa baadhi ya vikundi na spishi zinaweza kuwa zimepona haraka kuliko zingine, na kwamba sio mifumo yote ya ikolojia ya sayari iliathiriwa kwa njia ileile. "Licha ya kuongezeka kwa umakini ambao kutoweka kwa Cretaceous ya marehemu (ambayo ilisababishwa na athari ya meteorite, sababu ya nje) imepokea katika miaka ya hivi karibuni, sasa tunajua kwamba katika kesi nyingi waendeshaji wakuu wa matukio ya kutoweka ni matukio ya ndani, kama vile milipuko mikubwa ya volkano. """ Mlipuko huo mkubwa, ambao huathiri maeneo yote au hata kuwa na matokeo ya sayari, huanzisha michakato tata ambayo inaweza kutishia kuwepo kwa aina nyingi za uhai. <unk>Kwa mfano, fikiria ongezeko la joto duniani linalosababishwa na uzalishaji usio wa kawaida wa gesi, na kuongezeka kwa asidi ya bahari. Matukio haya mawili pia yanatokea leo mbele ya macho yetu; lakini tofauti ni kwamba wakati huu, michakato hiyo imesababishwa na shughuli za spishi moja tu: sisi. Mabadiliko hayo ya ghafula ya hali za kimazingira yanahatarisha uhai wa spishi nyingi. """Tuko kwenye mwanzo wa kutoweka kwa wingi kwa mara ya sita, kwa mara ya kwanza kusababishwa sio na matukio makubwa ya kijiolojia, bali na spishi ya Afrika ya vijana, ya uvamizi na yenye kubadilika sana." "Paleontolojia ni sayansi ya kihistoria", anaendelea Benton. <unk>Haiwezi kutabiri wakati ujao lakini, kwa kuelewa taratibu zilizotokea katika siku za nyuma, inaweza kutusaidia kuelewa sasa na kujitayarisha kwa kile kinachoweza kutokea katika siku zijazo. Leo, kwa kutumia teknolojia, tunaweza kutengeneza mifano inayoweza kutoa makadirio sahihi kwa kutegemea habari za wakati uliopita. Kwa hiyo, kutabiri wakati ujao leo ni kidogo chini ya uhakika <unk>. Kwa kuangalia wakati uliopita, tunaweza kupata angalau hitimisho moja, kwamba maisha yatashinda mgogoro wa sasa kwa njia moja au nyingine. "Uhakika wetu mkubwa unahusu jinsi uhai utakavyoendelea kuishi", anakiri Bernardi. "Kama Georges Cuvier alivyosema, ""kuangamia kwa jamii kubwa ni mapinduzi ya kweli, kwa sababu hubadili kabisa hali iliyopo, na hakuna kitu kinachobadilika baada ya mshtuko huo.""" Kwa mfano, kwa kuzingatia uwezo wetu wa kukabiliana na mabadiliko kama spishi, ni vigumu kwamba sisi wanadamu tutakufa; swali ambalo halijibiwa hadi sasa ni ni ni sehemu gani ya utofauti wa sasa wa maisha itaendelea kuishi pamoja nasi, na ulimwengu utaonekanaje katika miaka michache ijayo. Huenda tusiwe tukishuhudia "mwisho wa ulimwengu", lakini bila shaka huu ni badiliko kubwa. Kwa sababu hii, tunahitaji kuzingatia jinsi ya kukabiliana na hali hii ya ghafla.