Shortly after Earth’s formation, the planet was barren of life. Then, proteins combined in just the right way, and life appeared. For billions of years, it was simple and uninteresting, oceans full of simple, single-celled organisms floating for millennia after millennia. Suddenly, life got a lot more interesting. Organisms became more complex, with more than one cell. And they got much bigger — 10,000 times bigger by volume, Nick Lane, professor of evolutionary chemistry at University College London, wrote in his 2015 book The Vital Question.

The importance of this step — of this sudden increase in size and complexity — cannot be overstated. Without it, complex life (like humans, for example) would not exist.

How exactly this step happened is one of the big questions in evolutionary biology. There are a number of theories about how, exactly, life got so much more complicated. One of the prevailing theories, from Lane himself, focuses on energy. Here’s the thinking: Cells need more energy to build more complex structures. To do that, according to Lane’s theory, single-celled organisms merged with bacteria we now know as mitochondria, which have an electrical charge and bring power to the cell. It’s possible, though unlikely, for the two bacteria to fit together, and even less probably that the two were able to survive and live symbiotically. This occurrence that made possible all other forms of complex life is rare, to be sure.

But in Lane’s opinion, it only happened once.

“It comes down to one merger between two cells that made one cell, then everything comes from that. You, me, the redwood tree or the hummingbird, a fungus, a piece of algae growing in a pond, every form of life we can see with our naked eyes and many that we can’t come from that single cell,” Nick said in an episode of the science podcast Radiolab on his work.

The debate around how life got much more complicated is important to understand the history of life on Earth, but it also could inform our search for life on other planets. If complex life is exceedingly rare, does that make it less likely for us to find intelligent life in the universe? Should we instead be looking for something much smaller and simpler? If the universe outside of Earth only populated by single-celled organisms? Because these specific conditions are so unlikely, might complex life only exist on our own planet?

Futurism got in touch with a few experts to ask if they agree with Lane’s theory, if they believe intelligent extraterrestrial life is possible, and to get their perspectives on whether we should somehow alter our search.

Mohamed Noor, a professor of biology at Duke University:

To the best of my knowledge, Lane’s version of what happened is likely true: Acquiring mitochondria happened [only] once, long ago in the ancestor of plants, fungi, and animals. It greatly facilitated the evolution of multicellular organisms. However, it’s impossible to know how to assess the need for something like that in the context of life that is totally unrelated to life on Earth. All life on Earth has a single common ancestor. This ancestor (and all life we on Earth) was presumably carbon/water-based, replicated using nucleic acids, and lived in conditions that existed on ancient or modern Earth. If life arose on a much colder world, for example, many other [environmental] parameters may be totally different as well. In such a case, life there may use liquid ammonia rather than water as a solvent. It may not use nucleic acids for heredity. But some aspects may be general to life — carbon may intrinsically make sense for life, given its abundance and ability to make long chains. A “cell membrane” of some sort to insulate that life from its environment also seems probable. Finally, there are other things we never think about. How “fast” does this life metabolize and interact? Are there generations occurring in the blink of an eye? Or single interactions over millennia, moving so slowly we wouldn’t even notice? Honestly, I feel like we cannot have any estimate of the probability of life of any kind (intelligent or not) until we move from a sample size of 1 (related to life on Earth) to a sample size of at least 2 [that is, until we discover at least one more example of life in the universe]. Back to your question, any “life” needs a source of power, but mitochondria need not be the only solution to that problem, especially if life is starting with a completely different basis. Still, my best guess is that microbial-sized life is way more likely to exist than something as large as us on other worlds. If our desire is to “find life,” I speculate that we’re much better off closely examining acquired samples from Europa or other worlds than waiting to receive a radio signal.

Pierre Pontarotti, the director of research at the Mathematics Institute of Marseille, wrote:

The symbiosis between bacteria and eukaryotes has occurred many times during the evolution of life on Earth. For example, cyanobacteria [merged] with the ancestors of plant cells — the cyanobacteria became the chloroplast. Therefore, if organisms like bacteria and eukaryotes are present on another planet, the symbiosis should happen. We, of course, have no information about the kind of life that exists on other planets. But because galaxies and planets have evolved many times during the history of the universe, why shouldn’t life do so as well?

John Rummel, senior scientist with the SETI Institute: