The universe is teeming with earth-like planets.
In February, three were discovered circling a star, Trappist-1, 40 light years away. Last November, one even closer yet was found — four light years away — orbiting our next-door neighbor, Proxima Centauri. By one estimate, there are as many as 40 billion planets similar to Earth just in our Milky Way galaxy.
With so many potentially habitable exoplanets (as they are called), many believe that it is inevitable that life, even intelligent life, must have arisen on some of them.
But what would those life forms be like?
If we're to believe Hollywood, quite a lot like life here on Earth. From Star Trek to Star Wars to Valerian and beyond, most interplanetary science-fiction movies populate their worlds with lifeforms quite similar — in general appearance and biology — to what has evolved here on Planet Earth. Guardians of the Galaxy takes this approach to a new extreme, including Groot, a humanoid evolved from a botanical ancestor (perhaps explaining its limited vocabulary).
Yet, not all films agree. Last year's Arrival introduced hexapods, organisms with seemingly little affinity to any species here on the home planet.
So, what should we expect? An Avatar-like ecosystem full of species slightly different from those on Earth, or a world composed of unfamiliar organisms?
The great astronomer Carl Sagan was in Arrival's camp, proclaiming "extraterrestrials would be very different from us." Paleontologist and evolutionary biologist Stephen Jay Gould expressed similar sentiments, but felt that there was no way to scientifically study the question other than finding life on another planet.
Yet not everyone agrees and, in recent years, the question of how predictable evolution is has become a topic of great scientific inquiry. Convergent evolution is the phenomenon of species independently evolving to be similar. Usually it results from the species adapting to similar situations, natural selection favoring the same solution to the same problem posed by the environment. Convergent evolution was known to Charles Darwin but, until relatively recently, we thought it was uncommon, a great example of the power of natural selection, but not commonplace. We now know, however, that convergence is far from rare; rather, it is pervasive, occurring all around us. Think, for example, of fast-swimming marine predators: dolphins, sharks, tuna and ichthyosaurs (extinct marine reptiles from the Age of the Dinosaurs) all evolved a very stream-lined body shape and powerful tails for rapid and efficient locomotion. Or consider Euphorbia plants from dry parts of Africa. Tough-skinned, often green, with spines instead of leaves, they look like cacti, but they're not — the Old- and New-World doppelgängers have independently evolved the same traits to cope with water loss and herbivores in arid regions.
The pervasiveness of convergence has led some evolutionary biologists to proclaim evolution deterministic, the outcome downright inevitable (see two books by Simon Conway Morris, Life's Solutions and The Runes of Evolution, and also Convergent Evolution by George McGhee). If the environment repeatedly poses the same challenges, and if natural selection repeatedly produces the optimal solutions, then evolution is repeatable. And, as a corollary, we can predict what life would be like on an Earth-like planet — pretty much the same as here. The argument can be taken one step further — the Homo sapiens species is supremely adapted to life on Earth, the adaptations we forged as we emerged on the savannahs of Africa proving a brilliant stepping-stone to global dominance. Consequently, if evolution is so deterministic, the expectation for life on planets like our own is clear: Humanoid life forms should evolve and dominate, just like here. Hollywood has it right.
Unfortunately, there's a problem with this argument. Although the list of examples of convergence is impressive, it wouldn't be hard to make an equally impressive list of non-convergence. Off the top of my head, here are some evolutionary singletons, types of animals that have evolved just once, without a close match: sauropod dinosaurs, like Brontosaurus (Dinosaur purists may note that the name Brontosaurus was long ago discarded, replaced for quirky scientific reasons with Apatosaurus. To those killjoy know-it-alls I respond, "Haha! Thanks to new scientific discoveries, the name Brontosaurus was resurrected in 2015."); elephants; the kiwi; sloths; and the world's greatest animal, the duck-billed platypus. Each of these types of animals has evolved a single time, with no close evolutionary match, now or ever (True: Sauropods and elephants are similar in being huge, lumbering herbivores — but I'm focused on much more similar evolutionary matches).
If evolution is so deterministic, its outcome so predictable, it's hard to understand why there are no matches for these evolutionary singletons. Streams like the ones platypuses inhabit are found on every continent except Antarctica, yet the duckbill hails only from Down Under. Suitable tropical tree branches occur around the world, but sloths only evolved in South America. Why sauropods in the Mesozoic and not today?
The reason is simple: There are actually multiple different ways to solve a problem posed by the environment. Consider the woodpecker and the aye-aye, two completely different animals that live a similar lifestyle, tapping on wood to detect the tunnels of wood-eating grubs, chiseling into the wood to get to the tunnels, then extracting the grubs. But the species have evolved completely different tools to do so, the bird a tough beak, an extremely long tongue covered in prickles, and a skull reinforced against concussions to withstand the repeated jackhammering. The aye-aye, on the other hand, has a long skeletal finger that can twist in any direction and protruding incisors to do the excavating.
We don't need to find life on other planets to test the convergence hypothesis. All we have to do is go to New Zealand, an island on which life has diversified in the absence of terrestrial mammals. If the outcome of natural selection is deterministic, then a world dominated by birds would look pretty much like life elsewhere on the planet. But of course, it doesn't. The kiwi may live a lifestyle similar to a badger, but it doesn't look at all like one. The dominant herbivore is, or was, a 10-foot tall bird (the moa), quite different from deer or bison. Throw in flightless parrots, carnivorous parrots, bats that forage by walking around in the leaf-litter and many more, and we can throw the convergence hypothesis out the window. New Zealand is a distinct evolutionary world, the evolutionary outcome unique.
The question is no longer whether convergence or lack of convergence is common: We know now that both are. Rather, scientists are interested in understanding why convergence occurs in some cases, and not others. It's still early days, but one conclusion is clear: Closely-related species (or populations of the same species) tend to adapt in the same way, not surprisingly because they start out so similar in so many ways — natural selection is likely to modify them in similar ways. By contrast, distantly-related species, initially different in so many attributes, are much more likely to find different ways to adapt to the same situation. Think about the difference between birds and mammals: The former have beaks, the latter teeth and fingers. It's not surprising that woodpeckers and aye-ayes found different ways to solve the same problem.
Of course, life couldn't be more distantly related than if it occurred on another planet. With all the differences that such life forms must exhibit, natural selection (if it occurred — who's to say that evolutionary processes would be the same?) might very well sculpt well-adapted species, but they wouldn't look at all like us and our earthly compatriots. The aye-aye and the kiwi tell us that. And that means that the minority opinion in Hollywood is almost surely correct.
If you want to think about what extraterrestrial life might be like, watch Arrival, not Avatar or Guardians of the Galaxy.
Jonathan Losos is a professor of organismic and evolutionary biology and curator of herpetology at the Museum of Comparative Zoology at Harvard University. His research concerns the origin and maintenance of biological diversity. His latest book, Improbable Destinies: Fate, Chance and the Future of Evolution was published in Aug. 2017. You can follow him on Twitter: @jlosos.