Famous paleontologist Stephen Jay Gould (1941–2002) was sure that, if the deck were reshuffled, humans would never evolve — even on this planet — again. As Paul Parsons puts it at BBC’s Science Focus Magazine,
His reasoning was that evolution is driven by random sets of genetic mutations, modulated by random environmental effects, such as mass extinctions, and that it would be extremely rare for the exact same set of effects to crop up twice.Paul Parsons, “Could humans be the dominant species in the Universe, and we just don’t know it yet?” at Science Focus (November 19, 2021)
But as very large telescopes, capable of peering into exoplanets, are under development, current analysts are rethinking that approach. There are good reasons for thinking that extraterrestrial life forms would share basic characteristics with terrestrial ones (convergent evolution). Cambridge paleobiologist Simon Conway Morris, for example, told Science Focus that
“One can say with reasonable confidence that the likelihood of something analogous to a human evolving is really pretty high. And given the number of potential planets that we now have good reason to think exist, even if the dice only come up the right way every 1 in 100 throws, that still leads to a very large number of intelligences scattered around, that are likely to be similar to us.”Paul Parsons, “Could humans be the dominant species in the Universe, and we just don’t know it yet?” at Science Focus (November 19, 2021)
[K]angaroos are marsupial mammals, not placentals. Yet their genes are close to humans. Researchers: “We thought they’d be completely scrambled, but they’re not.”
Kangaroos? Shark and human proteins, meanwhile, are also “stunningly similar.” Indeed, sharks are genetically closer to humans than they are to aquarium zebrafish. Researchers: “We were very surprised…”
Birds are said to have evolved ultraviolet vision at least eight times. Two different species of deadly sea snake, with “separate evolutions,” were found to be identical. Dolphins and insects, we are told, share components of a hearing system.
The smartest invertebrates, the molluscs (including squid, octopuses, and cuttlefish), seem to have evolved brains four times. From one study we learn, “The new findings expand a growing body of evidence that in very different groups of animals — and mammals, for instance — central nervous systems evolved not once, but several times, in parallel.Denyse O’Leary, “Evolution Appears to Converge on Goals — But in Darwinian Terms, Is That Possible?” at Evolution News and Science Today (July 27, 2015)
Given that many different life forms have tended to evolve similar solutions, based on the constraints of physics, Morris thinks that intelligent extraterrestrials would be fairly similar to humans in important ways. Arik Kershenbaum, author of Zoologist’s Guide to the Galaxy, (Harvard, 2021), agrees with him, noting the underlying similarities of the solutions to problems:
Yes, bat wings and bee wings are different, but only in detail, not in principle. Both consist of a membrane supported by rigid structures. Both generate lift by creating airflow over that membrane. In fact, the main difference between bee wings and bat wings is not in their structure, it’s in the way they use them. The small size of insects means that they cannot simply flap their wings like bats and expect to fly. They need to buzz, generating lift both on the forward stroke of their wings and on the backward stroke — something that neither birds nor bats do.Dan Falk, “Why Extraterrestrial Life May Not Seem Entirely Alien” at Quanta (March 18, 2021)
The common ways that life forms on Earth solve their problems — starting from very different points — gives Kerschenbaum confidence that, while extraterrestrial life forms will be different in some ways, “many things will be the same.”
Some argue that convergent evolution is a “law” of evolution:
In short, convergent evolution theory posits that evolution itself is a law of nature — and, as a logical endpoint, it’s likely that evolution would operate the same way on different planets as it does here on Earth. In other words, it’s theoretically possible that the blue and green alien humanoids you see on “Star Trek” could be, well, actually out there.Noor Al-Sibai, “Scientists Say There May Be “Humans” All Over the Universe” at Futurism (November 22, 2021)
But that’s a dicier claim. What we are really looking at here is a constraint on evolution, not a law. Physics offers only a limited number of solutions. In many cases, the life forms that find them are not driven by common ancestry but by a common goal.
Exactly how each of them does it is still unclear. What seems likely, however, is that life forms that fly on exoplanets will mostly face the same constraints as birds, bats, and insects do here.
You may also wish to read:
Zoologist: Law of evolution can predict what aliens will be like. Arik Kershenbaum’s new book argues that convergent evolution on Earth helps us understand what to expect from extraterrestrial life. Kershenbaum’s argument fails when he addresses human culture: It just isn’t true that co-operation among humans is governed wholly by genes.
If we find life on exoplanets, some of it might be “crabs”. Over millions of years, many crustaceans gradually grew to look more and more like crabs, a process called convergent evolution. In an environment similar to Earth’s, we might expect life forms to converge on similar solutions. “Crabbiness” might be one of them.