Science fiction, from Star Wars to Star Trek, often presents aliens as looking somewhat (or entirely) like us. Of course, this may be a budget issue, it's much more expensive to operate an alien suit with seven arms than to ask audiences to believe that a long time ago in a galaxy far away life evolved into Mark Hamil.
But there are some scientists out there who think that alien species could – or are likely to – evolve into something very close to humans. And though they might not be in the majority, they aren't kooks shouting from the sidelines, either. It all comes down to convergent evolution, which you may know as the answer to the question "why does everything keep evolving into crabs?"
Convergent evolution is when similar features evolve in species from different periods or regions that have a similar form or function, despite the last common ancestor of the animals or plants not having that particular feature. Think how echolocation has evolved in both whales and bats, and mechanisms for flight evolved in birds, insects, pterosaurs, and bats. (Get your own evolutions, bats, quit hogging up everyone else's).
Think also of how several different animals have evolved prickly protrusions, including echidnas (of the monotremes), porcupines (rodents), and hedgehogs (erinaceinae). Despite appearances, the last common ancestor of the three was likely in the time of the dinosaurs, they just ended up with similar characteristics.
Convergent evolution essentially happens when animals and plants have to adapt to similar environments or ecological niches and end up with similar solutions. Crab-like forms are thought to have happened independently at least five times in decapod crustaceans, including porcelain crabs, hairy stone crabs, and coconut crabs.
If this happens on Earth, it stands to reason that it could happen elsewhere in the universe. With similar environments, it's possible that alien animals will evolve similar adaptions that suit those environments. If there are enough planets out there similar enough to Earth, and given how well humans are adapted to life on Earth, it could be that out there human-like aliens are dominating their own planets, too.
“One can say with reasonable confidence that the likelihood of something analogous to a human evolving is really pretty high," Professor Simon Conway Morris of the University of Cambridge recently told Science Focus. "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.”
So it could be that there are many human-like creatures out there in the universe waiting for us, as well, of course, as a buttload of crabs. Not all are convinced.
"Although the list of examples of convergence is impressive, it wouldn't be hard to make an equally impressive list of non-convergence," Professor of organismic and evolutionary biology, Jonathan Losos wrote in 2017, citing everything from sauropod dinosaurs to the duck-billed platypus. Of particular focus for Losos is New Zealand, where there are no native land 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," he writes. "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."
"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 matter, of course, is not settled, and likely won't be until we actually are able to find alien life itself.