Form Equals Form Equals Function

Why is it that the most enduring, recognized form for aliens to take in our stories is “humanoid”? Walking upright, two legs, two arms, two eyes, etc. The most stereotypical term to refer to visiting aliens is “little green men“. Why do we so often assume that aliens will be so similar to us?

Part of it is undoubtedly because there is a comfort in familiarity; if they look kind of like us, they must be like us, right? (Conversely, we feel more confident when the bad, violent aliens look nothing like us.) A certain lack of imagination on the part of those who craft the stories also plays a role. And let’s not discount the practicality of putting a human actor in a roughly human-shaped costume.

But there’s actually a reasonable scientific justification for an alien life form to have a similar form to ours. It’s called convergent evolution.

Convergent evolution occurs when two different species evolve a similar physical characteristic independently of each other. For example, the honey possum of Australia has a pointed snout and a long, thin tongue that it uses to gather nectar, very like a hummingbird.

Typically convergent evolution occurs when the differing species each fill a similar ecological niche. The aye-aye of Madagascar and the striped possum of Australia each have a single long, skeletally thin finger which they use for prying grubs and other invertebrates from under the bark of trees. The particular food source was available because both Madagascar and Australia are free of woodpeckers. (The late Douglas Adams described convergent evolution as it relates to the aye-aye and striped possum in his magnificent nonfiction book Last Chance to See, with Mark Carwardine. Go out and buy a copy now, it’s an amazing read!)

Humans have evolved into our current form for various reasons, but each change we’ve undergone has helped us survive and thrive. And it’s not unreasonable to assume that another species on some distant Class M planet went through a similar evolutionary progression. Hence, aliens that look very similar to us.


One side note. I’m hardly the first person to talk about convergent evolution as it relates to humans and humanoid aliens. My fellow SiMF blogger Athena Andreadis covered the topic in some detail in her book To Seek Out New Life: The Biology of Star Trek. Author Joan Slonczewski has written about it too. And there are others.

Both Andreadis and Slonczewski mention the virtual impossibility of two alien species, no matter how physically similar, being able to interbreed. As superficially similar as the two species may appear, the genetic differences would almost certainly be too great to enable procreation.

This demonstrates a very common problem in science fiction writing, the ignoring of reality in favor something cool, or to make the story work. While to a certain extent this isn’t a problem, pretending facts don’t exist makes it difficult to suspend disbelief. Giant bugs couldn’t really exist, they’d be crushed under their own weight. There are plenty of ways to create fascinating stories and aliens while remaining grounded in fact.


Surprisingly enough, human/alien convergent evolution on its own doesn’t add a new depth to extrapolation in science fiction. Any physical trait we speculate on for ourselves, we can also consider occurring in an alien species; so long as we invent an environmental or ecological reason for the change to occur, we can assume the same reason could occur for the aliens. In other words, anything we can imagine for ourselves, we can imagine for aliens.

But where extrapolation can get very interesting is when we consider what evolutionary traits we might share with an alien species because of our interactions with them. Meeting aliens would absolutely be a dramatic shift in our understanding of the universe, and could lead to any number of interesting changes down the line:

  • If we visited the aliens’ homeworld, we would be forced to evolve and acclimatize: survival against viruses we’d never encountered, ability to sustain ourselves on native foods, etc.
  • We and the aliens could encounter a language/speech barrier, and overcome it by evolving psychic abilities which allow us to convey complex emotions to each other. (this is admittedly somewhat fantastical.)
  • In a slightly more science-based vein, we and the aliens could develop a new shared sense that we’d previously had no use for, that enables us to communicate.
  • If aliens visited us and shared their knowledge of space travel, we could send a large population of humans into space. Those humans could then evolve bodies more suited to space travel – something the aliens should already have done.

That last speculation touches on another interesting concept. If a large population of humans went out into space, and evolved to accommodate for space travel, they would end up drastically different from the humans who stayed on earth. That would be divergent evolution – and that will be the topic of my next post.

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