In science fiction movies and TV shows, intelligent aliens are usually the same basic shape as humans: two arms, two legs and a head. But why would creatures that evolved on a completely different planet look so similar to us? We asked some experts, and they told us the most likely explanations for humanoid aliens.

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Top image: Artwork by Wayne Douglas Barlowe

The truth is, aliens tend to look like us in science fiction for a couple of basic reasons: budget, and relatability.

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"Most aliens in SF are humanoid because humans produce SF," says Michael H. New, an Astrobiology Discipline Specialist at NASA. "While we are interested in the 'other,' our conception of otherness is often limited."

And a lot of experts firmly believe that aliens would not look at all like humans. For example, Stephen Jay Gould claims that life that evolved elsewhere would look totally different from us — and in fact, if you "reran the tape" from the beginning of life on Earth, you wouldn't end up with humans on this planet either. The emergence of humanoids on Earth is a totally random event that was a fluke, even with the exact conditions that we arose from.

But let's say that we do meet aliens, and they turn out to be bipeds with a roughly human-like shape... how do we explain that?

Panspermia

This is the most common explanation for creatures that look sort of like us turning up all over the universe. Either humanoid aliens spread their DNA across the galaxy to give rise to creatures in their image, or the DNA just spread through the galaxy on its own, on asteroids and stuff.

Star Trek: The Next Generation reaches for this explanation in the episode "The Chase," pictured above. And it's the centerpiece of the recent movie Prometheus, as well.

"I'm of the strong opinion that if humanoid aliens exist, they must have some genetic heritage in common with human beings," says Mark A. Bullock with the Southwest Research Institute. He'd find that easier to believe than the notion that humanoids could evolve independently elsewhere. Plus "it's been shown that panspermia is quite a viable mechanism, so the interchange of genetic material between worlds is not out of the question."

If the galaxy really did turn out to be full of humanoid aliens, "some kind of panspermia wouldn't be a bad explanation," New tells io9. "We're bilaterally symmetric and bipedal because our ancestors were." It's entirely possible that if certain events had played out differently, the dominant species on Earth would have had a very different shape.

The Burgess Shale, which is roughly 500 million years old, "displays a wide range of body plans, only some of which are still seen on the modern Earth," adds New. So he believes you'd need some outside intervention to account for humanoid aliens.

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Bullock sounds a similar note, saying that the Cambrian explosion, 600 million years ago, "saw a great deal of evolutionary experimentation with body plans," some of which could be a glimpse of life forms that we might see on other planets.

At the same time, panspermia is only really likely at the microbial level, cautions Joan L. Slonczewski, a biology professor at Kenyon College and science fiction author whose books include A Door Into Ocean and The Highest Frontier. Beyond microbes, panspermia doesn't really make much sense as an explanation for humans' own development.

"Humans on Earth are so obviously a part of Earth's evolutionary program," says Sclonczewski. "From the molecular and cellular level, to the shape of organisms, we humans evolved here."

Convergent Evolution

Or maybe humanoids just evolved on other planets, separately from us, because they just arrived at the same destination via other paths?

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There are certain things about humans that helped us rise over other primates, says James Kasting, a distinguished professor of Geoscience at Penn State University. Our opposable thumbs helped us grasp tree branches, and also hold tools. And walking upright was useful, as well. Finally, being warm-blooded helped us to power our big brains.

"I would think that there's a good chance that intelligent alien life evolved in more or less the same way and would thus bear some resemblance to humans," says Kasting. "Not necessarily a close resemblance, though."

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The upright-walking, bipedal, two-armed posture "seems to have evolved independently in various unlikely contexts, from meerkats to velociraptors," notes Slonczewski. "Maybe it just makes sense to have two feet to move, two hands to manipulate something, and a sensory 'head' with as wide a view as possible. Then again, that's what we have, so it makes sense to us."

We've seen enough examples of convergent evolution on Earth to believe that it could happen on other planets as well, notes Steven J. Dick, the 2013-2014 Baruch S. Blumberg NASA/Library of Congress Chair in Astrobiology at the Library of Congress. "For example, the eye has been reinvented many times independently, as have wings in insects, birds and bats. Fish and marine mammals such as dolphins have evolved streamlined shapes for their water environment."

Dick recommends the 1981 book Life in Darwin's Universe: Evolution and the Cosmos by Gene Bylinsky, which argues that "a limited number of engineering solutions" are possible when it comes to successful life forms.

But Dick adds that you can't discount environmental factors which would ensure that life on other planets would look at least somewhat different, including gravity. Dick tells io9:

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Because they would have been shaped by their own unique planetary environments, organisms would be different from us in the particulars, just as there is great diversity of life on Earth, including the different requirements of land and water organisms. More generally, gravity imposes size limitations on life; from the cell to the whale is a large range indeed, but the food system of the whale (and the dinosaur on land) must strain to feed such a large structure, even as the heart struggles to sustain its blood flow. Life on a low-gravity planet might be free to soar upward both in the plant and animal kingdom, while life on a high-gravity planet would be correspondingly stifled.

Bilateral Symmetry

Let's say that the notion of aliens separately evolving bodies that have more or less a human silhouette is kind of unlikely — it's still possible that bilateral symmetry could be a constant among intelligent life forms, say some experts. This refers to the fact that your left and right sides are more or less the same, with an eye, an ear, an arm and a leg on either side.

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"Bilateral symmetry appeared independently several different times in the evolution of larger organisms on Earth," says Bullock. "So bilateral symmetry may be a common feature of intelligent life, regardless of whether its specific body plan."

And once you get bilateral symmetry, you are going to start drifting in the direction of a vaguely humanoid body plan, argues Bjørn Østman with Michigan State University. The symmetry means you'll have an even number of limbs — which is most likely going to be four, rather than six or more, which don't convey enough of an advantage to justify the extra limbs.

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"Even on earth there are lots of animals that have more than two pairs of limbs," concedes Østman. "But I think that the reason why we have lots and lots of animals that hva four limbs is that that's highly advantageous. It just happens to be mechanically a very good solution to traversing a rugged landscape."

And once you have a lot of quadripeds on land, one of those quadripeds is going to start using its front limbs to manipulate tools. "If you can free two limbs to manipulate tools, then it becomes very advantageous to develop high intelligence," notes Østman.

So assuming an intelligent alien is symmetrical and has some of its limbs devoted to tool use, then it might end up being roughly bipedal, says Østman. And the sensory organs, like eyes, will have to be forward-looking and not too far away from the tool-using limbs. Which means you end up with something like a head, because the nervous system will be close to the sensory organs for maximum efficiency.

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Thus those two factors — symmetry and tool use — may lend themselves to something at least vaguely similar to a human shape, in Østman's view.

"If we were to eventually find other intelligent life in the universe, they would be humanoid, I think," Østman concludes. "I find that a high probability." But at the same time, he admits he's in the minority, and most other scientists agree with Gould that humanoid life is unlikely to evolve elsewhere.

Follow Charlie Jane on Twitter at @CharlieJane