Pharyngula

I am not a fan of the convergent evolution argument for humanoid aliens. I can well believe that it’s likely that intelligent aliens exist out there in the universe, but I’m not even going to try to predict what they look like: there are too many alternative paths that are possible. But for some reason, many people like to insist that it’s reasonably likely that they’d resemble us in general, if not in detail, and they’ll then go on to extrapolate that behaviorally and culturally they’ll share many properties with us. Usually, as with Simon Conway Morris and now George Dvorsky, this argument relies entirely on the premise of convergent evolution.

It annoys the hell out of me, because it requires waving away or consciously ignoring basic principles of evolution. Here’s how Dvorsky illustrates the concept:

convergent_evo

I’ve seen versions of this illustration a thousand times. Gosh, look, dolphins and ichthyosaurs all have paddle-shaped fins and streamlined bodies! Therefore, this is evidence that aquatic forms all converge on similar morphologies. And therefore, intelligent, terrestrial organisms will also converge on an ideal form for their niche, which is ours also, therefore we represent a morphological ideal.

I hate the argument because it isn’t applicable to alien species. In both cases shown above, the organisms involved belong to the same subphylum, the Vertebrata, and the same superclass, the Tetrapoda. They share a common ancestor, and the same starting point. When you start with a terrestrial creature that looks like this, with four limbs and a suite of similar traits…

Eryops

…and then let it re-adapt to a marine life as a free-swimming, active predator after long detours into mammalian and reptilian forms, is it any surprise that they converge on similar structures? They are both constrained by their ancestry! The four-finned torpedo does not necessarily represent an ideal form that will be recreated on every planet we ever someday visit, but is instead a compromise, a form that can be generated from a four-limbed vertebrate with a minimum of fuss.

But what if we look at organisms with more remote ancestry? A whole different phylum, perhaps? If we start down the evolutionary road with a conchiferan sort of creature, a mollusc…

limpet

…which later evolves into a free-swimming, active predator, you get this:

squid

That’s something that looks completely different from a fish-like ichthysaur or dolphin; it’s got a completely different shape, a completely different set of feeding behaviors, a completely different internal organization.

We could ask the same question of other phyla. Where are the sleek torpedo shaped crustaceans sporting a nice dorsal fin and quartet of paddles? Show me a marine annelid that has followed this same path.

Now keep in mind that life on another planet will share no ancestry with anything on Earth. If our history is any example, they will be the product of a few billion years of single-celled tinkering, with a riotous adaptive radiation of multicellular forms that will explore a small fraction of morphospace…and every step will be contingent on prior states.

You can only make this ludicrous convergence argument if you think 1) contingency is relatively unimportant, that 2) adaptation is extremely powerful and will always drive a species towards an optimum, and that 3) the shape of a relatively tiny subset of species on this planet represent that optimum. There is a fourth requirement as well: you must be oblivious to the fact that (2) and (3) contradict each other.

Comments

  1. #2 John
    United States
    November 9, 2013

    I once read a book which speculated what intelligent aliens would look like. It was full of pictures of potential aliens. Every single one was a humanoid hybrid of some earth animal, because obviously they’re going to be bipedal, etc., and all possible morphologies already exist on earth. If aliens descended from something like a koala, they might look like this…

  2. #3 Critical Dragon1177
    November 9, 2013

    PZ Myers

    I think it might still not be completely unreasonable to think that aliens might look something like us based on convergent evolution since we don’t know what else is out there, and we don’t know exactly what the limits of evolution are, other than everything having to obey the same natural laws, but I see what you mean by this.

  3. #4 Critical Dragon1177
    November 9, 2013

    #2 John

    About non humaniod aliens. If you can find it, check out the documentary / sci film, “Alien Planet.” It aired on the Discovery Channel awhile ago, and if I remember correctly its still on Net Flix.

  4. #5 G
    California USA
    November 10, 2013

    Some years ago an artist (I think he was in Canada) created a sculpture based on using a piece of software to show what dinosaurs might have evolved into had they not been rendered extinct. The result looked surprisingly like a greenish version of a “Gray”, the mythical ET of modern “contactee” lore.

    Our cultural views of ETs are biased by depictions in fiction that in turn are driven by the needs of the stories: humanoids, insectoids, and so on.

    In reality it will probably be hundreds of years before we establish sufficient communications with another civilization to enable an exchange of photos of life on our respective worlds. Think of the c-lag before technical protocols are agreed beyond the level of sending basic numerical data to establish our existence to each other.

    OTOH if the new Kepler findings are correct, and we can extrapolate to quantity of intelligent civs, it might only be a matter of decades from initial contact, rather than centuries.

    And has anyone else here noticed the responses to the Kepler story in various online media, that appear to show an increase in interest in space exploration?

  5. #6 Thomas P
    November 10, 2013

    G, sending pictures is pretty easy. Just send a string of numbers with a length that adds up to a product of two primes and any mathematically proficient species is likely to figure out that it represent some kind of 2D data, which will be confirmed when they try and get an obvious pattern. It’s slightly more tricky to give a description of scale, but this should be doable without any exchange of protocols.

    To learn how another intelligence thinks on the other hand, their language, moral, organization of their society etc, that is going to be really difficult and dangerous.

  6. #7 Tim Tyler
    November 10, 2013

    George only says that aliens “may look eerily familiar to us”. To go from there to “humanoid” seems like a bit of a leap.

  7. #8 G
    California USA
    November 11, 2013

    Re. Thomas @ #6: That’s just beautiful, but since I’m not a math nerd, I never would have thought of it. It assumes we’ve managed to figure out each others’ numbering systems, but that step would have had to come first. One of the great joys of science & math is the fact that it all works everywhere in the known universe.

    Know what’s really cool? The offspring of every other technically capable species in the universe get to learn the same Newtonian physics as we learned, though obviously with different names and numbering systems.

    One of the things we could probably exchange with ET pretty quickly, would be that set of equations. At the same time we may as well include Einsteinian relativity and QM, and then ask ET if they can help us with unified field theory. (Right, and after another century or two for message propagation, we get back “Sorry, you’re not ready to handle that yet. Stop killing each other for another hundred orbits around your star, and then maybe we’ll tell you.”;-)

  8. #9 Wesley Dodson
    November 11, 2013

    Seems like human brain development is an indirect consequence of bipedalism. Not only did freed, sensuous, dextrous hands require more brain power to better utilize, they allowed us to do things like cook, which provided more and easier nutrition from a variety of sources. If intelligence is a matter of brain/body ratio, what else is going to push that ratio to the extreme except an undedicated pair of graspers and the development of external pre-digestion?

  9. #10 Wesley Dodson
    November 11, 2013

    There is also the biblical aphorism that man is made in God’s image, and although nothing from the Bible should be taken literally, I do allow that the old book may contain some figurative wisdom. Phrased in a more modern way, the form of man reflects the form of the universe; we are, after all, the product of its forces and materials. And the universe, though diverse, is repetitively patterned and in a sense homogenous. Thus it doesn’t seem too far-fetched to expect other technologically savvy life-forms to be human-like. Not the merely successful life forms; there could be a ruling species on a planet as evolved as ours that has not depended on a relatively massive brain for ascendancy, and so could look like a croc, a squid or whatever. These creatures are made in the Universe’s image too. But if you are looking for an intelligent, tool-making, agricultural and social animal that lives on land, why wouldn’t it have a body plan a lot like ours? The distrust of anthropocentrism is healthy, but it is not we who project our form onto the cosmos, it is the cosmos that projects its form through us.

  10. #11 Graham Jones
    November 11, 2013

    G, the idea is that you send a message like

    -***-*—**—-*—-*–***—*-***-

    This has 35 symbols in the most common current Earthly notation for numbers. It doesn’t matter how the aliens write the number, as long as they see that it is

    *****

    multiplied by

    *******

    and that both those are primes. It could be 5 rows of 7, or 7 of 5. It is unlikely that the picture resulting from rearranging this in a 2D grid means much to aliens either way, but it might to you.

  11. #12 Barney Wolfe
    USA
    November 12, 2013

    #5 G
    I think we know what dinosaurs look would look like “if they had not been rendered extinct”. I feed them in my backyard year around.

  12. #13 David Marjanović
    Museum für Naturkunde, Berlin
    November 13, 2013

    I think it might still not be completely unreasonable to think that aliens might look something like us based on convergent evolution since we don’t know what else is out there, and we don’t know exactly what the limits of evolution are, other than everything having to obey the same natural laws, but I see what you mean by this.

    It’s not completely impossible, but it’s exceedingly unlikely. That’s the point.

    Some years ago an artist (I think he was in Canada) created a sculpture based on using a piece of software to show what dinosaurs might have evolved into had they not been rendered extinct.

    It wasn’t “some” years ago, but 31 years ago; no software was involved whatsoever; and the sculpture was based on the very assumption that the human form is somehow obviously inevitable. Read this Tet Zoo post and its 82 comments to learn everything you never thought of asking about it. :-)

    And the universe, though diverse, is repetitively patterned and in a sense homogenous.

    Be more specific. I see no reason not to think our form is historically contingent. Again, compare ichthyosaurs, dolphins and squid.

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    November 21, 2013

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