How inevitable was it that body plans with bilateral symmetry would dominate?

(Assuming for the moment that they do indeed dominate - it certainly seems to be the case that some kind of majority of extant animal species have body plans that are bilaterally symmetrical, or based on that form)

Would it just have been some tiny little watershed event - some lucky shot back in the Precambrian, or is there some inherent advantage to bilateral symmetry that means once it’s here, it’s taking over?

If it is an inherent advantage, is that advantage local?

Or, putting it another way, would we expect bilaterally symmetrical alien lifeforms to dominate, assuming we were able to visit a bunch of other worlds?

For the record, I do understand the role of evolutionary legacy in this issue; that the today’s bilaterally symmetrical organisms share a common ancestor that was bilaterally symmetrical; that isn’t the question; the question is: how much of the proliferation is down to luck, and how much is down to optimal configuration?

Bilateral symmetry does mean that you only need half the genes, and half the genes means there are only half as many places where a fatal mutation could happen.

This is not true - certainly as far as “only needing half the genes” goes! There is probably little genetic cost if any to repeat a developmental unit several times - that is, an organism with five-fold radial symmetry would not necessarily require any more genes than one with bilateral symmetry. In any case, most organisms that are bilaterally symmetrical externally are not symmetrical internally.

Bilateral symmetry is a consequence of an actively moving lifestyle. It means, basically, that your top side is different from your bottom side, and that you have a front (head) end and a back (tail) end.

In the case of a primitive organism that moves along the sea floor, it will increase its efficiency of movement if it specializes its bottom side for locomotion along the substrate. Therefore the top and bottom will become differentiated. Likewise, it will probably be more efficient at finding and consuming prey if it concentrates most of its sensory organs (eyes, olfaction, etc) and its feeding apparatus together in one part of the body, and moves with this part of the body always in the lead, where it will be the first to encounter food. Therefore the front and the back ends of the body will become differentiated. Since in most cases there is no need to differentiate left and right sides of the body, the result is external bilateral symmetry.

The animal groups that are not bilaterally symmetric - anemones, jellyfish, echinoderms - are all slow moving or sedentary, with little sensory development. (The echinoderms are thought to be secondarily radially symmetric, descended from a bilateral ancestor.)

In sum, one would expect bilateral symmetry to dominate even in alien environments, as it is simply a consequence of active movement.

Thanks Colibri; that’s exactly the sort of explanation I was after… but taking this a little further then; it seems like a good place to put your visual apparatus (if you have it) is at the front (that is, the common direction of travel); to have your digestive opening (if indeed you have a digestive tract) near the visual apparatus, so that you can eat things when you see them, but below the visual apparatus, so that you don’t have to stick your eyes in the mud to eat.

Extending that kind of reasoning onward a bit, it seems not at all unlikely that some of the forms that haul themselves out onto the land would end up as bipedal or centaur-like, with a pair or more of free upper appendages, plus some kind of face.

At the very least, symmetry makes running more efficient. Consider trying to run with one long leg and one short leg. One of the Paul Bunyan stories describes a breed of mountain goat whose left-side legs were much shorter then the right side. This made it very efficient when clambering along the sides of steep mountains, but if you caught one, all you had to do to keep it was put it on level ground, where it could only run in a circle.

Bilateral symetry is a result of two forces…cephalization and gravity. Gravity means that an organism will have a top side and a bottom side. A hypothetical spherically symetrical ancestral organism would therefore tend to evolve into a radially symetrical organism, with different sides speciallizing in different functions, even if the organism lives in the water column and doesn’t go near the substrate. Of course if you live near the substrate, the side near the substrate is going to have different needs than the side exposed to the water column, even absent gravity.

Now add in cephalization, the tendancy of sensory organs to be clustered near a central nervous system cluster, the the tendancy for organisms to favor moving in the direction where their sensory organs are clustered, the tendancy for mouths to be near the sensory organs, and the tendancy for excretory organs to be located away from the direction of movement. So cephalization turns the radially symetrical organism into a bilaterally symetrical organism.

On preview, I see Colibri already explained it. Yes, this means that most mobile critters have something like a face. But it doesn’t follow that there will be a pair or more of free upper appendages. How many organisms have free upper appendages (or “hands”, to use the technical term)? Not many, unless you count the eating appendages of arthropods. And of course, many organsims don’t really have appendages at all.

True enough, although I was thinking primarily of anything that ends up becoming a tool-user; free upper appendages are quite a boon in this case.

I think what I’m saying is that, in the very unlikely event of us contacting (or much more likely, while still really remote - becoming aware of) a race of intelligent extraterrestrials, it isn’t at all beyond the bounds of possibility that they would be humanoid, to a certain degree, at least.

I don’t know what the thinking is on the relatively late development of our primary adaptation - our large cerebral hemispheres - but it seems to me that such a development might have begun much earlier under other scenarios, such as those that might otherwise have happened here but just didn’t, and those that might occur on another planet. It may be that bilateral symmetry and cephalization should be pretty early developments in the evolution of living things anywhere - even on another planet, but I don’t know why you’d expect humanoids from that. Why not, say, lizards, with large brains, or fish - or dolphins?

Well, define “humanoid”. If that means two legs, two arms and a head, then sure. But that’s a far cry from Star Trek’s bumpy forehead style humanoids. Under your definition a velociraptor, a raccoon, an ostrich, a kangaroo, and all primates would be humanoids. And those critters are all vertebrates with various recent common ancestors with humans.

If you mean erect spine, short neck, plantigrade feet, wide but flattened chest, no tail, and naked skin, then no, “humanoids” aren’t exactly likely. Those traits are mostly due to our unique evolutionary history as climbing apes that secondarily evolved to live on the ground. A velociraptor-like body plan is much more sensible.

to me it seems odd that very few animals have progressed BEYOND the stage of being symmetrical. Lobsters and crabs have non-symetrical “arms” and it makes sense One large “hand” for grabbing, cutting with full strength, and one smaller hand for manipulating things.

Ape-like homnids could have evolved that way–one large muscular arm for hanging onto tree trunks , or giving deadly blows to enemies, and one smaller and more delicate arm with long fingers for picking fruit or prodding insects out of their nests.

Even for us tool-using creatures with big brains , it would have been logical to evolve that way. A strong arm on the left to pull down a tree limb, and a more dexterious right hand for peeling the bark and cutting a sharp point to make a spear.

Then we would declare that any creatures from alien planets would also be non-symmetrical.After all, its clearly the best way, because it’s just like us.

(and think about sexual evolution : “Wow,man, did you see that girl? What a right side she’s got!”)

Active animals on other planets would be likely enough to resemble those on Earth at least in being bilaterally symmetrical, in having a head with a sensory cluster and a mouth located below most of the sense organs (to avoid having food get in them), and having some kind of appendages attached to the body for moving around. They would not resemble starfish or amoebas. However, beyond that is anyone’s guess.

Intelligence is likely to be correlated with being active (since a sedentary life-style doesn’t demand much thinking), so intelligent extraterrestrials at least should have the characters mentioned above. However, intelligence does not necessarily have to require having manipulative appendages - dolphins are a case in point. Of course, the development of technology does require the ability to manipulate the environment, so any extraterrestrials that came to visit us would probably have to have some kind of hands, tentacles, or whatever.

In my opinion, odds are that intelligent extraterrestrials would not be “humaniod” to the degree that you would ever be likely to mistake one for a human even on a dark night. There are too many other possible body plans available, from velociraptoriods to centauroids to multi-tentacled elephants. While it’s not impossible that an alien race could have descended from a primate-like bipedal creature and hence resemble us somewhat, that would be an extraordinary coincidence.

You’re right; perhaps I was using rather too broad a definition of humanoid; I definitely don’t mean anything like the Star Trek alien humanoids, but merely something with recognisable parts in sort of similar arrangements to those found in Humans (and kangaroos, raccoons, velociraptors too, for that matter)

chappachula - I haven’t suggested that any particular plan is ‘best because it’s like us’ and I don’t want to - I’m very definitely trying to approach this from the standpoint of ‘what is most likely because it works’, or actually ‘what is less likely because it doesn’t work’ as well. Even lopsided animals such as crabs have a body-plan that is based on bilateral symmetry (as opposed to five-fold radial symmetry, for instance).

I’m sure all kinds of exotic body plans could occur under all kinds of exotic conditions (and indeed do occur, right here on Earth), but I’m chiefly interested in organisms that would or could become technological (i.e. that we’d have a chance of learning about, if they managed to show up to SETI) - dolphins might be bright (or they might not), but they face a lot of significant hurdles on the way to developing technology.

The question actually began in my mind when I read a book (can’t remember the title) about a race of huge, tree-like aliens with threefold radial symmetry which came to Earth and enslaved the human race - all the way through the thing it struck me that they should (at least if they were anything like they were described) have been beset by problems and would have been totally inept as conquerors, let alone interstellar travellers. Although of course this was just a work of fiction, so the limitations are just those imposed by the writer, intentionally or otherwise.

But that’s the question here; is bilateral symmetry so common because it is optimal for an actively moving, sensate creature, or is it just plain old luck and legacy? - Colibri dealt with this question very well earlier in the thread, I thought.

Let us have a moment of silence for our long-extinct trilateral land-dwelling bretheren, the Trichordates.

Better them than us, I guess.

In the late 80’s Willard Huyck’s movie “Howard the Duck” the protagonist evolved from ducks. :smiley: Terrible movie if you were over 14. Interesting concept though.

You may be thinking of the Tripods Trilogy, by John Christopher: The White Mountains, The City of Gold and Lead, and The Pool of Fire. I also had trouble imagining an environment in which the aliens’ body plan would make much sense.