Animal Legs


I was laying there trying to go to sleep, when a thought crossed my mind.

Why did (almost) all animal-types develop 4 limbs? I can’t think of anything outside of snakes and fishy critters that have another configuration.

Did all of these evolve from some single 4-limbed ancestor? Is that why about everything has a head in the front and an ass in the back (ie, we have such similar biology)?

Or does it just happen that 4 limbs is the most effective configuration and thus led to the most successful breeding?

Why nothing with 6 limbs? Is it just inefficient?

I recall an experiment with AI, where it was asked to create through random mutation the most efficient structures to move from point A to point B. While a very limited definition of “success” (there are obviously many more factors than locomotion than go into a successful critter), many of the good results had designs no one had seen before, and a few we have (one was basically a sidewinder), but none were 4-limbed.

The most “variety” I have seen with animals is deep ocean fish/misc watery thingies that take on all kinds of bizarre forms. Is this because the ocean is 1) isolated and 2) well, relatively stable, making a more “friendly” environment for random mutations?

What about insects etc? Why did they evolve with all kinds of bizarre body types when animals didn’t? Shorter lifespan and larger numbers so greater chance of mutation over time?

First of all, insects are animals too. As are all sorts of other critters–like worms, sea anemones, etc.–that have completely different body forms. What you’re talking about is basically just vertebrates (animals with backbones).

Fish are the common ancestors of all vertebrates, and if you look closely, you’ll see they all have two pectoral fins (the ones closest to their heads; like little arms) and two pelvic fins (a little further back on their bodies). These four fins are what, when these critters started crawling up on land, evolved into the four legs we’re familiar seeing in most vertebrates. This image is a great diagram that shows what I’m talking about. (Note that the anal fin–right there behind the pelvic [the one on the tail is the caudal fin], was not paired, and was not retained as a limb in terrestrial vertebrates)

Apparently, nature seems to think having these four limbs is quite efficient for locomotion and other needs, as only few animal groups have given rise to legless varieties (ie, snakes, some lizards, and mayby a few other animals I’m forgetting). Why not 6 limbs? shrug That’s just not what evolved. If those early fish had 6 paired fins before making the move for dry land, then I’m sure we’d probably see most terrestrial vertebrates with 6 limbs as well.

Hope that answers some of your questions. Someone else will surely come along and provide more information and/or correct me.

Well … IANA geologist, and the only species I know a whole lot about are either mammals or bacteria. :cool:

However, nobody seems to be “stepping up to the plate.” Can’t have that. :stuck_out_tongue: There are several basic body plans in current use on this globe. Mammals and reptiles are all descended from some four-limbed ancestor (as you noted). Please be aware that snakes are descended from the same ancestor; they have vestigial remnants of hind limbs. As for six-limbed, all the insects fall into this category. Eight-limbed ones are the arachnids (spiders), and their relatives (mostly water dwellers). Then there are weird critters like centipedes and millipedes. I won’t Google them; I’ll let you do that. :slight_smile:

Here are some sites you may find helpful:

Animal Diversity Web

The very earliest animals now known.

Here’s an interesting site which explicitly discusses the origin of tetrapod (four-legged) critters.

Some of the weirdest, most interesting critters you never imgained lived during the Cambrian Period.

More interesting for its plant life developments (e.g., seeds) is the Devonian Period.

Maybe now somebody with qualifications will speak up?

Check out a book called “At Home in the Universe” by Stuart Kauffman. Toward the end, he compares two early extinction events. After the first one, when life began to reassert itself, the fossil record shows dozens of really bizzare new body types - there are many entire phyla that are only known from fossils of this era.

After the next big extinction event, however, when many of these bizarre (to us) phyla disappeared, the newly emptied niches were filled by adapting preexisting body types from the phyla that survived, rather than coming up with completely new types again.

The reasoning was that once you have an organism that works, it’s much safer, evolutionarily speaking, to stick with that and modify it as needed rather than to try to start from scratch. The book has a big long discussion about fitness landscapes and fitness peaks that explains it more precisely.

Presumably, the first successful land animals that weren’t insects had four limbs. They survived and passed that along to the rest of us.

Just a slight clarification. The current thinking is that legs evolved while the ancestors of land animals were still fully aqautic-- ie, before the colonization of the land. These “fish” as I suppose they must still be called used their legs to skoot about on the bottom of whatever body of water they evolved in. Their utility to a terrestrial lifestyle appears to be a good example of exaptation.

The common idea of some air breathing fish hauling itself up on land using its fins is most likely incorrect. That fish would already have legs complete with claws to do the job.

I’m sorry, you’re right. I didn’t mean to imply that, but after re-reading my post it seems I wasn’t very clear at all (It was late; I suppose I was lucky that I was even coherent). I do know the leading idea is that limbs came before the move to land, instead of the other way around. My professors would certainly kill me if I didn’t know that by now. Next time I’ll wait until I’m thinking more clearly before posting.

Dirx, aka :wally