Three-Legged Body Plan

"Since most, if not all, creatures on Earth have an odd number of limbs,"

Are you sure you wrote this the way you intended? I think you meant 'even'.


Anyway, you should consider reading some Stephen J. Gould, such as 'A Wonderful Life'. He is an expert on Earth's early life.

Virtually all the life on Earth today is descended from one branch of creatures that exsited. Therte was a huge explosion of diversity half a billion years ago, but only one major branch dominated. All that has since descended from that period represents only about 10% of the species in early history. Other branches had myriad body forms, 5-fold symmetry, 10-fold symmetry, twenty-fold symmetry...

As Gould claims, it is only dumb luck that the creatures that survived are anything like they are. Rewind the film of history, run it again, and we'd get a completely different result.

 

Maybe he thinks having two or four legs is strange :tongue:

 

Well, there certainly can be birth defects such that an organism is born with an odd number of limbs, but except for humans who aren't in too much danger of predation anymore, usually this leaves the odd-limbed critter a little less balanced and a little slower than the others of its species so it is more likely to get gobbled up by a predator. So, in this case, it is very plausible because it does happen, although it's a disadvantage in most cases, so not likely to increase in frequency in the population in any appreciable numbers.

 

:rofl: Yeah, if he thinks he's seeing lots of creatures with odd numbers of limbs, I think he should have his vision checked very soon.

 

My friend had a cat with three legs, it was an awesome cat. It ended up getting called Tripod.

It was an amputee though, not sure whether it counts.

 

i know they aren't really "limbs" but some starfish have an odd number of arms which they use similarily to limbs, (although some have even too)


It seems that Aristotle was asking the same questions too.

http://www.mlahanas.de/Greeks/Aristotle/OnTheGaitOfAnimals/OnTheGaitOfAnimals.html

 

2-fold; we are basically bilaterally symmetrical. Of course the symmetry isn't exact, but then I don't suppose those Burgess Shale critters were exactly n-fold either. BTW, other reconstructions of those beasties has them looking a lot more reasonable than Gould had them in Wonderful Life. In at least one case he had the fossil upside down. He makes all this noise over the various body plans but I only thought "1001 Clever Things a Bright Young Genome can do with a Homeo Box".

 

I'd have to take issue with that. Are you sure you don't mean something like "Virtually all life in the animal kingdom is descended from one branch
..."?

I know I'm beng pedantic, but virtually all life on the planet has no limbs at all. And the limbed variety... represents hardly any of the genetic diversity on the planet. Feel free to call me a nitpicker.

Patty

 

Well, maybe. I think it is worth mentioning that bacteria and archaea make up 50-80% of the biomasss on the planet.

Animals are only a few percent. And when you consider genetic diversity, animals got squat compared to bacteria. Gould painted a picture that life became dominated by one branch, but it's more of twig than a branch, if you ask me.

Nothing personal, I teach microbiology and get in a huff when people downplay these fascinating organisms.

 

I feel like I have walked into the twilight zone.

"No one is downplaying them."

"Virtually all the life on Earth today is descended from one branch of creatures that exsited. " (referring to animals.)

Wrong. Period. "Virtually all life" on the planet is not even in the animal kingdom. By a large margin!

I'm not trying to derail your thread. I'm not! I'm correcting a rather glaring error.

 

Yes, you're right. Thanks for picking up on that. It is an important clarification.

 

Doh. Macro animal life. Stupid of me to be so ... Kingdomist.

 

-- Okay, in order to prevent any misunderstanding: I am assuming that with "creatures" you particularly meant arthropods and vertebrates. --

I have no theory of why they have an even number of legs, but it goes hand in hand with bilateral symmetry. For each one on the left side there is another one on the right side.

(Many molluscs have only 1 foot; not an odd number of legs but at least an odd number of feet)

 

We'll forgive you.

 

I don't think rewinding the film of history would cause any different result, UNLESS the environment is somehow altered.

 

LOL. The Lorax is happy.

And I'm glad you weren't offended.

 

There are many discussions on the "direction" of evolution at the link below, including a discussion on why animals have an even number of limbs):

http://www.bbc.co.uk/cgi-perl/h2/h2.cgi?state=threads&board=science.evolution&&sort=T [Broken]

For example see the thread down the first page titled "human evolution" wrt to your email (I'd send you another reply but I'm close to 35 messages and need to figure out how to clear those out soon!). The limb question is right below that one, titled "tripedal animals."

 

Dolphins and whales IMO fit into the odd number of limbs category. I even remember some fossil evidence suggesting that a 4 legged land animal evolved into a dolphin type body form and took to the sea. There was evidence that later it reversed the process to return to the land with 4 legs again.

Then of course we have snakes, evolving from lizards, that abandoned all limbs.

How about animals with prehensile tails? Is that not considered another limb? I think it is.

Almost forgot to mention kangaroos...