Three-Legged Body Plan

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  • #1
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Since most, if not all, creatures on Earth have an odd number of limbs, do you think a three-legged body plan isn't very plausible?

This thread was inspired by the aliens in War Of The Worlds...:smile:
 

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  • #2
DaveC426913
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"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.
 
  • #3
Monique
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DaveC426913 said:
"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'.
Maybe he thinks having two or four legs is strange :tongue:
 
  • #4
Moonbear
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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.
 
  • #5
Moonbear
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Monique said:
Maybe he thinks having two or four legs is strange :tongue:
: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.
 
  • #6
brewnog
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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.
 
  • #7
Gold Barz
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Yeah I meant even number of legs, that's my mistake

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.

Although I don't agree with some of what Gould has to say, this is definitely true.

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....or maybe it was just a typo..
 
  • #9
Gold Barz
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DaveC426913 said:
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...
.

What is our "??-fold symmetry"?
 
  • #10
selfAdjoint
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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".
 
  • #11
Gold Barz
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So early Earth life had five-fold symmetry, ten-fold symmetry, twenty-fold symmetry kinds of creatures running around?, must have been a crazy place!
 
  • #12
pattylou
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DaveC426913 said:
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.


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
 
  • #13
Gold Barz
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Thats what he probably meant.
 
  • #14
pattylou
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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.
 
  • #15
Gold Barz
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No one is downplaying them, we were simply talking about animals, insects, etc...thats it.
 
  • #16
pattylou
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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.
 
  • #17
Gold Barz
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I don't know, anyways...pattylou I just sent you a PM, I'd appreciate it if you reply.

Thanks.
 
  • #18
Moonbear
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pattylou said:
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.
Yes, you're right. Thanks for picking up on that. It is an important clarification.
 
  • #19
DaveC426913
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pattylou said:
"Virtually all the life on Earth today is descended from one branch of creatures that exsited. " (referring to animals.)
Doh. Macro animal life. Stupid of me to be so ... Kingdomist.
 
  • #20
gerben
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Gold Barz said:
Since most, if not all, creatures on Earth have an odd number of limbs, do you think a three-legged body plan isn't very plausible?

This thread was inspired by the aliens in War Of The Worlds...:smile:

-- 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)
 
  • #21
Moonbear
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DaveC426913 said:
Doh. Macro animal life. Stupid of me to be so ... Kingdomist.
:biggrin: We'll forgive you.
 
  • #22
quasi426
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Gold Barz said:
Yeah I meant even number of legs, that's my mistake

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.
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....or maybe it was just a typo..

I don't think rewinding the film of history would cause any different result, UNLESS the environment is somehow altered.
 
  • #23
pattylou
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DaveC426913 said:
Doh. Macro animal life. Stupid of me to be so ... Kingdomist.

LOL. The Lorax is happy.

And I'm glad you weren't offended.
 
  • #24
pattylou
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Gold Barz said:
I don't know, anyways...pattylou I just sent you a PM, I'd appreciate it if you reply.

Thanks.


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."
 
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  • #25
GOD__AM
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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... :eek:
 
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  • #26
pattylou
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And opossums and elephants.
 
  • #27
pattylou
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Conceivably you could have bilateral symmetry and an odd number of limbs. Consider digits. We have an odd number, off each limb.

If the line of symmetry went down a central, single trunk - then selective cell death allowed an odd number of extensions that evolved to function as limbs, we could possibly see three "legged" animals etc.

The mechanics of locomotiong would obviously be very different, and conceivably the amount of energy required to locomote with such a design would be a disadvantage. Don't know about that.
 
  • #28
Gold Barz
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I am still shocked about the twenty-fold symmetry animals or creatures walking around...wow.

So two-legged animals is the most likely? and adding more legs or limbs will turn out to be a disadvantage?

What if the energy problem was erased because of abundance of food?
 
  • #29
Gold Barz
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Is there any advantages to having three legs?

Can a three-legged creature be more agile and faster than a two-legged creature?

quasi426 said:
I don't think rewinding the film of history would cause any different result, UNLESS the environment is somehow altered.

well its like one big chain reaction, tweaking something in the past could throw up something completely different.
 
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  • #30
GOD__AM
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Gold Barz said:
Is there any advantages to having three legs?

Can a three-legged creature be more agile and faster than a two-legged creature?



well its like one big chain reaction, tweaking something in the past could throw up something completely different.


Some animals have gained advantage this way. Consider the kangaroo mentioned above. It owes much of its speed to the fact that its large heavy tail helps balance it when its running/hopping forward. It also uses its tail to lean back on so it can kick and defend itself.

The elephant mentioned above is also an excellent example. While it's trunk can't be considered a "leg" it certainly qualifys as an extra limb that affords them some advantages over animals that have to use their mouth for a lot of things.

Monkeys and other animals with prehensile tails certainly gain an advantage using their tails as an extra limb.

Nearly every animal displays near perfect symmetry in body design. An animal born with three legs would most likely be due to a birth defect, and unless the brain was rewired to take advantage of it they probably would be at a disadvantage. If the leg grew out of a central location, and in that way maintained symmetry it could possibly work out ok... I think the thing you have to ask yourself is why would an animal that seems to do amazingly well with an even number of legs need one more. What good would it do?
 
  • #31
Gold Barz
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Yeah, what advantages could these extra legs offer?
 
  • #32
nipwoni
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Even today, there are many animals with radial symmetry, as opposed to bilateral, two-sided symmetry. The echinoderms are an example- they're the phylum closest to Chordata, our own. They include starfishes, sea urchins, sand dollars, sea cucumbers, and sea lilies. They can have five, six, seven or more "arms."
 
  • #33
DaveC426913
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IMHO, the elephant's trunk and monkey's tail et al, while technically fulfilling the role of an odd/extra limb, is a separate discussion.

I submit to the discussion that this topic's value would be increased if we agreed to narrow the scope of examination to odd number of limbs used for locomotion, and possibly even more specifically to gait.

I think that was the intended scope, but that's just MHO.


P.S. Sci-fi readers interested in tripedal locomotion might want to pick up A.C.Clarke's Rendezvous with Rama.
 

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