So why is it that the foot and hand are similar in design. It seems extremely unlikely that simple evolution of appendages could create a hand with a thick finger (thumb) and four slender fingers and a foot with a thick toe (big toe) and four slender toes. How did they both come up with 5 similar appendages?
From an evolutionary standpoint, I can understand that there was an organism which had appendages. It used these appendages for climing/walking/whatever. Eventually these appendages changed into what we know as arms and legs. But it seems odd to me that arms and legs would have evolved to have such similar features when they are not used for exactly the same thing.
It’s almost like there used to be an organism with 4 appendages with two joints and at the end of each appendage were 5 smaller appendages. I can understand that we have left/right symmetry, but how did it get 4 identical appendages to start with?
It makes me wonder if DNA has subroutines called MakeLongAppendage and MakeShortAppendages. So when it needed an arm or leg, it would call MakeLongAppendage. Then when it needs toes/fingers, it calls MakeShortAppendages.
So how do we have the same number of fingers as toes?
quote: it seems odd to me that arms and legs would have evolved to have such similar features when they are not used for exactly the same thing.
Yes, but they evolved FROM the same ‘thing’, more or less, so theyshould be similar, but adapted for their specialty which gave them a advantage over others who didn’t have them adapted, allowing for more of the adapted limb folks to survive long enough to reproduce in greater numbers.
Well, that’s one way of looking at it. I don’t know if you’ve done much reading in evolution, but I’ll try to give you as short an answer as possible.
Imagine a creature with pretty much identical limbs. Among the non-fatal mutations that occurs in a million years or so, one change is to give the front limbs’ “toes” a slightly better capacity to do something that helps the creature gather its food. So it does better than the others and has more offspring. After a while the “new and improved” front toes come to predominate and the old style dies out, because it can’t compete as well.
Continue this process over many, many millenia. Those whose changes work better become more numerous. Somewhere down the line, some of the critters are trying to climb trees, for example. Those who have good hands, or paws with claws, so that they can grip the tree better will survive longer since they can climb away from predators (or chase up the tree after their prey).
The basic differentiation between the front legs and the back legs I would guess happened fairly early on. An alligator’s front and back legs are not all that different. The front and back legs of rodents are somewhat different. The front and back legs and paws of monkeys are also kind of different, although they can grip things with their toes better than most of us can.
We have the same number of fingers and toes for the same reason that most mammals have the same number of toes on their front and back feet. They start out as the same “thing.” In some species, one or more of the toes (or fingers) has become vestigial, or blended into the other digits, but they are still there. Even in horses, which appear to just have one digit (the hoof) on each leg, the hoof is really a blending of the 5 digits its ancestor (eohippus) used to have.
Bear in mind that this is a highly simplified explanation of evolution. To really learn about it, go to a book store or site (amazon.com will do nicely). Look or search for “evolution” and you will find tons of good books on the subject. Ernst Mayr, Steven Jay Gould and Richard Dawkins are all authors that are worth a look.
So then there was an organism which had a bunch of long appendages, each with 5 smaller appendages on them? Eventually it learned to crawl and the appendages became what we know today as arms and legs?
Why is five digits the basic number for land vertebrates?
Why are the prehensile hands and feet of primates built on the same design, that is, with one large opposable digit on the inside vs. four smaller ones on the inside?
As Doug’s report indicates, the answer to 1) is because the ancestor of all surviving lineages had five digits on all limbs (even though some earlier forms, like Acantostega had up to eight digits.)
Our thumbs and big toes are part of our primate heritage: they were originally adaptations for climbing in our arboreal ancestors. (The big toe being opposable like the thumb in our ancestors.) It’s perhaps partly coincidence that we have the same design in both hand in foot. Opposums have opposable thumbs on the hind feet, and these are the innermost digits. (The hands do not have opposable thumbs.) Koalas, on the other hand (so to speak), have two thumbs on the hand (the two innermost digits are opposable) and one on the foot (the innermost one). Chameleons also have prehensile hands, but in their case the front feet have the three innermost digits opposed to the two outer ones (three “thumbs”), while the hind feet have the two inner digits opposed to the three outer ones.
The “identities” of our fingers and toes are controlled by the same developmental “master” genes: the Hox4 complex. Thus, whatever happens to one set of digits pretty much hapens to the other. The exact morphology of the particular digit is (WAG:) controlled by another set of genes, which would then differentiate “toe” from “finger”.
It is also thought that the Hox1 complex was previously part of the digit-identity scheme, which, together with Hox4, would account for the larger number of digits present in fossil critters like Acanthostega that Colibri mentioned. At some point, Hox4 became the sole master control complex for digit development.
I seem to recall reading somewhere that mutations in this master digit gene tend to cause sexual mutations leading to impotence, and thusly the number of digits has been highly conserved over the eons. Creatures that mutate into having more or less digits tend not to be able to reproduce, regards of any advantages or disadvantages incured from the extra digits.
In other words, the master digit gene has more than one function, and major mutations tend to cause infertility. This is a powerfully conservative force on the number of digits.
Thaumaturge , do you have any info. on this? I have tried to find it, but have had no luck so far.
Also, I thought I just read that some reasearch has been done recently that shows that many of our traits were the result of different organisms combining, not neccesarily one organism evolving. Would this have anything to do with it?
I would much rather have 6 or 7 digits on each hand and foot. It would greatly help out my bass playing.
Moles have five front toes and three rear ones, I tinks. Does that count? … OT for the bored: some pictures of whacked monkeys: http://www.duke.edu/web/primate/noclem.html
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I find the Slender Loris particularly disturbing, like a little martian or something.
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You might try locating the following paper (it doesn’t appear to be available online):
Podlasek, C.A., Duboule, D., and Bushman, W.A. (1997) Male accessory sex organ morphogenesis is altered by loss of function of Hoxd-13. Dev. Dynamics 208: 1-12.
Can’t guarantee that it is exactly what Thaumaturge is referring to, but it does appear to connect Hoxd-13 with male reproductive organs.
Note that “Hoxd” is the current name for the Hox4 complex. In 1992, Hox1, Hox2, Hox3 and Hox4 were renamed to HoxA, HoxB, HoxC and HoxD, respectively.
Also, HoxD genes have been implicated in the development of gut sphincters.
ElFoldo – so you had the idea sort of right, with the MakeLongAppendage and MakeShortAppendage subroutine. Remember that some variables have slightly different values when the appendage routines are called from the MakeFrontofTrunk and MakeBackofTrunk routines, so their results are slightly different.
Don’t take the analogy too far, though, unless you’re using a massively parallel, yet completely object and event-driven, computing language (which doesn’t exist yet, as far as I know).