A really cool study was just published in the journal Evolution, and what with getting my apartment ready for a New Year's party on the 31st, and my being completely incapacitated yesterday, I didn't get to read through it until today. Campbell Rolian and colleagues (in press) address the question: In human evolution, were hand and foot digital proportions each the targets of direct selection, or could hand/foot proportions have evolved as a byproduct of selection on only the hand or only the foot?
This is an interesting question. In your standard Anthropology 101 class, you learn about how humans (and hominins) are unique relative to apes. Two unique things about us are: a robust, adducted big toe for bipedalsim, and a hand adapted for tasks requiring a fairly high degree of dexterity, such as tool use. But something to keep in mind--indeed the authors of this study did--is that the hand and foot are serially homologous, each is a variant on a common theme. Because the developmental architecture behind the hand and foot are largely similar, an intuitive question is whether selection on the hand or foot only would effect the evolution of the element that wasn't under selection. Could developmental integration of the hominin hand and foot have led to evolutionary integration, do/did the hand and foot co-evolve?
Turns out this may well be the case. Authors looked at lengths and widths of hand and foot phalanges (finger bones) in a sample of humans and chimpanzees. Generally, in both Pan and Homo, homologous traits in the hand and foot are more highly correlated than expected by chance, even compared to correlations between traits within the hand and foot. Cool, and none too unexpected.
But then the authors did some crazy simulations, to see what kinds of selection regimes on the hand and foot may have led from a chimp-like morphology to the morphology we humans enjoy today. I'll need to reread this section a couple times, but it looks like selection on the big toe is one of the most important aspects of hominin hand/foot evolution. And it would not be impossible for evolutionary changes in the human hand to be largely by-products of selection on the foot, due to the nature of covariation (integration) of the hand and foot. Whoa!
The implication, which the authors seem to like, is this: given a chimp-like ancestral morphology for the hand and foot, it seems that the two major hominin/human traits given above (bipedalism and tool-use/manual dexterity) are largely due to selection simply on the foot. That is, because of the developmental integration of the hand and foot, selection for a bipedally capable foot indirectly induced the evolution of a hand conducive to manipulation. Ha, the hand was just along for the ride! Get it, because the feet move the body, and so the hand... but also evolutionarily... Dammit.
Anyway, that's nuts! Of course, another very interesting thing about the first digits of the human hand and foot, aside from the fact that the first digit on both is relatively large and robust, is that the mobility of these digits is just about opposite what it is in the apes. Whereas the big toe is very mobile/opposable in apes (and the 4.4 million year hold putative hominin, Ardipithecus ramidus), it is completely adducted in humans (and fossil hominins that aren't Ar. ramidus). Less extreme, the human thumb joint is allegedly more mobile than apes' thumbs. So this is the next step, I guess: what is the developmental basis for the wild evolution of the human hallux and pollex joints?
Rolian C, Lieberman DE, and Hallgrimsson B. Coevolution of human hands and feet. Evolution: in press.