Sunday, September 5, 2010

Our Hands Evolved Because Our Feet Evolved

A legitimately fascinating article (here) by Campbell Rolian, Daniel Lieberman, and Benedikt Hallgrímsson makes this claim, with substantially more precision. From the abstract:
Human hands and feet have longer, more robust first digits, and shorter lateral digits compared to African apes. These similarities are often assumed to be independently evolved adaptations for manipulative activities and bipedalism, respectively. However, hands and feet are serially homologous structures that share virtually identical developmental blueprints, raising the possibility that digital proportions coevolved in human hands and feet because of underlying developmental linkages that increase phenotypic covariation between them....
In particular, selection pressures on the feet led to longer, stronger thumbs, and slightly shorter other fingers. This makes it easier to hold something with the tips of our other fingers and thumb, in part because they are closer together, and in part because the forces on the thumb are dissipated over a larger surface area. This diagram might help you visualize:

modern human's pad-to-pad precision grasping, doi:10.1371/journal.pone.0011727.g001
Rolian et al go on to argue that:
[C]hanges in manual digital proportions that enabled digit tips to be brought into opposition may have been a prerequisite for the development of precision gripping capability in australopiths. Also, although pedal changes associated with facultative bipedalism probably provided australopiths with hands capable of producing and using Oldowan stone tools by at least 3.5 million years ago, it should be noted that manufactured stone tools do not appear in the archaeological record until about 1 million years later. Australopiths may have lacked the cognitive capacity for manufacturing tools and/or their technology was entirely nonlithic.... In short, there are several reasons to believe that selection on the foot caused correlated changes in the hand during human evolution, that selection on the hallux was stronger and preceded selection on the lateral toes, and that these changes in manual digital proportions may have facilitated the development of stone tool technology. (more here)
To over-simplify their claim: while feet were being selected for better load bearing and less mechanical work in running, it just so happened that this also made hands better suited for tool use.

So, the use of tools by early humans was due in large part to random chance. Since any event that relies on coincidence is less likely to be replicated, I read this as making the development of intelligence less likely / inevitable. If so, this means that the lack of other species in the night sky can be explained away in one more way--maybe most biological replicators don't get as lucky as we did in our evolutionary past to get a jumpstart towards tool use. Granted, this is rampant speculation and it's possible that we would have started using tools anyway.

But still, this makes it at least slightly more likely that we'll end up colonizing the known universe, instead of withering away on this doomed planet. And for today, I'll celebrate that.