This reference is fascinating. I'd love to learn more about the connection with the genito urinary tract. --- The argument that 5 is merely a holdover from 60 million years ago is not a good argument, because 50 million years is plenty of time to move up or down from 5 if it were important enough. A good argument why exactly 5 is that within the past 100,000 years (order of magnitude), there was a "bottleneck", where the number of humans was reduced to fewer than 1,000, and every human today is descended from that small group which presumably was pretty homogeneous. So there hasn't been enough time since that bottleneck for evolution to experiment with more or fewer digits. However, 5 digits must have been appropriate for the conditions during the bottleneck, because these individuals survived & reproduced & the others didn't. So we should look for "human" remains older than 100k years that have more or fewer than 5 digits/hand. One theory I've heard proposed is that the bottleneck was caused by massive flooding, in which case the ability to swim would become paramount. More fingers does seem to help with swimming, as even w/o webbing between the fingers, the turbulence around the fingers supplies enough resistance to be useful. At least one researcher has decided that the diaspora of humans around the Earth at some points required swimming in order to get across rivers, so even though swimming may not be used by very many humans very often, it appears to be a critical capability for the species as a whole. At 10:08 AM 12/20/2010, Hans Havermann wrote:
Henry Baker:
There must be a steep marginal cost associated with having 1 more finger, so that evolution calls a halt to digit inflation.
Michael Coates, professor in the department of Organismal Biology and Anatomy at the University of Chicago, might have addressed this a few years ago by positing that, on a genetic level, "the mechanisms involved in patterning the tips of our limbs include those involved in our reproductive success".
http://www.scientificamerican.com/article.cfm?id=why-do-most-species-have