I've just had a quick look at a paper on the likely role of genetic enhancers in the development of human thumbs. Not exactly rocket science has already done an excellent job of commenting on it, so this is really just a heads-up - go over there to read the whole thing.
The paper reports on an experiment that involved the insertion of a human enhancer region, HACNS1, into mouse embryos. The base sequence for HACNS1 is fairly similar in most vertebrates, but it shows rapid, recent divergence in humans & chimpanzees. What does it do? Well, in the mouse embryos it promoted the genetic activity in their paws - or more specifically, the part that will become the first digit: the thumb/big toe. But other primate versions of HACNS1 didn't do this. So maybe - maybe - this enhancer was involved in the evolution of human feet & hands. It's too early to tell for sure: this is a preliminary study after all (though I would bet that the media will present it as 'gene for thumbs discovered!) - but what a fascinating bit of research :-)