But aren’t there many mammals who have different number of digits on the front and rear limbs? Cats come to mind here, I think. I don’t think that having the same number on each is really a biological necessity.
I recently read, and I’m I sorry I can’t offer a cite, that finger and toenails also aid with tactile sensations, they offer the fingertips and toepads a backstop.
Not a necessity, but it would require additional information in the genotype to modify the digits during foetal development, in this case, to retard the development of the tissues that grow the keratin for nails, or the modification to dewclaws in dogs.
In other situations, you could simply add another, mostly independent phenotype. The panda’s thumb, for instance, is an extra digit that comes from an extension of an existing bone and is not a part of the specific genotype that forms the rest of the digits. This extra thumb is an advantage to pandas, in that it helps them grasp and tear bamboo, but there is no similar advantage for humans in reducing the number of toes. We don’t need them, or we could at least design something more efficient, but (metaphorically speaking) Nature doesn’t have any reason to spend the time to fix it, just as General Motors doesn’t want to spend the time to make radio dials that don’t fall off after two years.*
Sorry, couldn’t resist the jibe at my old automotive nemesis, the Chevy Corsica. GM products are much improved these days in terms of build quality.
My physical anthropology teacher in undergrad told me the same thing. By evening out the pressure on the back of the nail, tactile sensation on the fingerpad is heightened. Probably vestigial on the toenail, though, or mostly so.
*imagining Badmana bootin’ it through the sand barefoot, with huge long creepy toenails… :eek: