Thanks for reading this.  What did fingernails (specifically human) evolve from?  At first, I was thinking they must be evolved from modified hair and then I thought, no, our reptilian/amphibian ancestors had them too (in the form of claws) and, therefore, they must be modified scales.

So, do humans in essence have the remnants of 20 reptilian scales as finger nails?!?? 

Thanks again,
LW

Most primates have fingernails, in fact, fingernails - on at least the first digits - are characteristic of primates. Primates belong to the cohort Unguiculata - those blessed with claws or nails.
The 56-million-year-old fossil plesiadapiform, Carpolestes simpsoni, seems to have developed nails on its thumbs, judging from the shape of the distal phalanx of its first digit (the bone forming the end of the thumb).
Fingernails are useful to animals which specialise in grasping - they cover just the back of the digit, leaving the pad exposed as a good gripping surface. It seems that nails also enhance the sensitivity of the pad, and they also aid fine manipulation. Imagine the frustration of peeling an orange with claws (I mean peeling with claws, not an orange with claws - now that would be an evolutionary oddity!)

 

Last edited by Alice Roberts (21st Apr 2010 08:29:52)

Going a bit further back, the likely origin of claws (and therefore the homologous nails) is a bit tricky to identify. The African clawed-toad Xenopus laevis has claws (as the name suggests), but they do not seem to be homologous structures to those seen in 'reptiles' or mammals (http://www3.interscience.wiley.com/jour … p;SRETRY=0), which suggests that claws have evolved more than once in the tetrapods (which is no real surprise, since they're useful things to have). The important thing to keep in mind here is that amphibians don't  have scales, so there is not necessarily a homologous link between scales and claws.

The keratin that is used to make claws is a very versatile biological material (it's used in skin, scales, hair, feathers, horns, baleen, etc.). To the best of my knowledge all vertebrate skin is partly composed of keratinocytes (which are keratin secreting cells), which means that the mechanism for claw development could be as simple as an increase in keratinocyte density at the ends of the digits. This makes it tricky to track the evolution of claws and nails, since they can probably be easily lost and gained.