Nail (anatomy) - Evolution

Evolution

The nails of primates and the hooves of running mammals evolved from the claws of earlier animals.

In contrast to nails, claws are typically curved ventrally (downwards in animals) and compressed sideways. They serve a multitude of functions —including climbing, digging, and fighting— and have undergone numerous adaptive changes in different animal taxa. Claws are pointed at their ends and are composed of two layers: a thick, deep layer and a superficial, hardened layer which serves a protective function. The underlying bone is a virtual mould of the overlying horny structure and therefore has the same shape as the claw or nail. Compared to claws, nails are flat, less curved, and do not extend far beyond the tip of the digits. The ends of the nails usually consist only of the "superficial", hardened layer and are not pointed like claws.

With only a few exceptions, primates retain plesiomorphic (original, "primitive") hands with five digits, each equipped with either a nail or a claw. For example, all prosimians (i.e. "primitive" primates or "proto-primates", see also Plesiadapiformes) have nails on all digits except the second toe which is equipped with a so-called toilet-claw (i.e. important for grooming activities). The needle-clawed bushbaby (Euoticus) have keeled nails (the thumb and the first and the second toes have claws) featuring a central ridge that ends in a needle-like tip. In tree shrews all digits have claws and, unlike most primates, the digits of their feet are positioned close together, and therefore the thumb cannot be brought into opposition (another distinguishing feature of primates).

A study of the fingertip morphology of four small-bodied New World monkey species indicated a correlation between increasing small-branch foraging and

1. expanded apical pads (fingertips),
2. developed epidermal ridges (fingerprints),
3. broadened distal parts of distal phalanges (fingertip bones), and
4. reduced flexor and extensor tubercles (attachments areas for finger muscles on bones).

This suggests that whereas claws are useful on large-diameter branches, wide fingertips with nails and epidermal ridges were required for habitual locomotion on small-diameter branches. It also indicates keel-shaped nails of Callitrichines (a family of New World monkeys) is a derived postural adaptation rather than retained ancestral condition.