D.N.A. sequences change by mutations, and the idea behind the molecular clock is that those changes occur at, more or less, a constant rate, over time.

Lice have been with us and evolving with us for as long as we have existed.

D.N.A. sequences change by mutations, and the idea behind the molecular clock is that those changes occur at, more or less, a constant rate, over time.

The loss of body hair is interesting to anthropologists, because it is a feature that distinguishes us from our nearest living relatives, chimpanzees. They have body hair, we don't.