The key to the study by David Reed and colleagues, which appears in Molecular Biology And Evolution, is that there are two kinds of lice that hang around humans: the head lice that live on our scalp, and the body lice that live in our clothes. At one point in the past these two shared a common ancestor, Reed reasoned, and the body lice would have split off and become a separate group once they had human clothing in which to live.
The genomes of the two kinds of lice split somewhere between 83,000 and 170,000 years ago, which means that humans ran naked for hundreds of thousands of years without body hair or clothing. Clothing probably arose during an Ice Age, and eventually enabled humans to leave Africa to explore colder parts of the world. Link