The origin of head lice and its relationship with humans has long been debated. Some scientists believe the two species originated separately, although they are closely related genetically. Others believe the lice have evolved from primate relatives. Regardless of their origins, humans and body lice are infected with the same type of parasite. There is some evidence that the two species share the same ancestor, though the timing is unclear. Some experts maintain that head lice and body lice are the same species, but they fail to explain the differences in physiology.
The difference between head lice and body lice may be related to the way they adapted to different hair shafts. The former remained on the scalp, while the latter evolved to have claws and can cling to smoother clothing fibers. The lice are transferred from one person to another via close personal contact. In order to pass on a head lice infestation, a person must have head-to-head contact with an infected person. Sharing personal items can also spread the infection.
Lice are a form of parasites that are transmitted through the human bloodstream. However, chimpanzees are also known to pick lice eggs off each other to control the population. Scientists believe that lice evolved into two varieties approximately 100,000 years ago.