Human head lice come in two genetically distinct varieties, one that
is distributed worldwide and another that is found only in the Americas.
Examination of the DNA of both types of lice has suggested that they
probably diverged from a common ancestor nearly 1.2 million years ago
and have remained separated for most of the time since. This result has
leads to some fascinating implications regarding the trajectory of
human evolution. Firstly, this divergence happened quite early, around the time when our ancestors split off from our cousin species, homo erectus. Secondly, given that lice are parasites that have evolved to live exclusively on their human hosts, the two almost identical-looking strains were probably separated by the speciation of the humans themselves. Finally, when the lice did come back into contact this would have been when the two species of human came back into contact, perhaps as recently as 25,000 to 50,000 years ago.
This is quite a statement. It means that modern humans may have got infested by the second species of lice through direct contact with another species of human being, either by touching, fighting, cannabalism, mating or all of the above. Regarding the mating question, the researchers are now thinking of looking into the genetic histories of crab lice which can be only sexually transmitted. However despite this possibility, they are also supporting the dominant theory that when modern humans came into contact with other species, they completely replaced them.
There are also other interesting things to come out of the genetic data. Studies of human DNA have suggested that modern human evolution may have passed through a "bottleneck" where the entire world's population was found to have been descended from a population around 100,000 years ago that had collapsed to a total of less than 10,000 individuals. This was presumably because of some natural disaster or disease outbreak which had nearly wiped out the entire species. This bottleneck is also reflected in a similar collapse evident in the DNA of the world-wide variety of lice but not in the American one (which is thought to have been associated with the archaic human strain).
The fact that second kind of lice is restricted to the Americas, for me, also throws this possible discovery of homo erectus bones around a lake in Mexico into an even more interesting light.
More information on this research into lice DNA can be found in this press release from the University of Utah site.