Thu 8 Mar 2007
Humans have the dubious distinction of being host to three separate species of louse. The head louse, Pediculus humanus, or its direct ancestor has been with us for the longest ride, probably for the entire 12-13 millions of years that primates and head lice have existed. The body louse (also known as the clothing louse) and the pubic louse that prefers that coarser hair for its home developed or hopped aboard later.
The New York Times reported that David L Reed of the University of Florida has been poking around with the DNA of human lice and the lice of other primates and monkeys, and has found some intriguing clues to human development. He has specialized in using the clues in the DNA of host specific parasites to mark the changes in the host behavior or development. This is of particular value where the fossil record does not include these data. One way that DNA yields information is by counting the number of variations or mutations between the DNA of two organisms. The “junk” DNA between protein encoding sections does not have much effect on the organism but mutates at pretty much a constant slow rate with time. The more variations between two DNA samples, the longer since the organisms had a common ancestor.
We share the head louse with chimpanzees, but not gorillas whose hair is different. Lice are pretty particularly adapted for the specific host, and the head louse probably became extinct on the “gorilla habitat” about 7 million years ago when gorillas split from our common ancestor. Similarly the ancestor of the gorilla louse became extinct about the same time on the common ancestor of chimps and humans.
Well, that is the story told in the DNA for the head louse, but how did we get the other two pests? The body louse is adapted for hanging onto threads of fabric, not hair. The body louse is pretty similar to the head louse. Four years ago Mark Stoneking at the Max Planck Institute compared the DNA of the head louse and the body louse. The DNA divergence was pretty small – indicating that the body louse speciated from the head louse only 107,000 years ago. This means that humans wore sewn clothes starting some time not long before 105,000 BCE. The body louse took advantage of the new habitat, sewn clothing, and developed all the specialization needed to succeed in its new home, making a new species of louse.
The pubic louse has a very interesting history as discovered by Dr. Reed in Florida. At some time in the past, humans lost most hair on their bodies, leaving the fine hair on the head and a coarser hair in the pelvic regions. Head lice probably infested the hair all over our distant ancestors as they do with chimpanzees, but as their hosts became more naked their domain retreated to the head. Pubic hair is much coarser, and unsuited to the head louse.
David Reed got samples of gorilla lice from the Mountain Gorilla Veterinary Project and human pubic lice from a public health clinic. DNA comparison between these two species of lice indicate that the gorilla louse and the human pubic louse diverged about 3.3 million years ago by hopping species from gorilla to human. (Lets not talk about how)
This indicates that our ancestors would have lost their body hair by the time of the transfer of the gorilla louse to human hosts, as the hair louse would have been a better competitor in that environment, but with the areas of hair isolated it would have limited the competition.
This indicates a time line to confirm the split of the human-chimp line from the gorilla line at about 7 million years ago, that the Homo line lost most of its hair by 3.3 million years ago, and started wearing sewn clothing about 105,000 BCE.
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