Using lice to study clothing

By Rachel Whelan
April 3, 2007;  The Stanford Daily Online

Though humans’ early coverings were sturdy enough to shield us from the tribulations of nature, they were too delicate to be preserved in the fossil record - so scientists had nearly given up on determining when we started wearing clothing. That is, until Mark Stoneking and his colleagues at the Max Planck Institute in Germany decided to look at lice.

Humans are host to three species of lice, which differ mainly in where they live on our bodies. Head lice are perfectly adapted to latch onto the fibers of the scalp, whereas body lice grab onto clothing fibers. All human lice need constant contact with the body’s blood and warmth to survive, but each species is comfortable only in its own niche: a head louse transferred to an individual’s shirt will rush back towards her scalp.

Knowing that body lice originated from head lice, Stoneking and his colleagues assumed that the date when body lice appeared should correspond with when clothing became common. Early clothing would probably have been worn tightly enough against the skin to encourage some head lice to move out and set up camp in a new, wide-open ecological niche-cloth fibers. The ability to colonize clothing fibers gave this new species of body lice access to the rest of the human body that wasn’t covered in hair.

To date the origin of body lice, the researchers used a molecular clock approach. After using the fossil record to establish the mutation rate of lice DNA, they deduced how much time had elapsed since the two lice species diverged by counting up the number of differences between their DNA sequences.

Stoneking examined a global sample of head and body lice cells, looking at DNA in both the cells’ mitochondria and their nuclei. But in the end, it was the mitochondrial DNA that was key to dating species divergence.

Stored on chromosomes in the nucleus, nuclear DNA genes are rearranged by about 50% during the formation of sperm and egg cells, in a process called recombination. Mitochondrial DNA, on the other hand, does not undergo recombination, making mutations easier to track since the genes stay in the same place on each chromosome.

The earliest mitochondrial genes contain only head lice DNA, supporting the consensus that they existed first. Then, around 72,000 years ago, lice genes appear that are specific to body lice. Stoneking concluded that body lice - and by inference clothing - thus originated 72,000 years ago.

But though Stoneking’s results were aimed at solving a specific open historical question, like other archaelogical studies, it is hedged with a wide margin of error. In Stoneking’s case, the margin of error was 42,000 years - something that may throw a wrench in any future plans to record the speed of changes in prehistoric fashion.