Our ancestors may have caught nits from their primitive cousins.
Humankind's inexorable spread across the globe may have had an unwelcome side-effect - it brought us face to face with our ancient hominid relatives just long enough to catch their head lice. Researchers have discovered that there are two distinct lineages of modern lice, one of which presumably evolved on the scalps of another human species before making the jump to our own.
Homo sapiens probably picked up the second type of louse about 25,000 years ago as we swept through Asia, say David Reed of the Florida Museum of Natural History, Gainesville, and his colleagues. That would mean we bumped into our evolutionary cousin Homo erectus after the hominid had lived undisturbed in the East for more than a million years.
Reed and his colleagues compared the DNA of head and body lice (Pediculus humanus) collected from all over the world. Unexpectedly, they found two distinct genetic groups of head lice, despite the fact that they look almost identical.
By measuring the amount of genetic difference between the two groups, the researchers calculated that the two louse lineages split around 1.2 million years ago - about the time that the ancestors of H. erectus were leaving our birthplace, Africa, for pastures new.
The only way the two groups could have become so genetically different is if they were living not only on different heads, but on different species on different continents, the researchers argue. The most likely explanation is that by the time H. sapiens evolved in Africa about 200,000 years ago we had our own lineage of head lice, and then picked up more from H. erectus on our travels, says Reed. The study is reported in the current issue of PLoS Biology1.
The theory is bolstered by the fact that one of the louse groups is found on people all over the world and the other is practically exclusive to the Americas. This is consistent with the idea that the latter was acquired relatively recently by a population of H. sapiens, who ultimately found their way across the Bering Strait.
What might such an encounter between the two hominid species have involved? We may have fought, traded or reused clothes, or even mated with our distant relative, Reed suggests. Whatever, the encounter must have involved close contact with clothes or bodies.
"Some degree of human contact would have been necessary, although this was not necessarily extensive or prolonged," says Chris Stringer, who studies human evolution at the Natural History Museum in London. He adds that there is evidence that modern and Neanderthal humans may have intermingled in Europe at about the same time, although DNA evidence suggests that they did not interbreed.
The idea that H. sapiens met its relatives head-on without mating with them offers further support for the widely accepted "out of Africa" theory, which suggests that, from our African origins, we rampaged across the globe, either directly or indirectly displacing other hominid species.
One problem with Reed's idea is that the most recent H. erectus fossils found in Asia are about 50,000 years old, long before the supposed meeting. But fossil-hunters may yet find much younger ones if they redouble their efforts, he says: "We might find it if we sample Asia better."
Reed aims to study more parasites that might yield similar clues to our evolution and migration, such as bedbugs, tapeworms and pinworms. "We're only scratching at the surface of what parasites can tell us - pun intended," he says.
He also hopes to answer the question of whether we had sexual contact with H. erectus by studying the genetics of pubic lice (Phthirus pubis). Although these can also be spread when people swap clothes, the main route of transmission is sexual.
But Reed maintains that our ancestors' exploits in Asia were impressive enough anyway. "The thing that blows me away is the idea of seeing and touching another human with whom you've shared no common ancestor for a million years," he says.
ReedD. L., et al. PLoS Biology 2, e340. (2004).
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Hopkin, M. Early man left trail of lice. Nature (2004). https://doi.org/10.1038/news041004-4