Genetic Study of Lice Hints At Clash of Archaic Humans

Clayton even took a trip to Papua New Guinea, where he asked a village elder whether he would mind collecting some lice. Clayton "showed them what they looked like under a microscope," Reed said. "They were fascinated."

Genetic analysis revealed two distinct families of head lice -- one found worldwide and the other only in the New World. By comparing the degree of genetic difference between those two louse families to the differences among human, chimpanzee and baboon lice, the team calculated that the human louse families branched off from each other about 1.1 million years ago.

That's just about the time that, according to fossils, early humans evolving in Africa underwent a split into two lineages. One, which remained in Africa until 50,000 to 100,000 years ago, eventually gave rise to modern humans. The other, known as Homo erectus, migrated to Asia and eventually disappeared.

Scientists once thought that H. erectus went extinct about 400,000 years ago, but more recent evidence suggests they survived in Asia as an archaic form of humans, perhaps until as recently as 25,000 to 50,000 years ago -- when the human line that was soon to dominate the world was making its big expansion out of Africa.

And while few until now thought our migrating forbears made any contact with archaic Asians, genetic and geographic patterns suggest the two lines of lice began mixing their genes around then -- evidence that their hosts had some kind of direct contact.

Because that meeting was so recent by evolutionary standards, the two louse lines today remain quite distinct. Both apparently arrived in the New World on the heads of humans who made their way there from Asia. The rest of the world got just one louse line, from the final out-of-Africa expansion.

The meeting in Asia would almost certainly have been strange.

"Think of how scary it must have been to see this primitive-looking human you haven't seen in a million years," Reed said. "They were fit and robust, but it seems we had the brain power."

Exactly what kind of contact they had remains a matter of speculation. "It could have been stealing clothes or sleeping together communally, or it could even be they were mating," Reed said.

But a good bet, he said, is that they fought to the death -- especially given the similar disappearance of Neanderthals around the same time.

Climate change or other factors may have contributed to those extinctions. However, Reed said: "Suddenly our species comes on the scene and two others meet their demise. It certainly looks suspicious to me."