Scientists unraveling the genetic history of head lice have found startling evidence that early humans mingled with a clan of hairy, distant cousins as recently as 25,000 years ago in Asia -- a previously undocumented meeting that appears to have occurred after the two peoples had evolved separately for a million years.
The more brawny but less brainy branch of humanity apparently went extinct soon after that family reunion -- echoing the fate of Neanderthals, who also went extinct after making contact with the predecessors of modern humans around the same time in what is now Europe.
It is not clear what happened during those cross-cultural exchanges. "We may have mated with them; we may have eaten them. There's no way to know," said Alan Rogers, a University of Utah anthropologist who was involved in the new work, being published today. Until recently, scientists had thought that the archaic humans in Asia had died out far too early to have encountered their modern counterparts.
One thing is certain, though. By the time the dust settled, only one line was left: those that led to us.
The research is the latest to use the genes of parasites, including lice, bedbugs and tapeworms, to fill holes in the story of early human evolution left yawning because of gaps in the fossil record. The genes of those intimate if unwelcome guests bear the record, it turns out, of human migration, interaction and, in some cases, extinction.
A year ago, a team of scientists in Germany compared the genes of head lice (which lay their eggs on hair) and body lice (which lay eggs only on clothing) to deduce when people first started wearing clothes (answer: perhaps as recently as 30,000 years ago).
The new study found that the genetic relatedness and geographic distributions of the world's two families of human head lice can best be explained by their having evolved on distinct lines of early humans that went their own ways more than a million years ago -- and then ran into each other in the last few tens of thousands of years.
The more archaic of those two human groups disappeared soon after the encounter, but not before sharing with our forbears their private reserve of lice.
"We've discovered the 'smoking louse' that reveals direct contact between two early species of humans," said Utah's Dale Clayton, who led the study published in the journal PLoS Biology. "Kids today have head lice that evolved on two species of cave men. One species led to us. The other species went extinct."
The findings remain controversial, in part because the team's calculations are subject to wide errors.
"I have serious reservations about their conclusions," said Mark Stoneking, who led last year's study on clothing and whose own data is partially undermined by the new work. Stoneking's comments echoed others' acknowledgments that the art of using parasite genetics as a tool in anthropology is itself still in an early stage of evolution.
The field is promising, however, because parasites evolve in very specialized ways along with their hosts. Human head lice, for example, can scarcely survive a day without a person to feed on. So any person-to-person leap of a louse is almost by definition evidence of direct contact between the two. And the distribution of lines of head lice must mirror the movements of their human hosts.
Study leaders Clayton and David Reed, an evolutionary biologist at the Florida Museum of Natural History in Gainesville, started by collecting lice from around the world to get a measure of the insects' global diversity.
"We had a lot of collaborators that helped us get these lice," Reed said. "People in public health centers. Missionaries deployed around the world. We begged, borrowed and did everything but steal."
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."