By Paul Rincon
BBC News Online science staff
Some head lice infesting people today were probably spread to us thousands of years ago by an extinct species of early human, a genetics study reveals.
The evolutionary history of head lice is tied very closely to that of their hosts
It shows that when our ancestors left Africa after 100,000 years ago, they made direct contact with tribes of "archaic" peoples, probably in Asia.
Lice could have jumped from them on to our ancestors during fights, sex, clothes-sharing or even cannibalism.
Details of the research appear in the open access journal Plos Biology.
The evidence comes from a genetic analysis of the human head louse (Pediculus humanus). Researchers found two types living on humans today.
One group has a worldwide distribution, while another, less common, type is found only in the Americas.
Of lice and men
Because head lice are unable to survive more than a few hours or days away from a human, their evolutionary history is tied in very closely to that of their hosts.
Just like Homo sapiens, the group of head lice found worldwide underwent a so-called population "bottleneck". This is an event that cuts the amount of genetic diversity in a population.
One explanation is that the human population was reduced in size before it expanded again after 100,000 years ago, as small bands of hunters left Africa. As human populations mushroomed, so did those of head lice that lived on them.
However, the less common group of lice do not exhibit this signature. Their genes suggest population sizes in the past were much more stable.
The analysis revealed the two different groups of head lice diverged from each other around 1.18 million years ago.
The study authors propose that the less common group evolved on an extinct group of humans which remained isolated from our ancestors until some tens of thousands of years ago, when they re-established contact with each other.
David Reed, from the Florida Museum of Natural History and the lead author on the new research, commented: "We either battled with them, or lived with them or mated with them. Regardless, we touched them, and that is pretty dramatic to think about."
Professor Chris Stringer, head of human origins at London's Natural History Museum, told BBC News: "Some degree of human contact would have been necessary to reunite the two lineages of head lice found in recent humans, although the contact was not necessarily extensive or prolonged."
It is well known that our ancestors overlapped for thousands of years in Europe with that continent's original inhabitants, the Neanderthals.
"There is indirect evidence of contact between modern humans and Neanderthals in Europe in the form of exchanges of concepts or actual items of body decoration," said Professor Stringer.
"However, the lice divergence date looks too ancient for the inferred separation time of Neanderthals and modern humans."
The paper's authors agree and instead suggest that it may fit a more ancient separation between the human lineage that led to Homo sapiens and one that led to a species known as Homo erectus.
This split probably occurred anywhere between 1.8 million years ago and 1.2 million years ago.
"If you do the calibrations on the split, that's the only one that makes any sense," co-author Dr Vince Smith, of the University of Illinois at Urbana-Champaign, told BBC News.
By one million years ago, Homo erectus was established both in Africa and in East Asia. In Asia, erectus could have remained isolated until a second wave of migration out of Africa brought modern humans into contact with them - and their lice - after 100,000 years ago.
Dates on animal teeth recovered with erectus fossils at Ngandong on the island of Java, places them there between 53,000 and 27,000 years ago - at a time when our own ancestors were appearing elsewhere in the world.
However, this does not explain why the louse lineage that evolved on archaic humans is restricted to the Americas.
"There must have been some contact between archaic humans and modern humans in Asia. The modern humans then moved into the New World via Beringia (an ancient land bridge between Siberia and Alaska)," said Dr Smith.
"But there are still a lot of questions that need to be resolved."
The researchers now want to study other human parasites to discover whether similar patterns are written in their DNA.
In particular, a genetic study of pubic lice would help support or refute theories of intimate contact, says Smith.
But Frank Huffman, an anthropologist at the University of Texas at Austin, US, is cautious about making the link between Homo sapiens and H. erectus in Asia.
A 6th century nit comb from Egypt
"There's good evidence to place Homo erectus and modern humans together in time, but not necessarily spatially together," he told BBC News.
The research has pleased proponents of the Out of Africa theory of human evolution, in which Homo sapiens replaced earlier human groups throughout the world.
It would also, therefore, seem to reject the so-called Multiregional Hypothesis, in which modern humans evolved from "archaic" populations like Homo erectus and the Neanderthals already established throughout the Old World.