A study of head lice has led researchers and scientists from the University of Utah and elsewhere to find that modern humans fought with, ate with, shared clothing and mated with a more primitive human species between 25,000 and 30,000 years ago.
Genetic differences in two species of head lice found on people are the basis for the conclusion, which the study's authors admit is not chiseled on stone plates.
The study is being published online by the Public Library of Science journal called "PLoS Biology."
Lead researcher was David Reed, former U. postdoctoral fellow who works at the Florida Museum of Natural History, University of Florida, Gainesville. Co-authors include Dale Clayton, professor of biology at the U.; Alan R. Rogers, U. professor of anthropology; Vincent Smith, University of Glasgow, Scotland; and Shaless Hammond, who as a high school student worked in Clayton's laboratory.
Two different species of head lice parasitize today's humans, according to the report. Analyzing the rate at which genes change, they calculate that the two species diverged about 1.18 million years ago.
That's close to the time paleontologists postulate that archaic humans left Africa. Modern humans evolved in Africa around 200,000 years ago, the researchers say.
The lice diverged when the groups separated.
Although the earlier humans died out, the lice they carried continue to thrive on human heads, having infected modern people that the earlier species came into contact with.
The lice from the earlier humans only live on people of the New World, who also have the same lice that infect other modern humans.
This has led the scientists to speculate that ancestors of today's Indians came into contact with the earlier type of humans. Head lice jumped from the earlier to modern people 25,000 to 30,000 years ago, Rogers believes.
That date is not based on genetic studies but on when scientists think modern humans reached east Asia and ran into pockets of the earlier types, he said.
Asked how he knows one species of head lice originally was carried by a now-extinct form of human, he replied, "The short answer is that we don't." The argument for believing that may have happened "is more involved."
Lice are found on chimps, humans and other primates. Genetics can give some idea of when the common ancestors of these lice split from each other, he said. Knowing how fast the lice evolve, that gives the figure that they split apart roughly 1 million years ago.
The type of louse carried by modern humans is found worldwide.
Its genetic variations show the same form of bottleneck, then broadening, of a population base that scientists think modern humans went through about 100,000 years ago. That's when homo Sapiens left Africa and began spreading around the world rapidly, the scientists say.
The other type of lice, found only in the New World, shows no such bottleneck and branching out. "So it must not have been on the ancestry of today's humans," Rogers said.
The paper's authors think the ancestors of today's Indians came into contact with the earlier type of humans "that inhabited east Asia" before the Indians arrived in North America, he said.
"We're not going to feel confident about any of these hypotheses until we come up with different ways to test them," Rogers added. "For the moment, everyone should be skeptical."
One hole in the theory is that people in Asia no longer carry the louse that they believe was a parasite on earlier human types. "So what happened to the folks in east Asia?" he asked.
"Why don't the modern people in east Asia have both kinds of lice, too? And I don't have the answer." Rogers said that is a weakness in the theory, which needs to be better understood.
Clayton said a strength of the study is that "it gives us a completely independent data set" to try and understand the history of our species.
With this study, "we don't have to look at human DNA or their fossils. We can look at completely unrelated organisms that happened to be riding along on humans because of their peculiar life history."
If the genetic background of the lice seem to be telling the same sort of tale about diversion that scientists in other fields ascribe to humans, that makes them confident about the findings, he said. Do they? "Well, that piece of it is very similar," Clayton said.
"It's all very speculative," he added. "But the scenario we're painting is the most consistent, simplest explanation, given the data."
He hastened to add that this doesn't mean this theory is necessarily right.
"We need further research," Clayton said. The researchers are planning repeat the study with another group of human lice. "If these show the same patterns, then we can be even more confident with our explanation."