The story of modern humans’ final interactions with the extinct Homo erectus may have been revealed by research into the evolution of lice.
The genetic differences between two types of human lice – which share a common ancestor – suggest that they spent most of the last 1.18 million years living on two distinct hominid species, but places their divergence into two distinct lice species slightly later than the split between Homo sapiens and Homo erectus.
The results suggest that our ancestors must have interacted with H. erectus either through fighting, sharing clothes or having sex, since both types of lice are now found on modern man and lice can only jump from one species to another if there is direct contact.
“We know that Homo erectus and Homo sapiens were both living in Asia 50 to 25 thousand years ago,” says lead author David Reed of the Florida Museum of Natural History in Gainesville, Florida.
New World louse
Of the two types of lice found on humans today, one lives on the body or the head of people all around the world, while the other is found only on the head and is unique to the Americas.
Reed and colleagues found that the two louse lineages were very different, making it extremely unlikely that they evolved to their current state on the same species. It is probable, he says, that the louse found only in the Americas evolved on H. erectus while the worldwide louse evolved on the line that eventually became H. sapiens.
Relatively recently - approximately 100,000 years ago - the New World louse found its way back onto H. sapiens in Asia who then carried it with them to the Americas. The researchers say there must have been some contact between the two hominid species before H. erectus finally died out.
But there are alternative explanations for the findings. Mark Stoneking, an expert on human evolution from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, says the findings depend on the last common ancestor of the two species of lice having lived almost 1.2 million years ago. The calculation to obtain this figure depends on the size of the lice population at that time. So if the population estimates are inaccurate, the results may be misleading, he says.
“I have serious reservations about their conclusions,” says Stoneking. “While contact between modern and archaic humans is still a possible explanation for their results, other explanations are more likely in my view.”
Lice and other parasites are useful for studying human evolution because, although their life span is much shorter, they often only get a chance to breed with lice on a different host when both hosts mate, says Reed. “Whatever forces of nature humans are exposed to, lice record the same history,” he adds.
Work with pubic lice could provide an even better insight into the possible interactions of early humans. “I think there are a lot more stories to tell from lice - and parasites in general,” says Reed. “Pthirus pubis could lead us to the holy grail of anthropology: whether or not Homo sapiens and Homo erectus interbred.”
Journal reference: Public Library of Science Biology (DOI: 10.1371/journal.pbio.0020378)