Lice suggest species of humans interbred
Modern humans made direct contact with a now-extinct early human species, according to a new study of DNA from head lice.
The study published online this week in the Public Library of Science journal PLoS Biology argues the species was probably Homo erectus, a chinless individual with a large projecting face, a prominent brow ridge, and a brain that was smaller than ours is today, the study said.
The lice research, which involved both observational and DNA analysis, also provides a mini-history of human evolution from 5.5 million years ago to the present.
Scientists believe the historical evidence is recorded in the genetic makeup of lice that can be found on the heads of many kids and others infected with the pesky parasites.
The first, found worldwide, evolved on our Homo sapiens ancestors. The second, only found in the Americas, evolved on another species of human that researchers believe was Homo erectus.
While the researchers did not directly find a "smoking louse", they did discover that the family tree for lice that infest people closely mirrors the evolutionary family tree for humans.
Analysis of lice DNA mutations revealed that the two identical-looking, yet genetically distinct, varieties of head lice that thrive today diverged 1.18 million years ago.
Humans diverged from other archaic humans at nearly the same time, 1.2 million years ago. According to the press releases, this indicates each of the two kinds of head lice infested a different species of early human.
"The DNA differences in the two lineages of lice found on modern humans could only have evolved in groups of humans that were not interacting for a period of a million years or more," study leader Professor Dale Clayton, of the University of Utah said.
"Otherwise, the two lineages would have interbred, preventing differences of this magnitude from arising in the first place."
"The best candidate groups of hosts for such isolation would have been the lineage that left Africa very early, leading to H. erectus, and that which stayed behind, leading to H. sapiens, eventually."
The scientists think the lice that once exclusively fed on Homo erectus jumped to Homo sapiens during contact in Asia 25,000-30,000 years ago.
"As you know, H. erectus eventually went extinct, but its lice did not," Clayton said.
"The only way to explain this is that some of the H. erectus lice moved onto H. sapiens before H. erectus went extinct. That requires direct contact because human lice cannot survive off the body of a host for more than 24 hours."
What form the sapiens-erectus contact took remains unclear.
"It could have been fighting, stealing skins or (stealing) clothing," Clayton said.
Dr David Reed, first author of the PLoS Biology paper and an assistant curator of mammals at the University of Florida's Florida Museum of Natural History, said the two types of humans and lice could have met in communal housing.
"It could be that in harsh climates the number of suitable refuge sites were limited, increasing the likelihood of either aggression to secure them, or the desire to share them," Reed said.
Dr Kevin Johnson, an expert on the use of DNA from lice in inferring the geographic and evolutionary history of the parasite's hosts said: "In general, this is a very exciting paper that leads to lots of new questions."
But Johnson, from the Illinois Natural History Survey, thinks that the lice's mitochondrial DNA (which is passed on maternally) show that the two types of lice diverged 300,000 years ago, which does not match up with the proposed Homo sapiens and lice divergences 1.18-1.2 million years ago.
In fact, he said, it is possible modern humans never met Homo erectus.
"Pediculus humanus (the human body louse) is one of the most transmissible pathogens in humans, so it could be easily transmitted between species without direct contact," Johnson said. "There are several cases of bird lice being found on multiple host species that have little direct contact."
Questions surrounding the possible sapiens meeting with erectus may soon be resolved, however, as Reed is next planning a study of pubic, or crab, lice, which are primarily spread by sex.
The upcoming study should confirm or disprove the possibility that modern humans mated with Homo erectus.