Interbreeding: louse study leads to head scratching

A challenging question about the origin of fully modern humans is whether or not Homo sapiens interbred with archaic species, such as the Neanderthals or H. erectus.  That modern humans occupied the same territory as both, at the same time, is well established for Europe and Asia.  The likely time for the first major migration of moderns from Africa is about 70 to 100 thousand years ago, and archaic humans did not become extinct in Eurasia until 30 ka at the earliest.  Genetic material from extinct humans is rare and difficult to analyse because of degradation.  A couple of mtDNA samples from Neanderthal remains give results that are sufficiently different from ours to rule out retention in modern human populations of the genetic outcome of any interbreeding between ancestral moderns and the population to which the two Neanderthals belonged.  Yet it does not rule out such interactions with other archaic groups.  We have no idea of the genetic diversity of Neanderthals, whose lineage probably split from that of our own (through that of H. heidelburgensis) as long ago as 700 ka.  If they lived in isolated bands of a small population, that diversity could have become substantial over such a long time.    So far, no genetic material has been recovered from H. erectus remains.  Another approach to the matter has emerged from a genetic study of human head and body lice – Pediculus humanus (Reed DL. et al. 2004.  Genetic analysis of lice supports direct contact between modern and archaic humans. Public Library of Science Biology, v. 2, e340through www.plos.org). The louse Pediculus humanus is unique to humans, and genetic comparison with that which infests chimpanzees suggests that these two species diverged at about the same time as the split that led to modern humans and chimps, at about 5.6 Ma. That is remarkably similar to molecular timing that uses primate DNA.  The interesting feature of the louse genetic analyses by the team from the Universities of Florida, Utah and Glasgow is that there are differences between the lice that leap on us.  There are two strains which originated before 1 Ma ago, according to the molecular clock.  One has a global distribution, and infests both head and body, whereas the other is exclusively a head louse and only occurs in the Americas.

Around 1 Ma there seems also to have been a major divergence among early humans between a strand of H. erectus, which survived until as recently as 20 ka in Asia, and one that led to European Neanderthals and the modern humans who began to migrate from Africa to Eurasia around 100 ka.  The unique occurrence of the head-only louse in the Americas (along with the other strain) suggests that the modern humans who crossed the Bering Straits to colonise the Americas came into direct physical contact with beings who carried that particular strain, en route.  The likely candidates would have been Asian H. erectus.  Contact had to be direct, because, unlike the flea, the louse cannot leap, and it can only survive on humans.  The lack of the New World Pediculus humanus in Eurasia suggests two things: if moderns were “in touch” with archaics, the latter carried the other variant (Neanderthals?); the present Asian population (and that of New Guinea and Australia) possibly did not have close contact with archaics who were alive at the time of colonisation (were there by then very few?).  All very interesting, but it does not resolve the question of interbreeding; intimate contact could have been through fighting, trading or interbreeding.  There is another, very different human-only louse, Pthirus pubis, which infests pubic hair only, and about which there is very little genetic information, so far…

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