One of the hottest debates in the study of human origins centers on whether modern Homo sapiens interbred with archaic humans. Although genetic data have provided insight into recent human evolution, fossils remain the only available evidence for many archaic human species, but this is notoriously sparse, leaving the data open to multiple interpretations. In the open access journal PLoS Biology, David Reed and colleagues circumvent the lack of human data by analyzing the next best thing - head lice - and provide intriguing evidence that H. sapiens might have been in contact with H. erectus.
Lice require direct physical contact between hosts for transmission and consequently their genetic sequence can mirror the evolutionary fortune of their hosts. Reed and colleagues reconstructed the evolutionary history of the head/body louse Pediculus humanus, by comparing the appearance and genes of this and related species of lice. They confirmed previous results that P. humanus comprises two evolutionary lineages-one contains both head and body forms and has a worldwide distribution; the other contains only the head louse and is restricted to the New World. However, their analyses also provide evidence that P. humanus must have originated long before its H. sapiens host and that the New World lineage remained isolated from the worldwide one for most of the past 1.18 million years. It is unlikely, the authors argue, that two ancient louse lineages could embark on such different evolutionary histories on the back (or head) of a single host. More likely, the New World louse evolved on an archaic form of humans and subsequently transferred to a modern version.
The split between modern humans (Homo sapiens) and one of our archaic forerunners, Homo erectus, took place about 1.8 million years ago but is statistically consistent with the more recent divergence time of the lice (at 1.18 mya). Reed and colleagues therefore propose a scenario in which H. sapiens and H. erectus carried distinct types of lice. As the first waves of modern humans left Africa about 100,000 years ago and modern humans replaced archaic forms, the two forms engaged in enough contact-whether in the form of fighting, swapping clothes, or interbreeding-for archaic lice to make the switch to modern human hosts. Tackling the question of interbreeding, the authors suggest, might now best be pursued by studying the other human louse P. pubis, which requires sexual contact for transmission.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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