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). Ree
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