History of Head Lice

Using nuclear and mitochondrial DNA markers, Reed et al. [2] estimated the divergence of chimp and human Pediculus lice at 5.5 million years ago (MYA) and provided evidence of cospeciation with their hosts. A recent revision to 4.1 MYA for the most recent common ancestor of chimps and humans [13] may require a similar adjustment of the louse molecular clock. What is more remarkable, however, is that Reed et al. [2] found that human lice split into two quite distinct clades, A and B, about 1.18 MYA. There is a worldwide clade (which includes both head and body lice) and a New World clade (exclusively head lice). So how can humans harbor two clades of louse that diverged from each other over one million years ago, when that separation is tenfold older than the emergence of Homo sapiens?

The answer, Reed et al. [2] suggested, is that the separation took place around the time of divergence of the ancestors of modern humans from Homo erectus.
These two hominid lineages then co-existed for about one million years until the demise of H. erectus. When modern humans radiated across Asia they might have had contact with H. erectus, just as in more recent millennia H. sapiens met H. Netherlands in Europe, as dramatized by the Nobel laureate William Golding [14]. There is no evidence that different human species interbred, but they may well have exchanged ectoparasites. Thus, the New World clade of head louse may have crossed horizontally from H. erectus to H. sapiens within the last 100,000 years.

Zinsser [1] noted that the hair of ancient Peruvian mummies and the scalps of pre-Columbian Native Americans contained nits or lice. Recent DNA analysis of lice from similar remains indicates that they belong to the worldwide clade A, so this clade must have been present in pre-Columbian American populations [15]. A third clade of head lice has been delineated in Ethiopia and Nepal and this clade, C, diverged from clades A and B about 2 MYA [16]. If Reed et al. [2] were correct to postulate that clade B came from H. erectus, one must wonder in which hominid population might clade C lice have maintained their separate identity.


By:Robin A Weiss
Division of Infection and Immunity, University College London, 46 Cleveland Street, London W1T 4JF