"The record of our past is written in our parasites..."

Genetic analysis of lice supports direct contact between modern and archaic humans


1, Department of Biology, University of Utah, 257 S 1400 E, Salt Lake City, UT 84112. USA.
2, Present address, Florida Museum of Natural History, Dickinson Hall, University of Florida, Gainesville, FL, 32611. USA.
3, Graham Kerr Building, DEEB, IBLS, University of Glasgow, Glasgow, G12 8QQ.
4, Present address: Illinois Natural History Survey, 607 East Peabody Drive, Champaign, IL 61820-6970, USA.
5 , Department of Anthropology, 270 S 1400 E, University of Utah, Salt Lake City, UT 84112. USA.

First & Corresponding Author: David L. Reed
e-mail: dreed@flmnh.ufl.edu

Status: Published in Public Library of Science (PLoS) - Biology

Mini Press Release

"The record of our past is written in our parasites..." (Alan Rogers, professor of anthropology at the University of Utah)

Questions concerning exactly how and when modern humans arose from their primate ancestors are amongst the most intensely debated topics in evolutionary biology. Fragmentary fossils and patchy genetic data provide key insights but fail to address a key question - did modern and archaic human species come into direct contact? Our study published in PLoS-Biology claims that not only did they meet, but that our ancient human relatives brought with them an unwelcome side effect - their head lice!

In this collaborative project between scientists at the universities of Florida, Utah (USA) and Glasgow (UK) we compared the evolutionary history of parasitic lice with that of humans and their primate ancestors. We show that human head lice are composed of two distinct lineages, one of which evolved on the scalps of another human species before making the jump to our own. One group has a worldwide distribution, while another, less common type is found only in the Americas. Just like modern humans (Homo sapiens), the group of head lice found worldwide underwent a so-called population "bottleneck" - an event that cuts the amount of genetic diversity in a population. These different groups of head lice diverged from each other around 1.18 million years ago. We propose that the less common group evolved on an extinct group of humans (perhaps Homo erectus) which remained isolated from our ancestors until some tens of thousands of years ago, when they re-established contact with each other. One million years ago, Homo erectus was established both in Africa and in East Asia. In Asia, Homo erectus could have remained isolated until a second wave of migration out of Africa brought modern humans into contact with them and their lice, less than 100,000 years ago. The research also supports the 'Out-of-Africa' theory of human evolution, in which Homo sapiens replaced earlier human groups throughout the world.


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Parasites can be used as unique markers to investigate host evolutionary history, independent of host data. Here we show that modern human head lice, Pediculus humanus, are composed of two ancient lineages, whose origin predates modern Homo sapiens by an order of magnitude (ca. 1.18 million years). One of the two louse lineages has a worldwide distribution and appears to have undergone a population bottleneck ca. 100,000 years ago along with its modern H. sapiens host. Phylogenetic and population genetic data suggest that the other lineage, found only in the New World, has remained isolated from the worldwide lineage for the last 1.18 million years. The ancient divergence between these two lice is contemporaneous with splits among early species of Homo, and cospeciation analyses suggest that the two louse lineages codiverged with a now-extinct species of Homo and the lineage leading to modern H. sapiens. If these lice indeed codiverged with their hosts ca. 1.18 MYA, then a recent host switch from an archaic species of Homo to modern H. sapiens is required to explain the occurrence of both lineages on modern H. sapiens. Such a host switch would require direct physical contact between modern and archaic forms of Homo.


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Literature Citation
Reed, D.L., Smith, V.S., Rogers, A.R., Hammond, S.L., Clayton, D.H. 2004. Molecular genetic analysis of human lice supports direct contact between modern and archaic humans. Public Library of Science (PLoS) - Biology. 2 (11): e340.

Data Files

Data matrices are available from here in NEXUS format:

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Morphological Dataset

(No character state labels, observations or images)

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Molecular Dataset

Aligned molecular sequence data
(Yield Figs 1 & 2)

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Molecular Dataset

Aligned molecular sequence data
(Yield Fig. 4)

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Queries concerning these datasets should be directed to David Reed or Vince Smith.

Media Coverage

The findings of this paper have been reported in the New York Times, Washington Post, USA Today, The Times (London), amongst many others. In addition, Nature News, Science Now, and New Scientist, along with BBC News on-line have covered the story. The paper was also reported in Science, featured on the front page of the National Science Foundation website, and was the most downloaded paper from PLoS-Biology website for a period after its publication. A compilation of PDF files documenting the coverage within the first few days of the papers release can be downloaded from here (note this is a very large file - 7.7MB).

Selected Radio Interviews (Available to download in MP3 format)

(2.8 Mb, 3 min. 5 sec.) BBC World Service, News Hour, broadcast 6 October, 2004
(3.8 Mb, 8 min. 24 sec.) CBS Radio Canada, Quirks and Quarks, broadcast 16 October, 2004

Publicity Images*

Promotional Fig. 1 (High Res TIFF File)
Battling an ancient enemy. Methods of head louse control haven't changes much over the centuries, neither have the lice. The picture shows a 6th century wooden nit comb from the Egyptian city of Antinoe, alongside its modern plastic counterpart. Inset is a close-up of a human head louse. Photograph of Egyptian comb used with permission from Te Papa, Wellington, New Zealand, negative number F.003884/5. Copyright information available in the first instance from Vince Smith.

Promotional Fig. 2 (High Res TIFF File)
Enlarged image of a human head louse (Pediculus humanus). Copyright information available from Vince Smith.



* Note that these figures do not constitute part of the publication and are intended for promotional use of this paper. High resolution images are available in TIFF format from the links above. Copyright permission must be sought from the relevant contact listed by each figure BEFORE use. Requests for use of multiple figures can be directed to Vince Smith in the first instance.

Figures from the Paper

Figure 5 (High Res TIFF File- available from David Reed.)
Temporal and geographical distribution of hominid populations redrawn from Stringer (2003). This figure depicts one view of human evolutionary history based on fossil data. Other interpretations differ primarily in the taxonomy and geographical distribution of hominid species. The temporal distribution of the two divergent lineages of human lice (Pediculus humanus) is superimposed on the hominid tree to show host evolutionary events that were contemporaneous with the origin of P. humanus. Whereas the New World lineage is depicted on Homo erectus in this figure, several alternative hypotheses are consistent with our data when other evolutionary histories of hominids are considered (not shown). The World wide clade of lice is shown in red and the New World clade in blue (see paper for descriptions of these groups).

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