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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.