Tiny travel companions

Since the first modern humans left their birthplace in Eastern Africa and slowly spread to populate every inhabitable place on the planet, Homo sapiens has been a migratory animal. Over thousands of years, these travellers unknowingly carried excess baggage on their journeys: countless microbes in and on their bodies, which evolved along with their host. By applying the tools of modern genomics to reconstruct the phylogenetic trees of these viruses, bacteria, fungi and protozoa, scientists hope to provide new insights into how our ancestors ventured throughout the world. Answering the questions of when and how humans started moving out of Africa or when the first settlers populated the Americas has so far relied on genetic markers from mitochondrial DNA (mtDNA) or the Y chromosome. As mitochondria are inherited from the mother, analysis of mtDNA identifies ancestors through the maternal line, whereas Y-DNA can trace origins using the paternal line. These studies, together with archaeological, anthropological and linguistic evidence, have generated a now widely accepted “standard model for evolution of modern humans” (Cavalli-Sforza & Feldman, 2003). According to this model, our species moved from its original homeland in Africa to the Near East approximately 60,000– 70,000 years ago, later spreading to southern Asia and Australasia, then to Europe and Central Asia, and finally entered the Americas through the Bering Strait about 14,000–20,000 years ago (Fig 1).