Traditionally, researchers have used variation among Homo fossils to define different species. But a new study suggests that early Homo fossils, originating in Africa, actually represent variation among members of a single, evolving lineage of Homo erectus.
The 1.8-million-year-old skull, known as Skull 5, was unearthed in Dmanisi, Georgia and features a small braincase with a long face and large teeth. The skull was found near animal fossils and stone tools from the same period.
David Lordkipanidze from the Georgian National Museum, along with colleagues from Switzerland, Israel and the United States, say that the differences between these Dmanisi fossils are no more pronounced than those between five modern humans.
"Had the braincase and the face of Skull 5 been found as separate fossils at different sites in Africa, they might have been attributed to different species," said co-author Christoph Zollikofer from the Anthropological Institute and Museum in Zurich, Switzerland.
Other fossils from the Skull 5 site, showing diverse traits, can be compared to various Homo fossils, including those found in Africa, dating back to about 2.4 million years ago, and fossils unearthed in Asia and Europe dated between 1.8 and 1.2 million years ago.
"[The Dmanisi finds] look quite different from one another, so it's tempting to publish them as different species," explained Zollikofer. "Yet we know that these individuals came from the same location and the same geological time, so they could, in principle, represent a single population of a single species."
The Dmanisi fossils represent ancient human ancestors from the early Pleistocene epoch, soon after early Homo diverged from Australopithecus and dispersed from Africa.
The findings, published in the journal Science, suggest that rather than a variety of species including Homo habilis, Homo rudolfensis and Homo erectus, a single, highly adaptable Homo species emerged from the African continent.