An analysis of a complete 1.8-million-year-old hominid skull found at the archaeological site of Dmanisi in Georgia suggests the earliest Homo species – Homo habilis, Homo rudolfensis and so forth – actually belonged to the same species.

The skull fossil, called Skull 5, is the world’s first completely preserved adult hominid skull from the early Pleistocene.

Unlike other Homo fossils, Skull 5 combines a small braincase with a long face and large teeth. It was discovered alongside the remains of four other early human ancestors, a variety of animal fossils and some stone tools – all of them associated with the same location and time period – which make the find truly unique.

The archaeological site of Dmanisi, located in the Kvemo Kartli region of Georgia about 93 km southwest of the capital Tbilisi, has only been partially excavated so far, but it’s already providing the first opportunity for anthropologists to compare and contrast the physical traits of multiple human ancestors that apparently coincided in the same time and geological space.

“The differences between these Dmanisi fossils are no more pronounced than those between five modern humans or five chimpanzees,” said Dr David Lordkipanidze from the Georgian National Museum in Tbilisi, a lead author of a paper in the journal Science and co-author of a paper published in the Proceedings of the National Academy of Sciences.

Traditionally, researchers have used variation among Homo fossils to define different species. But in light of these new findings, Dr Lordkipanidze and his colleagues suggest that early, diverse Homo fossils, with their origins in Africa, actually represent variation among members of a single, evolving lineage – most appropriately, Homo erectus.

“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 Dr Christoph Zollikofer from the Anthropological Institute and Museum in Zurich, Switzerland, a co-author of the Science paper.

That’s because Skull 5 unites some key features, like the tiny braincase and large face, which had not been observed together in an early Homo fossil until now.

Given their diverse physical traits, the fossils associated with Skull 5 at Dmanisi can be compared to various Homo fossils, including those found in Africa, dating back to about 2.4 million years ago, as well as others unearthed in Asia and Europe, which are 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,” Dr Zollikofer said.

“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 fossils from Dmanisi represent ancient human ancestors from the early Pleistocene epoch, soon after early Homo diverged from Australopithecus and dispersed from Africa.

The jaw associated with Skull 5 was found five years before the cranium was discovered but when the two pieces were put together, they formed the most massively built skull ever found at the Dmanisi site. For this reason, the team suggests that the individual to whom Skull 5 belonged was male.

The braincase of Skull 5 is only about 33.3 cubic inches (546 cubic cm), however, which suggests that this early Homo had a small brain despite his modern human-like limb proportions and body size.

“Thanks to the relatively large Dmanisi sample, we see a lot of variation. But the amount of variation does not exceed that found in modern populations of our own species, nor in chimps and bonobos,” Dr Zollikofer said.

“Furthermore, since we see a similar pattern and range of variation in the African fossil record… it is sensible to assume that there was a single Homo species at that time in Africa. And since the Dmanisi hominids are so similar to the African ones, we further assume that they both represent the same species.”

Skull 5 seemingly indicates that, rather than several ecologically specialized Homo species, a single Homo species – able to cope with a variety of ecosystems – emerged from the African continent.

And accordingly, our classification system for these early human ancestors may never be the same.

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Bibliographic information: David Lordkipanidze et al. 2013. A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo. Science, vol. 342, no. 6156, pp. 326-331; doi: 10.1126/science.1238484

Ann Margvelashvili et al. Tooth wear and dentoalveolar remodeling are key factors of morphological variation in the Dmanisi mandibles. PNAS, published online October 7, 2013; doi: 10.1073/pnas.1316052110