Chris Stringer: Skull 5 team proposes a radical rethink, but there are doubts to be overcome

The site of Dmanisi in Georgia has been famous for its fossils since a human jawbone was discovered there more than 20 years ago. Since then, David Lordkipanidze and his team have found hundreds more fossils, including the most complete collection of skulls and jaws of the early human species Homo erectus found so far.

They date from about 1.8 million years ago, and were recognised as particularly primitive examples of the species, small-brained and associated with very simple stone tools. In 2000, a human jawbone far larger than any other from Dmanisi was discovered, leading some scientists to classify it as a distinct species from the rest. But now that the corresponding skull (Skull 5) has been found, the Dmanisi team have not only rethought the classification of their human fossils but propose a radical rethink of all the human fossils from this time period.

Skull 5, like its previously known jawbone, is far larger than any of the others from Dmanisi, suggesting that it belonged to a robustly built and relatively old male individual, with a long, projecting face, and large teeth. Yet it has the smallest brain capacity of the series, no larger than those of much more primitive African fossils and even living gorillas.

This particular combination of features has not been seen in a relatively complete skull before, and the Dmanisi team use the new information to propose that their fossils are demonstrating normal variation in a population of Homo erectus 1.8 million years ago.

Moreover, when they enlarged their comparisons to include early African fossils that had been attributed to distinct human species such as Homo ergaster, Homo habilis and Homo rudolfensis, they felt able to extrapolate the newly enlarged variation of Homo erectus to include many of these other so-called species.

Thus these apparently diverse early Homo fossils, with their origins in Africa, actually represented variation among members of a single, evolving lineage most appropriately called Homo erectus, according to Lordkipanidze and his team.

Personally, I think the team have made an excellent case that this remarkable new skull, with its huge jawbone, is part of the natural variation of the Dmanisi population 1.8 million years ago, and these can all be attributed to a primitive form of Homo erectus.

I think they will be proved right that some of those early African fossils can reasonably join a variable Homo erectus species. But Africa is a huge continent with a deep record of the earliest stages of human evolution, and there certainly seems to have been species-level diversity there prior to 2 million years ago, so I still doubt that all of the “early Homo” fossils can reasonably be lumped into an evolving Homo erectus lineage. We need similarly complete African fossils from 2-2.5 million years ago to test that idea properly.

Professor Chris Stringer is head of human origins at the Natural History Museum, London