Complete skull of 1.8-million-year-old hominin found

An entire skull belonging to an extinct hominin that lived 1.8 million years ago has been found in Georgia – the earliest completely preserved specimen ever found and confirmation that the species it belonged to, Homo erectus was far more variable in appearance than originally thought. So much so, in fact, that its discoverers argue that the ancient human family tree should be pruned of many of its species, which may simply be different forms of H. erectus.

“It’s the most complete skull of an adult from this date,” says Marcia Ponce de León of the Anthropological Institute and Museum in Zurich, Switzerland, who studied the fossil. Anthropologists have found much older hominin fossils dating back several million years, but entire skulls comprising the brain case, the face and the lower jaw are rare.

“It’s a fantastic specimen, unprecedented in preservation for that time period,” says Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who was not involved in the discovery. The oldest previously discovered hominin remains in comparable condition were the famous Turkana Boy skeleton, which is 1.5 million years old

The skull was unearthed at the Dmanisi site in southern Georgia. The lower jaw was found in 2000, and the matching brain case turned up in 2005.

This is the fifth – and best-preserved – skull found at Dmanisi, which has also yielded simple stone tools and many animal remains. Together, the skulls point to a radical idea, according to David Lordkipanidze of the Georgian National Museum in Tbilisi, who led the study.

All one species?

Lordkipanidze and his colleagues say that the new skull supports the idea that the many species of hominin thought to have coexisted during this period are, in fact, a single species, H. erectus, which is simply more variable in appearance than previously thought.

To back up this claim, the team measured facial differences between the five Dmanisi hominins. Because they all lived in the same place at roughly the same time, this shows the extent of variation among H. erectus populations. Then they compared the Dmanisi population with a range of fossils belonging to ancient African hominins alive at the same time, and used modern humans and chimpanzees as control groups.

Although the hominin fossils were clearly different from modern humans and chimpanzees, the analysis found the rest of the fossils fell into a single, highly variable group. The team says they all belong to one species, meaning hominins like H. habilis and H. rudolfensis simply belong to H. erectus.

“We are not against the idea that there might have been multiple species 2 million years ago,” says team member Christoph Zollikofer of the Anthropological Institute and Museum in Zurich, Switzerland. “But we don’t have sufficient fossil evidence to make the distinctions between species.”

No, probably not

Other anthropologists are unconvinced. Spoor agrees that the specimens from Dmanisi are all H. erectus and that the species was variable, but he does not believe that all the African fossils belong to H. erectus. He points out that Lordkipanidze’s analysis suggests even the much more ape-like hominins in the genus Australopithecus belong to the H. erectus group. It is not surprising, then, that the new analysis misses the more subtle shape differences between Homo species.

“I think they will be proved right that some of those early African fossils can reasonably join a variable H. erectus species,” says Chris Stringer of the Natural History Museum in London, UK.

“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 H. erectus lineage.”

The team’s claim rests in part on the assumption that the hominin fossil record is incomplete, and that anthropologists have been forced to define entire species on the basis of single specimens, which are often incomplete. Spoor says this is misleading.

“There is this popular myth that everything fits in a shoebox,” says Spoor. “Actually, the fossil record is remarkably good. There are hundreds of fossils.” He says there are at least 30 complete braincases known from H. erectus.

H. erectus everywhere

While the team’s more radical claims are in doubt, the new skull does help confirm the importance and success of H. erectus. Hominins are generally thought to have evolved in Africa, and H. erectus is widely regarded as the first species to leave – long before modern humans did so.

“The hominins from Dmanisi are the earliest representation of Homo outside Africa,” says Lordkipanidze. Yet compared with modern humans, they had small brains and could only make the simplest tools. That suggests it didn’t take any great intelligence for them to go global.

“1.8 million years ago, there was another single global species,” says Zollikofer. H. erectus never made it to Australia or the Americas, but other than that they colonised most of Earth’s land mass. They probably also gave rise to the diminutive “hobbits” found on the Indonesian island of Flores.

The new skull tells us that H. erectus evolved in a mosaic fashion, with some aspects of the skull changing before others, says Spoor. The shape of the brain case was the first to change: the new skull has a braincase shaped like a typical H. erectus despite its small size. “The face and teeth remained more H. habilis-like initially,” he says. Later H. erectus had smaller faces and smaller jaws.

Journal reference: Science, DOI: 10.1126/science.1238484