Hominin skull discovery fuels debate about early human evolution

The face of Dmanisi Skull 5.

Credit: 

Courtesy of Guram Bumbiashvili, Georgian National Museum

The Dmanisi Skull 5 early Homo cranium and herbivore fossil remains in situ at the discovery site.

Credit: Courtesy of Georgian National Museum

Artist's reconstruction of the original Homo erectus owner of Dmanisi Skull' 5.

Credit: 

Courtesy of J.H. Matternes

The Dmanisi site (in the upper right of the photograph), lies beneath a small medieval town about 85 kilometers from Georgia's capital Tbilisi. The site sits on top of a lava flow and covers some 50,000 square meters.

Credit: Courtesy of Fernando Javier Urquijo

Five different hominid skulls have been found at Dmanisi so far, all exhibiting slightly different traits. The newest one, Skull 5 on the far right, is believed to be Homo erectus. The question is whether all five are Homo erectus or are different species.

Credit: Courtesy of M. Ponce de León and Ch. Zollikofer, University of Zurich, Switzerland

Hailed as a find for the ages, a rare skull of a 1.8-million-year-old human relative could provide answers to longstanding questions about the lineage of our species. It also fuels debate over what differentiates one hominin species from another, and could mean that Homo habilis, Homo rudolfensis and other early bipedal hominins may all be members of Homo erectus, rather than distinct species.

A jawbone found at an archaeological site in Dmanisi, Georgia, in 2000 was matched to a skull uncovered in 2005 just 2 meters away. Together, they represent the most completely preserved adult skull yet discovered from that period, according to paleoarchaeologist David Lordkipanidze of the Georgian National Museum, who reported the discovery with his colleagues this week in Science.
What makes the skull most interesting to the team is the difference in its size and shape compared to four other skulls found thus far at the site, which is about 85 kilometers southwest of Georgia’s capital, Tbilisi. Some of the skulls — two from adolescents and a third from an elderly individual — have larger or rounder braincases, or more pronounced eyebrow ridges.

The authors describe two possibilities to explain the dimensional variation between the skulls, which all date to the same time period. Either they belong to separate species and ended up in the same place at the same time, or they are members of the same species exhibiting differences between individuals.

Because the five individuals lived in the same community, and because the variability in measurements of their skull dimensions falls within the range seen in other primates, the researchers concluded that they belong to the same species, Homo erectus.
“The five Dmanisi individuals are no more different from each other than five [modern] humans or chimpanzees,” said co-author Christoph Zollikofer, a paleoanthropologist at the Anthropological Institute and Museum of the University of Zurich, at a press conference this week. Homo erectus lived in Africa and spread across the world between 1.8 million and 143,000 years ago.
Their conclusion could push other species, thought to be distinct members of the Homo lineage, out of the accepted story of human evolution, the authors said. Homo habilis and Homo rudolfensis are early hominins that lived in Africa at about the same time as the Dmanisi examples. Both species are known by only a few fossils and their skulls are more ape-like than human, two reasons their classification has been complicated.

Many scientists have praised the find, but some have reservations about the report’s conclusions. “It should be said before anything else that it’s an absolutely fabulous specimen. There’s nothing as well-preserved for that time period,” Fred Spoor, a paleontologist at University College London who helped find and describe Homo rudolfensis and habilis and wasn’t involved in the new research, told EARTH. Nonetheless, he says he isn’t convinced that the discovery spells the end for the other hominin species.

Spoor points to specific features of Skull 5, as the new example is known. “The overall shape of the braincase is something that only Homo erectus has, and you can’t find it in Homo habilis or rudolfensis,” he says. Instead, “what you have in my view is a fascinating skull with a number of primitive features combined with a number of very typical erectus features,” he says. “The braincase takes on Homo erectus features whereas the face is still sort of ancestral.”
In response, Zollikofer returns to the group’s argument in favor of individual differences within species. He told EARTH that the only example of a Homo rudolfensis skull has been reconstructed from fragments, so there are questions about it. In addition, he says, “it is straightforward to find two modern human braincases that differ more in shape than those of habilis, erectus or rudolfensis.”
Among the details the team provided about the skull is that its braincase volume — 546 milliliters — is about a third the volume of an average modern human. Skull 5’s original owner also suffered an injury to its right cheekbone, which broke and healed during its lifetime.

Based on how and where the five Dmanisi skulls have been found, archaeologists think that carnivores dragged the corpses into their dens, all likely within a few hundred years of each other. Sabre-toothed tigers and giant cheetah bones have also been found at the dig. “It seems that they were fighting for the carcasses and unfortunately for the hominins, but fortunately for us, they were not always successful” in fending off predators, Zollikofer said at the press conference.
The Dmanisi site, which lies beneath a small medieval town, sits on top of a lava flow and covers some 50,000 square meters, according to the group. Archaeologists suggest that early humans lived by a river there, and they have found stone tools likely used for butchering animals and other tasks.