Five remarkably preserved, 1.8-million-year-old skulls are are opening a unique window on early human evolution. In the process, they hint that several early human fossils found elsewhere and identified as different species may actually all be members of this same species, according to a new study.
Scientists discovered the skulls and other skeletal remains as they were excavating a long-studied site at the small town of Dmanisi on the southern border of the Republic of Georgia.
All the specimens represent individuals who lived at various times within the same roughly 100-year span, suggesting they belong to the same species, the researchers say.
An analysis of the skulls shows a range of features whose variety falls within the range of different looks found in modern humans, chimpanzees, and bonobos. This further reinforces the notion that these early humans were members of the same species, researchers say.
Armed with information on these individuals' range of looks, scientists may be able to prune some limbs from the human family tree if the team's analysis – that more fragmentary early-human fossils scientists have found in Africa may belong to the same species as the Dmanisi Five – holds up, the researchers suggest.
The discovery represents "a significant contribution to our understanding of the origins and evolution of our own genus, homo," says Donald Johanson, founding director of the Institute for Human Origins at Arizona State University in Tempe.
Scientists have worked for decades to uncover well-preserved specimens from the same geological period and same location, he says. It's a bit surprising that such a find was discovered on the Eurasian continent, given that Africa is widely seen as the birthplace of the genus Homo. This discovery does not challenge that view of human origin, but enriches it, Dr. Johanson says.
The five individuals the skulls represent "are the earliest representatives of our own genus outside of Africa," says David Lordkipanidze, a Georgian anthropologist who led the international team reporting the results in Friday's issue of the journal Science.
Of special interest is a virtually complete skull, which the team has dubbed Skull 5. Scientists discovered the lower jaw in 2000 and found the rest of the skull five years later and seven feet away. The team undertook painstaking analysis to establish that the two belong together.
The skull is "an extraordinary find in many respects," says Marcia Ponce de Leon, an anthropologist from the Anthropological Institute and Museum in Zurich and a member of Dr. Lordkipanidze's team.
Unlike other fossilized skulls from this period found elsewhere, Skull 5 is well-preserved, remains perfectly formed, and hasn't been broken into fragments.
The skull's odd proportions were unexpected, Dr. Ponce de Leon notes. The skull's brain case is much smaller than expected, but the face is large, with large, thick jaws that jut forward, large teeth, and thick bony ridges crossing the face just above the eye sockets.
"This is really an important specimen," writes Ian Tattersall, anthropologist and curator emeritus at the American Museum of Natural History in New York, in an e-mail. "It underscores that the history of the hominid family was one of vigorous evolutionary experimentation."
At the moment, there may be no better place on the planet to look in on that process than Dmanisi, suggests Adam Van Arsdale, a paleoanthropologist at Wellesley College in Massachusetts.
Some 1.8 million years ago, the site's location was arid, but with ample water flowing down from the nearby mountains, says Dr. Van Arsdale, who was not a member of Lordkipanidze's team but who works extensively at the site.
With plenty of water available, the location would have attracted animals large and small, in addition to early humans, he says. Fossilized remains include saber-tooth cats, medium-size deer, and wolves, as well as rodents and other small animals that live in arid terrain. These smaller animals help define the climate locally because they don't travel far, Van Arsdale explains.
Fossilized animal bones from this period at Dmanisi show cut marks from stone blades, indicating that the early humans at the site took advantage of the meat available, either through hunting, scavanging, or both. It's probable that this cut both ways, with these early humans sometimes becoming prey, although there is no evidence yet for that at Dmanisi.
Lordkipanidze notes that some 12 acres have yet to be excavated at the site, suggesting that much more about the environment and its early human and nonhuman inhabitants remains to be learned.
Understanding the environment will help uncover the evolutionary pressures early humans faced at the site, pressures that may help explain the variety in appearance displayed by the five individuals, Van Arsdale says.
Although the paper in Science focuses on the skulls, researchers also have uncovered other skeletal elements that speak to variations in other body features. Those include partial remains of at least one adolescent individual and at least three adults, "all of which is consistent with the cranial material," he says.
The physical variation Lordkipanidze's team sees at Dmanisi has led it to suggest that Homo ergaster falls within that variation and is really a subspecies of H. erectus. Based on fossil fragments in Africa from different locations and different periods, researchers had identified H. ergaster, a contemporary of H. erectus, as its own species. The researchers suggest that Homo habilis and Homo rudolfensis also are subspecies of H. erectus, although the researchers acknowledge that this proposition needs to be tested.
Others are reluctant to declare these species as members of one happy family, however.
Skull 5 is unusual compared with the other four, and it's unusual compared with H. erectus specimens found in Asia, notes William Harcourt-Smith, a paleoanthropologist at Lehman College, one of the colleges that make up the City University of New York.
The skull is "much closer to what is in Africa," he writes in an e-mail. "Personally, I feel the African material is distinct enough to be a different species, and in that respect so would Skull 5."
There is too much variation among the other three species to lump them in with H. erectus, adds Johanson, who says efforts to reclassify other finds based on the Dmanisi fossils is premature.
Still, it's not too early to raise the issue, says Van Arsdale. While researchers debate whether or how to prune the Homo family tree based on the Dmanisi finds, the early-human specimens from that site and the rich evidence of their environment could help unlock the ecological influences that helped shape early-human evolution, he says.