Putting together the pieces of a 1.8 million-year-old skull from the former Soviet republic of Georgia has led researchers to a surprising conclusion: Specimens that supposedly represent several early human species might be merely different-sized individuals from the same species.
If the conclusion holds up, the skull discovery would require a major rewrite for the story of early human evolution. Such species as Homo habilis and Homo rudolfensis, long a part of humanity's "bushy" family tree, could be folded into a wide-ranging species known as Homo erectus.
"It is really an extraordinary find in many respects," Christoph Zollikofer of Zurich's Anthropological Institute and Museum, one of the researchers behind the study published in this week's issue of the journal Science, told reporters during a teleconference.
The key to the claim is the assembly of a fossil called Skull 5. The specimen was discovered in separate pieces at a sprawling excavation in Dmanisi, about 50 miles (80 kilometers) from Tbilisi, Georgia's capital. Over the past eight years, Skull 5's jaw and the cranium were painstakingly matched up and compared with four other hominid skulls unearthed at the site.
The researchers were struck by the fact that Skull 5's braincase was relatively small, while the face was relatively large. What's more, other skeletal fossils associated with Skull 5 suggested that the individual's body proportions were much like a modern human's.
"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," Zollikofer said in a news release. He and his colleagues also noticed size variations among all five of the Dmanisi skulls — which led them to wonder whether different species in the genus Homo were being defined too narrowly.
An analysis of the various early Homo skulls from Africa, dating from 2.4 million and 1.2 million years ago, found that the size variations were no wider than the variations found in modern humans. The size differences were also in the range for chimpanzees and bonobos, the modern species that are considered closest to humans on the evolutionary tree.
"Since we see a similar pattern and variation in the African fossil record ... it is sensible to assume that there was a single Homo species at that time in Africa," Zollikofer said. "And since the Dmanisi hominids are so similar to the African ones, we further assume that they both represent the same species."
That claim will have to be debated over the months and years ahead. Science quoted other experts as saying Skull 5 and the other fossils from Dmanisi may represent yet another new species in the genus Homo, or perhaps Homo habilis. One paleontologist, Fred Spoor of the Max Planck Institute for Evolutionary Anthropology, told Science's Ann Gibbons that Skull 5 may well represent Homo erectus. But he balked at the idea that all the early Homo fossils from Africa should be classified as Homo erectus as well.
The study's lead author, David Lordkipanidze of the Georgian National Museum, told reporters that the Dmanisi site could provide further insights into the migration of early humans and how they interacted with their environment. "It's a real snapshot in time, and maybe a time capsule which preserves the whole ecosystem which existed 1.8 million years ago," he said.
Although the researchers' hypothesis would trim back the earlier branches of the human family tree, it doesn't address what happened during later eras of human evolution. The conventional wisdom is that the descendants of early Homo species differentiated into Neanderthals, Denisovans, so-called "hobbits" and modern Homo sapiens.
"There is a big gap in the fossil record," Zollikofer told NBC News. "I would put a question mark there. Of course it would be nice to say this was the last common ancestor of Neanderthals and us, but we simply don't know."
Update for 3:25 p.m. ET Oct. 17: Arizona State University paleoanthropologist Donald Johanson, who discovered the famous 3.2 million-year-old Lucy fossil skeleton almost four decades ago, said Lordkipanidze and his colleagues have produced "a beautiful little paper" — but he doesn't buy the claim that all of the earliest human species should be lumped together.
"I think it's probably premature to dump everything into Homo erectus," Johanson told NBC News. "This is what you're going to find the most opposition to."
Johanson said the entire collection of specimens of early Homo species from East Africa shows "considerably more variation than you see in this sample [from Dmanisi], which is not surprising, because you're looking at fossils from very different regions."
He said the Dmanisi skulls reminded him of Homo ergaster, an African species that's similar to Homo erectus specimens found farther east. "It strengthens the view that many of us have held, that [Homo] ergaster was the species that got out of Africa to give rise to this Dmanisi population, and that ultimately evolved into Homo erectus in Java," Johanson said.
Skull 5's small braincase also raised interesting questions, because humans living in Africa during the same time period had larger brains. "That may suggest that these populations in Africa and in Georgia were under different selective pressures," Johanson said.
He expected that "there'll be the normal bickering" over how to classify the Dmanisi skulls. Do they represent an existing fossil species, a new species, a subspecies or a sub-subspecies? In any case, Johanson said the newly reported findings will make a significant contribution to the study of early human evolution.
"It's just marvelous to have a real sample from a single locality of the same geological age, and such a comprehensive sample," Johanson told NBC News. "It's unsurpassed for the genus Homo of this antiquity."
More about human evolution:
- How climate shifts sparked human evolution
- Flash interactive: Before and after humans
- NBC News archive on evolution
In addition to Lordkipanidze and Zollikofer, the authors of "A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo" include Marcia Ponce de Leon, Ann Margvelashvili, Yoel Rak, G. Philip Rightmire and Abesalom Vekua.
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.