Ancient Skull Of Human Ancestor Implies All Homo Species Were One
October 18, 2013

Rare 1.8 Million-Year-Old Skull Fossil Suggests Early Humans Belonged To Same Species

[ Watch the Video: Ancient Homo Species Were Merely Variations On A Theme ]

redOrbit Staff & Wire Reports - Your Universe Online

An analysis of a 1.8 million-year-old human skull suggests that the earliest members of our Homo genus actually belonged to a single species, a finding that contradics previous beliefs that there were several different human species walking the Earth during that time.

The fossil, which was unearthed in Dmanisi, Georgia, is the first complete skull of an adult early hominid, the class that would ultimately give rise to modern humans. However, unlike other Homo fossils, this skull, known as Skull 5, combines a small braincase with a long face and large teeth. It was discovered alongside the remains of four other early human ancestors, a variety of animal fossils and some stone tools – all of them associated with the same location and time period, making the discovery truly unique.

Although the site has only been partially excavated so far, it is already providing the first opportunity for researchers to compare and contrast the physical traits of multiple human ancestors that apparently coincided in the same time and geological space.

“It’s a really extraordinary find,” said paleoanthropologist Dr. Marcia S. Ponce de León of the University of Zurich  during a press conference on Wednesday where she announced the findings. “For the first time, we can see a population from the early Pleistocene. We only had individuals before. Now we can make comparisons and see the range of variation.”

David Lordkipanidze from the Georgian National Museum in Tbilisi, Georgia, along with colleagues from Switzerland, Israel and the United States, said the differences between the Dmanisi fossils are no more pronounced than those between five modern humans or five chimpanzees.

Until now, researchers have used variation among Homo fossils to define different species. But in light of the current findings, Lordkipanidze and colleagues suggest that early, diverse Homo fossils, with their origins in Africa, actually represent variations among members of a single, evolving lineage – most appropriately, Homo erectus.

"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," said study co-author Christoph Zollikofer from the Anthropological Institute and Museum in Zurich, Switzerland, in   statement from the American Association for the Advancement of Science.

That's because Skull 5 unites some important features, like the tiny braincase and large face, which had not been observed together in an early Homo fossil until now. Given their diverse physical traits, the fossils associated with Skull 5 at Dmanisi can be compared to various Homo fossils, including those found in Africa, dating back to about 2.4 million years ago, as well as others unearthed in Asia and Europe, which are dated between 1.8 and 1.2 million years ago, the researchers said.

"[The Dmanisi finds] look quite different from one another, so it's tempting to publish them as different species," explained Zollikofer, co-author of a report about the findings published in the October 18 issue of Science. "Yet we know that these individuals came from the same location and the same geological time, so they could, in principle, represent a single population of a single species."

The hominid fossils from Dmanisi represent ancient human ancestors from the early Pleistocene epoch, soon after early Homo diverged from Australopithecus and dispersed from Africa. The jaw associated with Skull 5 was found five years before the cranium was discovered, but when the two pieces were put together, they formed the most massively built skull ever found at the Dmanisi site. For this reason, the researchers suggest that the individual to whom Skull 5 belonged was male.

However, the braincase of Skull 5 is only about 33.3 cubic inches (546 cubic centimeters), which suggests that this early Homo had a small brain despite his modern human-like limb proportions and body size.

"Thanks to the relatively large Dmanisi sample, we see a lot of variation," Zollikofer said. "But the amount of variation does not exceed that found in modern populations of our own species, nor in chimps and bonobos."

"Furthermore, since we see a similar pattern and range of variation in the African fossil record… it is sensible to assume that there was a single Homo species at that time in Africa.”

"And since the Dmanisi hominids are so similar to the African ones, we further assume that they both represent the same species."

While the current findings from Skull 5 would seem to indicate that a single Homo species – rather than severally ecologically specialized ones – emerged from the African continent, some anthropologists were skeptical of this conclusion.

Lee Berger, paleoanthropologist at the University of the Witwatersrand in Johannesburg, said he isn’t sold on the idea that this is the same Homo erectus from both Africa and Asia, or that individual Homo species from this time period are really all the same species.

"The specimen is wonderful and an important contribution to the hominin record in a temporal period where there are woefully too few fossils," he said in an interview with CNN.

However, the suggestion that these fossils prove an evolving lineage of Homo erectus in Asia and Africa, is "taking the available evidence too far.”

Paleoanthropologist Susan Antón of New York University praised the new analysis, but said the researchers didn't compare fossil features, such as the anatomy around the front teeth, that differ significantly between two different species of early humans.

Therefore, the hypothesis that there was only one lineage is not entirely convincing, she told USA Today.

Despite the criticism, the Dmanisi team believes that Skull 5 demonstrates that a single Homo species able to cope with a variety of ecosystems, rather than several ecologically specialized Homo species, emerged from the African continent. If this is true, our classification system for our early human ancestors may never be the same.

Image 2: (below): Computer reconstruction of the five Dmanisi skulls (background: Dmanisi landscape). Credits: Marcia Ponce de León and Christoph Zollikofer, University of Zurich, Switzerland