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Odd Fossil Skeletons Show Both Apelike and Human Traits

James Owen
for National Geographic News
September 19, 2007
 
Ancient humanlike fossils discovered at the site of a medieval castle fill crucial gaps in the story of our evolution, scientists say.

Researchers unearthed the 1.77-million-year-old skeletons at Dmanisi in the republic of Georgia.

The fossils of three adults and a teenager are thought to belong to Homo erectus, the earliest known Homo species found outside Africa.

But the remains suggest the individuals were particularly primitive, appearing to be noticeably different from populations of the same species elsewhere.

The scientists described the ancient hominins as small-brained, tree-climbing, meat-eating midgets that were both apelike and human in appearance.

"If this [group] is Homo erectus, it is the most primitive and oldest one known," said David Lordkipanidze of the Georgian National Museum in T'bilisi, Georgia, who led the study.

(Lordkipanidze is a grantee of the National Geographic Society's Committee for Research and Exploration. National Geographic News is a division of the National Geographic Society.)

The find is just the latest to come from the fossil-rich Dmanisi site, which was uncovered during archaeological work at a ruined medieval castle in 1991.

(Related news: "Skull Fossil Challenges Out-of-Africa Theory" [July 4, 2002].)

Small-Brained Tree-Dweller

The newfound fossils reveal that the Dmanisi ancestors had unusually small brains and bodies compared with early H. erectus fossils from Africa.

The team estimates that the individuals stood 4.7 to 5.4 feet (1.45 to 1.66 meters) tall and weighed 88 to 110 pounds (40 to 50 kilograms). The new fossils also reveal apelike arms and hands.

"Their hands were still well adapted to life [in the trees]," Lordkipanidze said of the four newfound specimens, adding that they likely slept in trees at night for safety.

But the species also had modern human features, the researchers said, including long legs and a spine suited for long-distance running and walking.

The human ancestor also had more humanlike body proportions than pre-Homo species.

(See an interactive feature on the Dmanisi fossils.)

Animal fossils unearthed at the site show the region was well populated with mammals, including extinct species of hyenas, wolves, bears, deer, and giraffes.

The fossil skulls of fearsome saber-tooth cats have also been found.

Carcasses of animals killed by these predators may have been an important source of meat for the ancestors, the team says.

"The Dmanisi people could have followed them as scavengers," Lordkipanidze said.

The hominins may even have hunted large animals, he added.

Broken fossil bones and remains marked by crude stone tools indicate some of the mammals were butchered for their flesh.

The team's study of the fossils will appear in this week's issue of the journal Nature.

Different Environment

The Dmanisi ancestors' smaller body and brain size are hard to explain, said anthropologist Daniel Lieberman of Harvard University, who was not involved in the study.

One possibility is that the Dmanisi were smaller simply because they were adapted to a different type of environment, Lieberman writes in an accompanying article in Nature.

Alternatively, he says, the Dmanisi fossils may represent a different species.

"My hunch," he writes, "is that the Dmanisi and early African H. erectus fossils represent different populations of a single, highly variable species."

Lordkipanidze said such variation may indicate the first human ancestors left Africa much earlier than previously thought and so were an earlier species than Homo erectus.

Erik Trinkhaus of Washington University in St. Louis, Missouri, however, said he doesn't find the new fossils so surprising.

The primitive features identified in the study fall within the range of variation previously seen in early human ancestors, he argued.

"They were little people with little brains—that doesn't really surprise me," Trinkhaus said.

"We have other specimens of H. erectus that were not very much larger."

He agreed that the species likely slept in trees.

"If you're a primate and you sleep on the ground at night, you don't wake up in the morning," Trinkhaus added.

"What is clear is that the overall anatomy is primarily for walking on the ground," he said.

Trinkhaus suggested environmental changes around the time of the Dmanisi enabled human ancestors to extend their range beyond Africa.

"Clearly something happened to allow [human ancestors] to exploit more varied and more seasonal environments around this time period, apparently associated with the emergence of H. erectus," Trinkhaus said.

"The Dmanisi material is one very rich reflection of that," he said.

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