Skull Fossil Challenges Out-of-Africa Theory
for National Geographic News
|July 4, 2002|
An exquisitely preserved skull of a tiny-brained human ancestor has been recovered from excavations beneath the ruins of a medieval castle in the republic of Georgia. The skull is about 1.8 million years old and belongs to the first group of humans to migrate out of Africa, reports an international team of archaeologists.
The find calls into question a widely held hypothesis that the evolution of big brains propelled the exodus of early humans out of Africa.
The fossil evidence from Dmanisi now includes three skulls, several jaw fragments, and hundreds of stone tools and animal remains. All of the material has been recovered from the same layer of sediment. It is forcing scientists to come up with alternative explanations for why humans were able to leave Africa.
"Before this find, the main reason was that at least these humans had big brains," said David Lordkipanidze, a paleoanthropologist at the Georgian State Museum in Tbilisi who led the excavation team. "Now this shows that [their brains] were quite small."
The brain of the new specimen from Dmanisi is about half the size of a modern human's brain. The two skulls found in 1999 at the site are also about 1.8 million years old and had room for substantially larger brains.
The research by Lordkipanidze and colleagues is published in the July 5 issue of the journal Science and is the subject of the cover story of the August issue of National Geographic magazine.
Dmanisi sits on a promontory formed by the confluence of two rivers between the Black Sea and the Caspian Sea about 50 miles (80 kilometers) southwest of Tbilisi. Archaeologists first began excavating the remains of a 1,000-year-old castle located on the site in 1936.
In 1983, while examining an ancient garbage pit, one of the archaeologists uncovered what Abesalom Vekua, a team member from the Georgian State Museum, identified as a tooth of a rhinoceros, an odd creature to have been wandering the Caucasus Mountains. Speculation about what lay in the sediments beneath the castle spurred further excavations.
Stone tools recovered in 1984 and the two skulls discovered in 1999 were all dated to 1.8 million years ago, making the site a rare treasure trove for scientists interested in human evolution.
"We can say this is the richest material from this time period from one site and from one strata," said Lordkipanidze. "Usually we have isolated finds. Now on one geological level you have three skulls and three mandibles. We have a chance to study isolated specimens and to study populations."
In addition, members of the excavation team have found thousands of plant impressions, carbonized plant remains, numerous animal bones, snail shells, and what appears to be plant pollen in a series of thick lake deposits just to the south of the Dmanisi site.
"This documents our ability to reconstruct an environmental history of the site before, during, and after the occupations with detail and precision that would be enviable in any archaeological setting," said Reid Ferring, an archaeologist at the University of North Texas in Denton and member of the excavation team.
"Also, the lake shore deposits provide us with a superb tool for locating other areas where those people camped or butchered animals," he added.
Wider Species Variation
Lordkipanidze and his colleagues have classified all three skulls as belonging to Homo erectus. However, they caution that the small brain and other features of the new skull suggest a close resemblance to Homo habilis, which was more apelike with a thin brow, huge canine teeth, and long, dangling arms.
The variation among the hominids recovered at Dmanisi makes it difficult to say exactly who these people were, said Lordkipanidze. He suggests that the variation may force scientists to rethink the definition of "Homo."
"The Dmanisi fossils show much more variation than we would have expected from any group of humans at that time," said Ferring. "But clearly, the notion that the African exodus had to await emergence of a tall, fast, strong, and clever species and that Homo erectus was our first ancestor to meet all those requirements was wrong."
The researchers conclude that the Dmanisi hominids are among the most primitive individuals attributed to H. erectus and that "it now seems that the first humans to disperse from the African homeland were similar in grade to H. habilis."
The range of material found at the Dmanisi site "will help us understand the evolutionary origins of these early humans, their variation, and their affiliation to other early human groups," said Chris Stringer, a paleontologist at the Natural History Museum in London, England.
Out of Africa
Prior to the discoveries at Dmanisi, scientists theorized that the first humans to migrate out of Africa had large brains and wielded advanced stone tools such as hand axes that allowed them to butcher and process meat.
The stone tools found with the hominid remains at Dmanisi, however, are simple choppers and scrapers similar to the Oldowan set found in the Olduvai Gorge, Tanzania. This implies that early humans with primitive technology were able to expand out of Africa, said Lordkipanidze.
Ferring added that although the stone tools are of the Oldowan set, "many of them show careful selection of the better raw materials located in the flanking valleys, and a number of the cores have been intensively reduced to maximize the number of flakes that were removedI don't think they were stupid."
The Dmanisi evidence calls into question older theories on who left Africa first and why they left, said Ferring. Now scientists must ask the question: If not brain power and tool technology, what did enable early humans to leave Africa?
Lordkipanidze and colleagues suggest that the answer lies in anatomy and ecology, but say that without further study and discussion within the scientific community no conclusions can be drawn.
"My feeling has always been that it was the acquisition of modern body form and the consequent emancipation of hominids from the forest fringes and woodlands that allowed the spread of early humans to begin," said Ian Tattersall, an anthropologist at the American Museum of Natural History in New York City.
Stringer suggests that 1.8 million years ago the environment at the Dmanisi site may have been much more like that of East Africa.
"Thus these early humans may have just expanded their range, remaining in a rather familiar environment," he said. "However, greater reliance on tools and meat eating probably also facilitated the expansion."
Lordkipanidze says he hopes that further excavations at Dmanisi, which are continuing this summer, will reveal skeletal remains that will help answer some of these questions. "It is feasible that we will find some part of the body," he said.
The excavations at Dmanisi are funded in part by a grant from the National Geographic Society's Committee for Research and Exploration.
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