Mr. Ples, a relative of early humans, didn't have such a big brain after all, a new study reveals. The results, which made the cover of the June 12 issue of Science, throw a wrench into ideas about brain evolution.
"This paper implies that something is very wrong with previous interpretations of early hominid brain capacity," said lead author Glenn C. Conroy, Ph.D, professor of anatomy and of anthropology. "Hopefully, it will launch more exact studies of other specimens."
The skull of Mr. Ples, found in South Africa in 1989, is 2.6 million to 2.8 million years old and is thought to have belonged to an australopithecine. These human-like creatures walked on two legs, ate tough vegetation, made primitive chopping tools and lived in a well-wooded, wetter Africa from about 3.5 million to 2.5 million years ago.
Conroy and colleagues named the fossil Mr. Ples because it came from Sterkfontein, the same site as Mrs. Ples, the first virtually complete adult australopithecine skull. Mr. Ples' skull is well-preserved on one side but partly open on the other. It earlier was thought to have a brain capacity of more than 600 cc -- about two and a half cups of water. "Such a large brain would be quite extraordinary because all of the other Australopithecus africanus brains are in the mid-400 cc range," Conroy said.
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In 1984, Conroy and radiologist Michael W. Vannier, M.D., developed a way of analyzing fossilized skulls that now is widely used: they imaged them with a computed tomography (CT) scanner. This allows researchers to see inside skulls that are filled with stone and to reconstruct hollow skulls that have pieces missing.
Moonlighting on a CT scanner in a Johannesburg hospital, Conroy made a virtual cast of Mr. Ples' skull. Analyzing every slice of the 3-D image, he calculated a brain size in the low-500s. "While that is still the largest brain known for A. africanus, it is nowhere near what has been bandied around by paleoanthropologists," said Conroy, who was too cautious to publish his surprising results right away.
In 1997, Conroy heard from Horst Seidler, Ph.D., at the University of Vienna, Austria. Seidler had used similar CT techniques on the famous Tyrolean Iceman, and he asked Conroy for the complete data set from the 2.6 million-year-old fossil.
Several weeks later, Conroy and South African anthropologist Phillip V. Tobias, D.Sc., Ph.D., visited Seidler and Gerhard Weber, Ph.D., in Vienna. The Austrian scientists had made a real 3-D model as well as a virtual model from Conroy's data. Their calculations -- and the simple act of measuring how much water the plasticine model could displace -- matched Conroy's. Mr. Ples' skull had a capacity of about 515 cc, the collaborators concluded. To confirm this result, they erased the skull bones in the computer image to create a virtual brain. Then they fitted the brain back inside the skull.
Back in St. Louis, Conroy analyzed an early hominid skull that sits on his desk. This A. africanus, also from Sterkfontein, turned out to have a capacity of 370 cc and not 440 cc as previously thought. "I was shocked," Conroy said, "because there was supposed to be a quantitative leap in brain size that distinguished early hominids from apes, yet 370 cc is the size of a chimpanzee brain. Our study of Mr. Ples clearly shows that somebody had a brain size of more than 500 cc 2.6 million years ago, however. So did brain size increase sharply in some australopithecines and gradually enlarge in their descendants to its present size of at least 1200 cc -- he capacity of a Harley-Davidson engine?"
Although the new study provides more questions than answers, it points to three important conclusions, the researchers said. "First of all, there is no evidence that any australopithecine had a brain capacity anywhere near 600 cc," Conroy said. "Second, some estimates of brain size in early hominids may be too high. And third, 3-D digital models provide a very accurate way of determining how much brain there was inside a fossilized skull."
-- Linda Sage
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