SALT LAKE CITY--A few years ago, the big head of an ancient apelike skeleton known as Mr. Ples challenged the standard view that the earliest members of the human family, the australopithecines, were relatively small-brained. Mr. Ples's massive, 2.6-million-year-old cranium suggested that his brain was as big as some of the first members of our own genus, Homo. Now, modern biomedical imaging tools have revealed a new estimate for Ples's brain capacity, reported here last week at a meeting of the American Association of Physical Anthropologists, that backs the idea that australopithecine brains did in fact start small and expanded only slowly and steadily.
Paleoanthropologists' view of hominid brain evolution starts about 3.5 million years ago, with members of Australopithecus afarensis, the species that includes the famed "Lucy." She and her brethren had brains the size of other great apes, averaging about 413 cubic centimeters (cc). Then between 3 million and 2 million years ago, two new species of more robust australopithecines--A. africanus in South Africa and A. boisei in east Africa--appeared, and their brain vaults were thought to be slightly larger. But the idea was that the real quantum leap in brain size came half a million years later, with the appearance of the first human, Homo habilis, which has an average brain capacity of 640 cc. Human brains then continued to get bigger, and today the average human brain capacity is about 1350 cc.
In 1996, the only published report on Mr. Ples's cranial capacity put it at a whopping 600 cc--startlingly high for his time and species. "The implications for models of brain evolution were profound," says Glenn Conroy, a paleoanthropologist at Washington University School of Medicine in St. Louis, Missouri. To recheck the size of Mr. Ples's brain, Conroy traveled to South Africa to CT-scan the skull for himself. Working on his computer in St. Louis, he reconstructed a three-dimensional model of the virtual skull and calculated its internal volume. His verdict: 513 cc. While that's decidedly larger than the roughly 385 cc brains of chimpanzees, it's also considerably smaller than estimates of early Homo.
To confirm the finding, Conroy joined forces with anthropologist Horst Seidler and his team at the University of Vienna, which creates physical models in resin from virtual computer models based on CT-scans. The Viennese group came up with similar measurements. "It's very reassuring that virtually identical results were given by the old and new methods," says co-author paleoanthropologist Phillip Tobias of the University of Witwatersrand in Johannesburg, South Africa.