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Scientists discover a genetic clue to how brains first beat brawn

Posted on: Thursday, 25 March 2004, 06:00 CST

Scientists discover a genetic clue to how brains first beat brawn

By JOHN NOBLE WILFORD New York Times

Thursday, March 25, 2004

At a pivotal time in human evolution, about 2.4 million years ago, a muscle gene underwent a disabling alteration. And scientists say this could have made all the difference, leading to the enlarged brains of the lineage that evolved into modern humans.

Researchers who made the discovery said this might be the first recognized functional genetic difference between humans and the apes that can be correlated with anatomical changes in the fossil record. They said the gene mutation may represent the beginning of the ancestral triumph of brain over brawn.

At the least, scientists said, the small mutated gene probably accounts for the more graceful human jaw, in contrast to the protruding ape jaw and facial ridges.

The discovery was made by scientists at the University of Pennsylvania School of Medicine and will be published today in the journal Nature. They also described the findings in interviews last week.

"We're not suggesting that that mutation alone buys you Homo sapiens," said Hansell Stedman, leader of the research team. "But it lifted a constraint that leads to brain growth."

Evolutionary scientists and paleoanthropologists not involved in the project said the interpretation of the findings is intriguing and provocative. A "seductive hypothesis," one of them said, while others cautioned that the explanation probably oversimplifies the causes behind the significant brain expansion that marked the emergence of the Homo lineage out of the more apelike Australopithecus species.

Even so, the findings are expected to encourage other scientists to investigate a whole range of genes that have decisive roles in making us distinctively human.

An analysis of DNA samples showed the mutation to be present in all modern humans worldwide. The analysis further traced the mutation's occurrence to between 2.1 million and 2.7 million years ago.

That happened to be just before the appearance of major evolutionary changes in hominid fossils, the research team notes in the journal article. Some hominids with protruding jaws and small brains were soon to evolve into the first species of the genus Homo, with significantly smaller jaws, larger brains and a modern human body size. After 2 million years, Homo erectus was able to strike out for lands far beyond Africa.

Stedman's group concluded that the findings "raise the intriguing possibility" that, as the strong, stoutly buttressed jaw muscles declined, this allowed the skull to develop a new shape and structure, giving the brain room to grow.

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