- The Washington Times - Wednesday, March 24, 2004

Human evolution may have hinged on a genetic mutation 2.4 million years ago that weakened the jaws of prehistoric man and allowed the development of bigger brains, say U.S. researchers.

This hypothesis was reported in this week’s issue of the scientific journal Nature by a team of biologists, anthropologists and plastic surgeons at the University of Pennsylvania Medical School and Children’s Hospital of Philadelphia.

Dr. Hansell Stedman, the study’s lead author, said he and his colleagues are “not suggesting that this mutation alone defines us as Homo sapiens.”

But he added: “Evolutionary events are extraordinarily rare. Over 2 million years since the mutation, the brain has nearly tripled in size. It’s a very intriguing possibility.”

Richard Potts, director of the Human Origins Program at the Smithsonian Institution, said the scientists clearly pinpointed a genetic mutation in early man that deactivated the big jaw muscles found in nonhuman primates that allow them to bite and grind tough food.

But Mr. Potts called the researchers’ premise that this ancient gene mutation may have led to brain growth and human evolution “way too speculative.”

In their published study, researchers described how they isolated a new gene on Chromosome 7 as part of a medical search for human genes linked to a family of proteins called myosins that make muscles contract.

They discovered a new myosin gene, called MYH16, that had one small mutation that made it inactive. This mutation was found to be present in all the humans they tested from Europe, Africa, Asia and South America.

But when they analyzed the DNA of seven species of apes, including chimpanzees, all the animals had the active, rather than the mutated, form of the gene. Further investigation revealed that the gene was only switched on in monkeys in facial muscles that control biting and chewing.

In an interview yesterday, Nancy Minugh-Purvis, also an author of the report in Nature, said the research showed that “modern humans have a [genetic] mutation that no other primates seem to have.”

In other words, unlike humans, chimps and other apes still carry the original big-jaw gene.

Researchers said their estimate of when this mutation first occurred — 2.4 million years ago ?? generally overlaps with the first fossils of prehistoric humans, featuring rounder skulls, flatter faces, smaller teeth and weakened jaws.

“This report is important and interesting … and it may very well be part of the explanation for differences between humans and chimpanzees [the closest relative of humans],” said William Kimbel, professor of anthropology at Arizona State University.

But Mr. Kimbel questions the date the researchers have assigned this genetic mutation and divergence. He says 2.4 million years ago is “one of the least well-lit” periods in the fossil records of hominids, or prehistoric humans.

However, Ms. Minugh-Purvis counters that the “date was very carefully calculated.”

University of Michigan biological anthropologist Milford Wolpoff hailed the findings and said that 2 million years ago was when people started making tools.

“This is a confluence of genetic and fossil evidence,” he said.

This article is based in part on wire reports.

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