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Jaw mutation may have led to human evolution

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

Igniting a scientific furor, scientists say they may have found the genetic mutation that first separated the earliest humans from their apelike ancestors.

The provocative discovery suggests that this genetic twist -- toward smaller, weaker jaws -- unleashed a cascade of profound biological changes. The smaller jaws would allow for dramatic brain growth necessary for tool-making, language and other hallmarks of human evolution on the plains of East Africa.

The mutation is reported in the latest issue of the journal Nature, not by anthropologists, but by a team of biologists and plastic surgeons at the University of Pennsylvania and the Children's Hospital of Philadelphia.

The report provoked strong reactions throughout the hotly contested field of human origins with one scientist declaring it "counter to the fundamentals of evolution" and another pronouncing it "super."

The Pennsylvania researchers said their estimate of when this mutation first occurred -- about 2.4 million years ago -- generally overlaps with the first fossils of prehistoric humans featuring rounder skulls, flatter faces, smaller teeth and weaker jaws.

And, the remarkable genetic divergence persists to this day in every person, they said.

But nonhuman primates -- including our closest animal relative, the chimpanzee -- still carry the original big-jaw gene and thanks to stout muscles attached to the tops of their heads, they can bite and grind the toughest foods.

"We're not suggesting this mutation alone defines us as Homo sapiens," said Hansell Stedman of the University of Pennsylvania School of Medicine. "But 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."

University of Michigan biological anthropologist Milford Wolpoff called the research "just super."

"The other thing that was happening 21/2 million years ago is that people were beginning to make tools, which enabled them to prepare food outside their mouths," he said. "This is a confluence of genetic and fossil evidence."

Other researchers strenuously disagreed that human evolution could literally hinge on a single mutation affecting jaw muscles, and that once those muscles around the skull were unhooked like bungee cords, the brain suddenly could grow unfettered.

"Such a claim is counter to the fundamentals of evolution," said C. Owen Lovejoy of Kent State University. "These kinds of mutations probably are of little consequence."

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