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 saying it was "super."
The researchers said their estimate of when this occurred -- about 2.4 million years ago -- overlapped 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 two-and-a-half million years ago was 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 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 Owen Lovejoy of Kent State University. "These kinds of mutations probably are of little consequence."
Others sought to find some middle ground in the debate.
University and commercial laboratories are rapidly comparing the human genome with that of chimpanzees to determine what makes people human, and how hominids split from apes and monkeys some 6 million years ago.
So far, perhaps 250 genetic differences have been flagged for further study.
Jaws have been a focus of evolutionary research since Darwin, and the mutation offers a tantalizing theory. But it is unlikely that one mutation -- even at a crucial evolutionary juncture -- would make a person, they said.
"They have successfully nailed a genetic mutation that works to deactivate these jaw muscles," said Richard Potts, director of the Human Origins Program at the Smithsonian Institution. "But their suggestion connecting it to the brain is way too speculative."
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