Smaller jaws would have fundamentally changed the structure of the skull, they contend, by eliminating thick muscles that worked like bungee cords to anchor a huge jaw to the crown of the head. The change would have allowed the cranium to grow larger and led to the development of a bigger brain capable of tool-making and language.
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, in the grasslands of East Africa, the cradle of humanity - generally overlaps with the first fossils of prehistoric humans featuring rounder skulls, flatter faces, smaller teeth and weaker jaws.
And the remarkable genetic mutation persists to this day in every person, they said.
Nonhuman primates - including our closest animal relative, the chimpanzee - still carry the original big-jaw gene and the apparatus enabling them to bite and grind the toughest foods.
"We're not suggesting this mutation alone defines us as Homo sapiens," said Dr. 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 2 1/2 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 were reduced, 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."
Others sought to find some middle ground in the debate.
University and commercial laboratories rapidly are comparing the human genome with that of chimpanzees to determine what makes people human, and how the earliest transitional creatures known as hominids split from Old World 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, some skeptics 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."
In their experiment, the Pennsylvania team isolated a new gene in an overlooked junk DNA sequence on chromosome 7. It belongs to a class of genes that express production of the protein myosin, which enables skeletal muscles to contract.
Originally the scientists were concentrating on determining the biological underpinnings of Duchenne muscular dystrophy, a muscle-wasting disease. But once they isolated the mutation, they spent the next eight months deciphering its evolutionary implications.
Different types of myosin are produced in different muscles; in the chewing and biting muscles, the gene MYH16 is expressed.
In primates like the macaque, the jaw muscles are 10 times more powerful than in humans. They contain high levels of MYH16, and the thick muscles attach to bony ridges of the skull.
But the Penn researchers discovered humans have a mutation that prevents the MYH16 from accumulating, and our jaw muscles are smaller.
As for when this genetic split occurred, the researchers came up with a calculation based on the widely held belief that genetic mutations occur at a constant rate.
Then they looked deep into the fossil record to determine when the jaws of human ancestors started looking smaller and more streamlined. What they found confirmed their estimate.
As far back as 2.4 million years ago, Homo habilis, or "handy man," emerged as the earliest known species to show distinctly human skull and jaw traits, while retaining an apelike physique.
Its brain grew by about 25 percent over that of its more primitive, nonhuman relatives Australopithecus and Paranthropus. This brain increase may have coincided with the first known use of manufactured stone tools, perhaps to extract marrow from animal bones.
The Homo line flourished, and finer-boned Homo varieties also developed over the years. Modern humans, or Homo sapiens, appeared about 150,000 years ago.
The Penn researchers said the jaw muscle mutation opened an evolutionary struggle in which brain conquered brawn, although it probably took another million years for Australopithecus and Paranthropus to disappear, leaving the world to the Homo lineage.
Critics said the study takes several wrong turns.
Under the pressures of natural selection, mutations occur at differing rates. So the jaw muscle mutation might have occurred far earlier.
Also, if large jaw muscles were eliminated, the plates that form the skull would have fused and the brain compartment would not have expanded.
Many additional - and simultaneous - mutations probably are needed to explain all the changes seen in Homo fossils, they said.
"The single mutation would have reduced the Darwinian fitness of those individuals," said anthropologist Bernard Wood of George Washington University. "It only would've become fixed if it coincided with mutations that reduced tooth size, jaw size and increased brain size. What are the chances of that?"
By Joseph B. Verrengia