Researchers say they may have discovered the mutation that caused the earliest humans to branch off from their apelike ancestors -- a gene that led to smaller, weaker jaws and, ultimately, bigger brains.
Smaller jaws would have fundamentally changed the structure of the skull, they contend, by eliminating the 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, allowing for a bigger brain capable of toolmaking 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 in the hotly contested field of human origins; one scientist declared it "counter to the fundamentals of evolution," while another pronounced 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."
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." 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.
Different types of myosin are produced in different muscles; in the chewing and biting muscles, the gene MYH16 is expressed.
In primates such as 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 that 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.