At a pivotal time in human evolution, about 2.4 million years ago, a muscle gene underwent a disabling alteration. And scientists say this could have made all the difference, leading to the enlarged brains of the lineage that evolved into modern humans.
Researchers who made the discovery said this might be the first recognized functional genetic difference between humans and the apes that can be correlated with anatomical changes in the fossil record. As they said: The gene mutation might represent the beginning of the ancestral triumph of brain over brawn.
At the least, scientists said, the mutated gene probably accounts for the more graceful human jaw, in contrast to the protruding ape jaw and facial ridges.
The discovery was made by scientists at the University of Pennsylvania School of Medicine and will be published today in the journal Nature. They also described the findings in interviews last week.
"We're not suggesting that that mutation alone buys you Homo sapiens," said Dr. Hansell H. Stedman, leader of the research team. "But it lifted a constraint that leads to brain growth."
Evolutionary scientists and paleoanthropologists not involved in the project said the interpretation of the findings was intriguing and provocative.
A "seductive hypothesis," one of them said, while others cautioned that the explanation probably oversimplified the causes behind the significant brain expansion that marked the emergence of the Homo lineage out of the more apelike Australopithecus species.
Even so, the findings were expected to encourage scientists to investigate a whole range of other genes that have decisive roles in making us distinctively human. This could enable molecular biologists to establish the chronology of important steps in human evolution with greater precision.
The Penn scientists were searching for remaining genes that govern myosin, a protein that makes up muscle tissue, when they came across a piece of the human genome sequence that had been overlooked.
The gene, MYH16, had apparently gone unrecognized because of a small mutation that had rendered it inactive for producing some jaw muscles for chewing and biting.
The scientists found that this myosin gene is still intact in other primates today, such as chimpanzees and macaques. They have correspondingly strong jaw muscles.
An analysis of DNA samples showed the gene-inactivating mutation to be present in all modern humans worldwide. The analysis further traced the mutation's occurrence to between 2.1 million and 2.7 million years ago, probably 2.4 million.
That happened to be just before the appearance of major evolutionary changes in hominid fossils, the research team noted in the journal article.
Some hominids with protruding jaws and small brains were soon to evolve into the first species of the genus Homo, with significantly smaller jaws, larger brains and a modern human body size. After 2 million years, Homo erectus was able to strike out for lands far beyond Africa.
"The mutation very possibly initiated an evolutionary cascade," said Dr. Nancy Minugh-Purvis, a paleoanthropologist involved in the project.
Stedman's group concluded that the findings "raise the intriguing possibility that the decrement in masticatory muscle size removed an evolutionary constraint on encephalization."
In short, as the strong, stoutly buttressed jaw muscles declined, the skull could develop a new shape and structure, giving the brain room to grow.
In an accompanying critique, Dr. Pete Currie, a developmental biologist in Sydney, Australia, who called the hypothesis seductive, wrote that the Penn researchers presented "convincing arguments as to how the mutation could have been responsible" for the acquisition of more humanlike traits by ancestral hominids.
"I'm amazed at what they came up with," said Dr. John Fleagle, a paleoanthropologist at the State University of New York, Stony Brook. "But I'd be surprised if the interpretation is that simple.
Dr. Alan Walker, an anatomist who specializes in human evolution at Pennsylvania State University, said, "The mutation is a very interesting finding, but what it means is a different matter."
Walker and others questioned the idea that jaw muscles of the more apelike hominids were a major factor in constraining brain size. "An extremely unlikely proposition," he said.