The evolutionary split between early humans and apes may have begun with a tiny mutation in a gene for jaw muscles - a lucky break that allowed the skull to grow and make room for the enormous brain that would eventually become the hallmark of Homo sapiens.
That's the controversial conclusion of researchers from the University of Pennsylvania, whose discovery of the mutation, announced Wednesday, has fanned the long-smoldering debate over how, exactly, modern humans evolved.
The Penn team's work suggests that early primate skulls - much like the skulls of modern gorillas and chimpanzees - were literally muscle-bound by powerful jaw muscles and cramped by the big bony spurs that anchored them. Only when a quirk of nature produced mutants with radically smaller jaw muscles was the skull free at last to expand over the generations, report Mercurynews.com
Hansell Stedman, the leader of the team said,"This is the first genetic distinction between humans and chimpanzees that can be linked to the fossil record". He and his colleagues claim that a tiny change to the myosin heavy chain (MYH16) gene weakened our ancestral jaw muscles.
As a consequence, the hominid skull had room to shift shape, enabling it to accommodate an ever-enlarging brain. In contrast, chimpanzees were stuck with big, powerful jaws and, by necessity, much smaller brains.
Pete Currie, geneticist and developmental biologist agreed that the nature and timing of the mutation "linked exquisitely" with the timing of evolutionary changes identified by paleontologists. "This is the first window for looking into and seeing what it is about our genes that makes us human," he said.
According to Boston.com 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."
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.