Touching off a scientific furor, 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 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 — generally overlaps 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 2½ 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 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 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 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, 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.”
Details of the study
In their experiment, the Penn 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 of the jaws, the gene MYH16 is expressed. But the Penn researchers discovered humans have a mutation in the gene that prevents the MYH16 protein from accumulating. That limits the size and power of the muscle.
In primates like the macaque, the jaw muscles were 10 times more powerful than in humans. They contained high levels of the protein, and the thick muscles were attached to bony ridges of the skull.
When did this genetic split occur? Scientists assume that the rate of genetic change a species undergoes is relatively constant over time. So the Penn group looked deep into the fossil record to determine when the jaws of human ancestors started looking smaller and more streamlined as compared to more apelike creatures.
Homo habilis was the earliest known species to begin showing skull and jaw differences from its more apelike cousins more than 2 million years ago.
The Homo line flourished, with the finer-boned Homo rudolfensis, ergaster and erectus lines soon emerging.
Meanwhile, the heavier-browed, long-jawed Australopithecus afaransis and Paranthropus robustus eventually disappeared.
Without the strong bands of muscle constraining the skull, the Penn researchers said the Homo skull changed shape and grew to accommodate a much larger brain, while the Australopithicine skulls did not.
The Penn researchers said mutation opened an evolutionary struggle in which brain conquered brawn, although it probably took another million years to complete.
The mutation also offers a glimpse of behavioral changes, the Penn researchers said. Apes use their powerful bites to maintain social control, while early humans may have had to rely more on cooperation.
Critics said the study wrongly assumes that evolution works so neatly.
The first early humans with the mutation probably would have had weaker mouths, but still had large teeth and jaws. Many additional mutations would have been needed.
“The 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?”