Posted on 03/24/2004 11:53:42 AM PST by Junior
Igniting a scientific furor, scientists say they may have found the genetic mutation that first separated the earliest humans from their apelike ancestors.
The provocative discovery suggests that this genetic twist — toward smaller, weaker jaws — unleashed a cascade of profound biological changes. The smaller jaws would allow for dramatic brain growth necessary for tool-making, language and other hallmarks of human evolution on the plains of East Africa.
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 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 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 (news - web sites). "But their suggestion connecting it to the brain is way too speculative."
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?"
More straw clutching
Interesting point. I wonder if anyone on this thread can come up with an explanation.
toward smaller, weaker jaws
and realized that I am a very advanced human. I have TMJ syndrome and wouldn't have much of chin without my beard inflating the poor thing.
The explanation is that it probably didn't make much, if any, practical difference to their ability to eat, and therefore had no impact on their ability to reproduce. Many people misconstrue evolutionary science to state that any mutation that is less clearly adaptive than another - however trivial and inconsequential to survival - will not perpetuate due to that initial reduction in fitness.
That is false. It's unfortunate that an anthropologist makes such an elementary error.
Close to the truth now, are we?
I think my sister used to date him!
LONDON (Reuters) - A genetic mutation that occurred 2.4 million years ago could be the reason why modern humans have such big brains and weak jaws, scientists said on Wednesday.
They discovered that a fault in a gene called MYH16 in modern humans happened at about the same time that their skulls started to change in shape from other primates, allowing their brains to increase in size.
But the trade-off was a smaller, less powerful jaw.
"The coincidence in time...may mean that the decrease in jaw muscle size and force eliminated stress on the skull which released an evolutionary constraint on brain growth," said Nancy Minugh-Purvis, a member of the team at the University of Pennsylvania School of Medicine, that made the discovery.
All humans have the MYH16 mutation but other primates, including chimpanzees and macaques, still have the intact gene. Over the past few million years, since the genetic fault occurred, human skulls have grown three times in size and the outwardly elongated jaws have receded.
Pete Currie, of the Victor Chang Cardiac Research Institute in Sydney, believes the research published in the science journal Nature could be the first functional genetic difference between humans and apes.
"Remarkably, the timing of the appearance of this genetic alteration, or mutation, roughly coincides with the appearance of "human-like" characteristics in the hominid fossil record," Currie said in a commentary in the journal.
Minugh-Purvis along with Hansell Stedman and other experts at the university pieced together the complicated puzzle after discovering that the gene was intact in primates but mutated in all humans.
A genetic fault is often linked with some type of inherited disease but the scientists were puzzled about what type of disease was common in all humans throughout the world.
Further research revealed that MYH16 was associated with muscles involved in chewing and biting and it encoded a protein in primate jaw muscles. This led the researchers to suspect the so-called disease in humans was a weaker bite.
Stedman and his colleague said the weaker bite would have lessened the force on the skull so it could grow larger and provide more space for a bigger brain.
"We can only hope that this study represents the vanguard of a new wave of analyzes that focus on the genetic basis of human evolution," Currie added.
A genetic mutation that occurred 2.4 million years ago could be the reason why modern humans have such big brains and weak jaws, scientists said Wednesday.
They discovered that a fault in a gene called MYH16 in modern humans happened at about the same time that their skulls started to change in shape from other primates, allowing their brains to increase in size.
But the trade-off was a smaller, less powerful jaw.
"The coincidence in time...may mean that the decrease in jaw muscle size and force eliminated stress on the skull which released an evolutionary constraint on brain growth," said Nancy Minugh-Purvis, a member of the team at the University of Pennsylvania School of Medicine, that made the discovery.
All humans have the MYH16 mutation but other primates, including chimpanzees and macaques, still have the intact gene. Over the past few million years, since the genetic fault occurred, human skulls have grown three times in size and the outwardly elongated jaws have receded.
Pete Currie, of the Victor Chang Cardiac Research Institute in Sydney, believes the research published in the science journal Nature could be the first functional genetic difference between humans and apes.
"Remarkably, the timing of the appearance of this genetic alteration, or mutation, roughly coincides with the appearance of "human-like" characteristics in the hominid fossil record," Currie said in a commentary in the journal.
Minugh-Purvis along with Hansell Stedman and other experts at the university pieced together the complicated puzzle after discovering that the gene was intact in primates but mutated in all humans.
A genetic fault is often linked with some type of inherited disease but the scientists were puzzled about what type of disease was common in all humans throughout the world.
Further research revealed that MYH16 was associated with muscles involved in chewing and biting and it encoded a protein in primate jaw muscles. This led the researchers to suspect the so-called disease in humans was a weaker bite.
Stedman and his colleague said the weaker bite would have lessened the force on the skull so it could grow larger and provide more space for a bigger brain.
"We can only hope that this study represents the vanguard of a new wave of analyzes that focus on the genetic basis of human evolution," Currie added.
Survival of the adequate? Actually, I see your point. If no other mutations were to ever occur, the primates with the stronger jaws would probably outlast the primates with the weaker jaw, over a number of generations. However, since other mutations do occur, the primates with the weaker jaws are in a position to profit from them, in the genetic long-run.
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