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?"
This makes a bit of sense. Observe this chimp skull:
Without the need for all that extra materal up there to hold the massive jaw muscles, a different -- and separate -- mutation for a larger brain case would then be able to survive. It could pass through the birth canal without killing the baby. One mutation, the relatively trivial one for a smaller jaw, makes the other mutation able to survive.
Yes, that's why the process frequently takes millions of years.
Since there can not be "self-generated" information in the DNA, the information that makes Homo Sapiens what they are had to have been in the DNA before the "split."
One can say that it was.
If it was there before the "split" why don't the chimps have the exact same body as we have?
Because they express different parts of the information than we do. Think of it this way. HTML includes all the information expressed in this thread. It also includes all the information expressed in this thread. The two threads are not the exact same.
I don't know the answer to that, but sometimes one mutation has broad spectrum of effects.
Another way to think about it is that all sorts of mutations occur and have no effect on Dariwinian fitness of the species until the environment changes and the mutation then confers an evolutionary advantage on its holders. A good example of a mutation which is just there and has yet to confer an evolutionary advantage (or disadvantage) is the ability to roll one's tongue. Some people have it, other don't and it doesn't seem confer a any kind of evolutionary advantage either way. Until an environmental change happens that favors (or disfavors) individuals with that ability, it just continues along in the genome.
By the way, the loosed jaw mutation wouldn't necessarily confer an evolutionary disadvantage on individuals with it in an environment with abundent "soft" food. If there was already a genetic pre-disposition in primates for larger brain growth, the occurrence of the jaw loosening could have been the key that unlocked the door to bigger brains in a relatively short timespan. That, in turn, would have conferred a significant advantage on the proto-humans.
There are folks who belive that big brains in mammals are a result of sexual selection, which is to say that smarter guys get more chicks. Consider your brain to be equivalent to a rack of antlers. Or in the case of democrats, a flaming red butt.
You can't have your cake and eat it too.
Simple. Environments change. A trait which confers an advantage in one environment may have no effect, or may be detrimental in another environment. For example, the human body's ability to efficiently convert calories to fat and store it and then slowly burn it off is a great trait in an environment which has abundant food supplies during the summer and little food during the winter. The same trait will cause obesity and heart disease in an environment with abundant high calorie food all year long.
The difference arose because of a change (mutation) in the genes. Mutations are not gross changes. Rather, they can be as simple as a flip-flopped gene, or a gene that's been turned off (such as our Vitamin C synthesizing gene), turned on, added to or deleted.
I've long been curious whether cosmic rays could mutate even a protein in a gene's structure in ways that might alter the replication of subsequent genes. Hence, evolution.
'Don't see any conflict in this with The Bible.
First? The hominids had been walking upright for a couple million years before this mutation came along. And their jaws had been smaller than chimp jaws for a long time, too, without any great increase in brain size.
I'm not saying that they haven't uncovered a significant step on the road from ape to man, but it looks like just one out of many.
One of two or three times in a move where I have shed tears. Of course I saw it on the big screen, before anyone knew it would be a classic. I think there were ten other people in the theater.
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