Thursday, March 25, 2004

Genes and Human Evolution

From reports like this one and this one, it seems clear that the decision to sequence the chimpanzee genome is quickly paying scientific dividends, just as I expected it would.

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, 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 food

[............]

University of Michigan biological anthropologist Milford Wolpoff called the research "just super."

"The other thing that was happening 21/2 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 were reduced, 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."

Under normal circumstances, the endorsement of anything by Milford Wolpoff would be enough to make me doubt its veracity. Wolpoff's stubborn defense of his multiregionalist theory, long after the genetic and archeological evidence against it has become crushingly overwhelming, and his overeager misinterpretation of the Mungo Man fossil find, have long marked him in my mind as the anthropological equivalent of Alan Feduccia, whose dogged insistence on denying the theropod ancestry of birds against all evidence brings him awfully close to the mindset of creationists.

Despite Wolpoff's enthusiastic endorsement of this paper, however, I think that there really is something to what the researchers have managed to find, Owen Lovejoy's claims to the contrary; whether that something is quite what Stedman et. al. think they've found is something else altogether. It isn't at all clear to me why jaw musclature should have been some sort of barrier to brain growth, and it seems a lot more likely to me that the actual direction of causality has been reversed here: it wasn't the loss of jaw muscles that enabled brains to grow, but an increase in intelligence that enabled our ancestors to get higher quality food than the tough vegetation that other primates must make do with, removing the selective restraint against a weakening of jaw muscles.

One good reason for believing this to be the correct order of events, rather than that posited by Stedman and co., is that without any increase in food gathering capacity, there would have been no good reason for strong jaw muscles to have been any less vital than they'd been for millions of years previously. If our ancestors were still no brighter than Australopithecines when this change is supposed to have occurred, how could they have possibly eked out a living in the interim while their brains were supposed to have been growing? Indeed, here's another paper that indicates that hominids were already using stone tools 2.5 million years ago, or about the very time when loss of function of the MYH16 gene is estimated to have taken place.

NB - An abstract of the actual research paper can be found here. A discussion of the paper is already underway at The Panda's Thumb