Weak jaws, bigger brains?

2004-03-25 14:07

Denver - 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, 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".

'Genetic mutation persists'

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.

Non-human 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 two 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.

Tantalising theory

Other researchers disagreed that human evolution could 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.

University and commercial laboratories rapidly are comparing the human genome with that of chimpanzees to determine what makes people human, and how the earliest transitional creatures known as hominids split from Old World apes and monkeys some six million years ago.

Jaws have been a focus of evolutionary research since Darwin, and the mutation offers a tantalising theory. But it is unlikely that one mutation - even at a crucial evolutionary juncture - would make a person, some sceptics said.

On the net:

www.mnh.si.edu

- AP