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Did Earliest Human Ancestors Have More Apelike Faces?

Stefan Lovgren
for National Geographic News
April 5, 2007
 
The earliest direct ancestors of modern humans may have looked more like apes than previously thought, a new study suggests.

But the findings, based on a reconstructed 1.9-million-year-old skull, are highly controversial among the anthropological community.

New computer-generated reconstructions suggest that the specimen had a smaller brain than scientists had believed as well as a distinctly protruding jaw.

"We see in this new reconstruction primitive features that are carryovers from what may be its Australopithecus ancestor," said study author Timothy Bromage, an adjunct professor at New York University College of Dentistry.

Australopithecus is an extinct apelike creature closely related to humans that lived up to four million years ago (see photos of fossils belonging to the oldest known Australopithecus child).

But other experts expressed skepticism about Bromage's argument that the repositioning of the specimen's face means its brain size must have been smaller.

"It's probably right that the face should stick far more forward. But to say that because they've changed the angle of the face, the brain size has to get smaller doesn't make any sense," said Robert Martin, a biological anthropologist at the Field Museum in Chicago, Illinois.

"The [specimen's brain] certainly isn't as small as [Bromage is] now arguing."

Bromage presented his study at the recent annual meeting of the International Association for Dental Research in New Orleans, Louisiana.

Half-Size Brain?

The skull used for the reconstruction was found in 1972 by Bernard Ngeneo, a member of a team led by anthropologist Richard Leakey, at Lake Turkana in Kenya's Great Rift valley.

Named KNM-ER 1470, the fossil skull has been at the center of much debate concerning its species.

After its discovery Leakey's team reassembled the fragmented skull by hand.

Their work suggested that the species had a large brain and a flat face, features similar to modern humans but unlike any early human species known to have existed two million years ago.

Originally thought to be a member of the species Homo habilis, the specimen was later categorized as Homo rudolfensis by scientists who believed it represented a separate species.

For the new reconstruction, Bromage used a deformable cast and computer-generated models to create replicas of the skull that could be shaped.

(Related image: "Earliest Known Human Ancestor" [April 7, 2005].)

He applied biological principles stating that the eyes, ears, and mouth must be in the same relationship to one another in all mammals.

According to Bromage, the cranial capacity of an early human can also be estimated based on the angle of the jaw's slope.

His reconstruction downsized the fossil skull's brain size from about 750 cubic centimeters to about 525 cubic centimeters. (Modern humans today have an average cranial capacity of about 1,300 cubic centimeters.)

"We now have a skull with a cranial capacity and facial prognathism [protruding muzzle] that actually fits with everything we see at that time," Bromage said.

Evolutionary Landscape

But Martin, the Field Museum anthropologist, says brain size cannot be inferred from jaw protrusion.

"To argue that because they've shifted the face forward, the brain has to decrease by 225 cubic centimeters is ridiculous," he said.

Martin also disputes the claim that a specimen living 1.9 million years ago could not have had a cranial capacity of 750 cubic centimeters.

He points out that a 1.6 million-year-old Homo erectus skeleton known as Turkana Boy had a cranial capacity of about 900 cubic centimeters.

"The 750-cubic-centimeter capacity sounds about right. It's certainly in that ballpark," he said.

Anthropologist Meave Leakey, who worked on the original reconstruction, declined to comment on the new study because it has not been peer-reviewed or published.

And Henry McHenry, an anthropology professor at University of California, Davis, said Bromage faced some sharp questioning from colleagues when he first presented his findings at a symposium on human evolution in South Africa last year.

"I am a bit skeptical that Tim's version is all that much superior and that the original reconstructions violated principles of craniofacial development," said McHenry.

Bromage, however, says his reconstruction calls into question to what extent Homo rudolfensis differed from earlier, apelike human ancestors.

"It's quite possible that Australopithecus is the ancestor to Homo rudolfensis," Bromage said.

He suggests that humans may have developed the larger brains and flatter faces characteristic of modern humans as recently as 1.6 million to 1 million years ago.

"People who want to reinterpret the human evolutionary landscape in that era," he said, "really ought to do that with this new reconstruction."

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