A skull with a prominent brow and small face is at the center of an ugly spat between anthropologists who disagree over whether it belonged to a gorilla or human ancestor.
If Michel Brunet of the University of Poitiers in France and his colleagues are right, the 6 million- to 7 million-year-old skull Brunet's student found in the sands of Chad in 2001 represents humankind's oldest ancestor.
If Milford Wolpoff of the University of Michigan in Ann Arbor and two French colleagues are correct, the skull is merely the remnants of a strange female gorilla.
At stake is scientists' understanding of human evolution, not to mention a few researchers' reputations.
"We don't really want to beat [Brunet] up about this," says Wolpoff, who was the lead author of an article criticizing Brunet's findings in this week's issue of Nature, "but his comparisons just don't make sense to us."
Skull Was a ‘Nuclear Bomb’
Brunet counters that he has presented enough evidence to prove the skull was a hominid, whereas Wolpoff has offered no proof that the skull belonged to a gorilla. He adds that the University of Michigan anthropologist never even viewed the skull or casts of the skull firsthand, but instead relied on descriptions from his co-authors who saw casts of the skull and from Brunet's original report on the find.
"I'm not at all surprised that this specimen has generated alternative opinions; that would be expected," says David Pilbeam, a professor of anthropology at Harvard University in Cambridge, Mass., who assisted in writing Brunet's rebuttal in Nature. "But I am surprised that people would write such radically different opinions without spending time looking at the specimen."
When Brunet announced his find this past July, it was described as having the impact of a "small nuclear bomb" in the field of anthropology. The age of the find and the skull's radically unique features suggested that human evolution was a much messier process than scientists had envisioned.
Fossil records show that the world was populated by apes 10 million years ago. Remnants of early humans then appeared on the fossil record starting at about 5 million years ago. The Chadian skull, named Sahelanthropus tchadensis, appeared to fill that gap. But because it does not easily fit into any of the known types of hominids or other families, it indicates there may have been several false starts in the course of human evolution.
"What this find does is say, 'Hey, you folks should realize that where humans are connected to the tree of life is not a simple connection,'" says Bernard Wood, an anthropologist at George Washington University in Washington.
But Wolpoff says, not so fast.
Walking the Walk
"Our key point is that for there to be a hominid line for us to define it, there must be some basis for that definition," says Wolpoff. "It's generally accepted that the basis for human lineage is bipedalism — walking on two feet — and he has no proof of that."
Wolpoff argues that in order to definitively prove the Chadian creature walked on two legs, more bones are needed — preferably leg or hipbones — that might indicate how joints were aligned. The only clue a skull can offer about walking style is evidence of muscle wear at its base.
An upright animal would likely carry its head on top of its frame and this position requires less neck muscle to keep the head upright. The skull of an animal that walked on four legs generally shows evidence of more muscle wear near the base because more muscles are needed to hold the head upright.
Wolpoff and Brunet disagree over what muscle wear indicate on the Chadian skull. In fact, other researchers (who asked to remain anonymous) point out that the creature could have been a true cross between chimp and human or ape and human and may have walked on two legs at times and four legs at others.
Wolpoff also argues that the teeth recovered and the skull's distinct brow line suggest the fossil might have been a female gorilla. Brunet says these features are common to human ancestors, plus the skull's long, narrow facial features are human-like.
The root of the arguments may actually be more political than scientific.
As Brunet points out in his published rebuttal, Wolpoff's co-authors, French researchers Brigitte Senut and Martin Pickford, claimed last year they had found a fossil called Orrorin tugeneniss, also known as the "Millennium Man," that was a direct ancestor of humans.
The Chadian skull appears to outdate Orrorin by about 1 million years. Senut and Pickford's finding was met by a frosty reception from researchers at the time who doubted their claims and criticized their decision to remove the remains from Kenya.
Brunet's team has kept their discovered remains in Chad.
Soon after seeing casts of Brunet's Chadian find, Senut and Pickford criticized his claim that it was a human ancestor.
Pilbeam is anxious for all sides to move on.
"There's very little to be gained from bunches of people shouting at each other," he says. "My suggestion is when the next set of descriptions are done, then it can be made available to everyone. Then more analysis can be done."