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."