"It would take thousands of hours of observation of each species, and you would have different people making the reports, so there's no fair comparison," he said.
But aggressive species tend to have a large disparity between the weight of males and females, and males tend to have longer canine teeth.
"Canine teeth are the main weapon when an animal bites you," Carrier said.
What he found was that in general shorter-legged species scored high in both markers for aggression.
All primates display some level of aggression when fighting for access to females. If shorter-legged australopith males were better fighters, they would have had better access to reproductively active females and thus have been able to pass their genes to the next generation.
(Related news: "Feet, Not Fists, Cause More Severe Injuries, Report Says" [December 26, 2006].)
But Carrier cautions that just because modern humans have relatively long legs compared to apes doesn't mean we're not aggressive.
It's probably not coincidence, he said, that our ancestors started developing longer legs at about the same time that they started using tools and making weapons.
"Once they had those," he said, "the outcome of a fight would be less dependent on physical strength."
But so far nobody has asked if short-legged humans make for better fighters.
"It would make sense," Carrier said, "but nobody's done a study."
He intends to look into it, however, as he continues his research. Meanwhile, he noted, martial arts bloggers are already discussing his study.
And from looking at the comments, he said, "I don't think those people know, either."
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