Creatures in the genus Australopithecus - immediate predecessors of the human genus Homo - had heights of about 3 feet 9 inches for females and 4 feet 6 inches for males. They lived from 4 million to 2 million years ago. "For that entire period, they had relatively short legs - longer than chimps' legs but shorter than the legs of humans that came later," Carrier said.

Previously, anthropologists believed that australopiths retained their stubby legs for such a long period to assist in tree climbing. While Carrier says his aggression hypothesis does not rule out the possibility that short legs aided climbing, he notes that supporting "evidence is poor because the apes that have the shortest legs for their body size spend the least time in trees - male gorillas and orangutans."

To arrive at his controversial hypothesis, Carrier analyzed leg lengths and indicators of aggression in nine primate species, including human aborigines. He compared Australian aborigines with eight primate species: gorillas, chimpanzees, bonobos, orangutans, black gibbons, siamang gibbons, olive baboons and dwarf guenon monkeys. Carrier used data on aborigines because they are a relatively natural population.

Carrier used available literature to obtain typical hindlimb lengths and data on two physical features that previously have been shown to correlate with male-male competition and aggressiveness in primates:

• The weight difference between males and females in a species. Earlier studies found males fight more in species with larger male-female body size ratios.
• The male-female difference in the length of canine teeth, which are next to the incisors and are used for biting during fights.

He found that hindlimb length correlated inversely with both indicators of aggressiveness: Primate species with greater male-female differences in body weight and length of the canine teeth had shorter legs, and thus display more male-male combat. Conversely, there was no correlation between arm length and the indicators of aggression. Carrier says this is because arms are used for fighting, but "for other things as well: climbing, handling food, grooming. Thus, arm length is not related to aggression in any simple way."

To verify his results, Carrier corrected for each species' limb lengths relative to their body size. Primates with larger body sizes tend to have shorter legs, humans excepted. Without taking that into account, the correlation between body size and aggression indicators might be false. Another analysis corrected for the fact different primate species are related. For example, if three closely related species all have short legs, it might be due to the relationship - an ancestor with short legs - and not aggression.

However, even with the corrections, short legs still correlated significantly with the two indicators of aggressiveness. The study also found that females in each primate species except humans have relatively longer legs than males. "If it is mainly the males that need to be adapted for fighting, then you'd expect them to have shorter legs for their body size," Carrier says.

He adds that humans "are a special case" and are not less aggressive because they have longer legs. There is a physical tradeoff between aggression and economical walking and running. Short, squat australopiths were strong and able to stand their ground when shoved, but their short legs made them ill-suited for distance running. Slender, long-legged humans excel at running. Yet, they also excel at fighting. In a 2004 study, Carrier made a case that australopiths evolved into lithe, long-legged early humans only when they learned to make weapons and fight with them.

"To some extent, our evolutionary past may help us to understand the circumstances in which humans behave violently," he adds. "There are a number of independent lines of evidence suggesting that much of human violence is related to male-male competition, and this study is consistent with that."

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