Stocky build gave ape-like ancestors a leg up

The short legs and squat physique of the ape-like human ancestors known as australopiths made them better fighters, according to a study published this month.

University of Utah professor of biology David Carrier said a stockier physique would have lowered the centre of gravity for the male australopith and given it more leverage when fighting over access to females.

The study analyzed leg lengths and indicators of aggression in nine primate species, including human aborigines, and appeared in the March issue of the journal Evolution.

Males from the genus Australopithecus — the immediate predecessor to the human species Homo — had heights of about four feet six inches, while females were about three feet nine inches tall. They lived from approximately four million to two 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 in a statement. "So the question is, why did australopiths retain short legs fortwo million years?"

Traditional explanations for short legs in primates have centred on their utility in climbing trees, which were an important part of the australopith habitat.

But the hypothesis breaks down because it doesn't explain why gorillas and orangutans — the two apes with the shortest legs for their body size — spend the least amount of time in trees, Carrier said.

Carrier found a positive co-relation between primate aggressiveness and shorter legs, and that females tended to have longer legs than males.

Humans are "a special case" excepted from this rule and are not less aggressive because they have long legs, he said. Carrier hypothesized in a 2004 paper that australopiths evolved into long-legged early humans only when they learned to make weapons and fight with them.

Long legs also had an advantage

But there was alsoan advantage to longer legs, according to a study released by the Washington University in St. Louis last week.

Longer legs also save on the rate of energy use required to move, according to a mathematical model developed by Herman Pontzer, an assistant professor of physical anthropology at the university.

"All things being equal, leg length is one of the major determinants of [energy] cost," Pontzer said in a statement.

"If two animals are identical except for leg length, the animal with longer legs is more efficient."