Ape-like human ancestors known as australopiths maintained short legs for 2 million years because a squat physique and stance helped the males fight over access to females, a University of Utah study concludes.
"The old argument was that they retained short legs to help them climb trees that still were an important part of their habitat," says David Carrier, a professor of biology. "My argument is that they retained short legs because short legs helped them fight."
The study analyzed leg lengths and indicators of aggression in nine primate species, including human aborigines. It is in the March issue of the journal Evolution.
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 says.
"So the question is, why did australopiths retain short legs for 2 million years? Among experts on primates, the climbing hypothesis is the explanation. Mechanically, it makes sense. If you are walking on a branch high above the ground, stability is important because if you fall and you're big, you are going to die. Short legs would lower your center of mass and make you more stable."
Yet Carrier says his research suggests short legs helped australopiths fight because "with short legs, your center of mass is closer to the ground. It's going to make you more stable so that you can't be knocked off your feet as easily. And with short legs, you have greater leverage as you grapple with your opponent."
While Carrier says his aggression hypothesis does not rule out the possibility that short legs aided climbing, but "evidence is poor because the apes that have the shortest legs for their body siz
Contact: David Carrier
University of Utah