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Anthropologists have long argued that humans evolved an upright posture to allow walking on two legs, but a University of Utah biologist now wonders whether standing up is really about throwing punches.

In a new study, David Carrier shows that men generate much greater hitting force from a standing position than from kneeling. And downward strikes carry more than three times the force than upward strikes from either posture, according to his findings, published online Wednesday in the Public Library of Science journal PLoS ONE.

His results support the hypothesis that ancestral humans adopted bipedal posture so that males could fight with the strength of their forelimbs, making their punches more dangerous, according to Carrier. He also believes his findings could explain why women are more likely to fall for tall guys. Taller men could deliver blows downward, thus giving them an edge in competing for mates and defending territory and resources.

"If this were true, females who chose to mate with tall males would have had greater fitness for survival," he said.

Carrier's study, supported by a grant from the National Science Foundation, is sure to stir debate in anthropology circles because it suggests that men are innately aggressive.

"It's a very different perspective. That's what's great about Carrier. He's a creative thinker," said anthropologist Dan Lieberman, a Harvard professor of human evolutionary biology who studies running in a variety of animals. "He's making the argument that there is a performance advantage for a biped in hitting other people. It's good to challenge old ideas and come up with new ones."

Carrier contends his findings could shed light on the roots of human aggression.

"We are a relatively violent species," Carrier said. "If a goal is to reduce violence we need to understand our evolutionary history. It probably is a combination of things we were selected for. I'm saying one of those things is aggression."

His insights were inspired in part by watching the dogs he put up for adopting after they were used in scientific research. They tended to converge in his kitchen and when two unruly young males fought, he noticed they would rear up on their hind limbs. Many others mammals also stand up to fight, including anteaters, lions, bears, horses and even rodents.

Did ancient pre-bipedal hominids do the same when males squared off in competition for women?

Carrier has long studied the biomechanics of animal locomotion and had previously argued that running ability drove human evolution toward an upright posture. He once tried to chase down pronghorn, North America's speediest land animal, in a quest to show that humans evolved into great distance runners because it enabled ancestral humans to capture four-footed game.

Scientists believe the upright gait was advantageous in the African savannah, the evolutionary crucible of humans up to 5 million years ago, because it allowed hominids to forage in tree branches, see over tall grass and shed body heat. But Carrier's perspective changed after he and others determined that bipedalism, or two-legged walking, is a less efficient way of covering ground.

"It cost more. There's no advantage in terms of speed, acceleration or endurance," Carrier said. "It's a complicated story because humans are exceptionally good endurance runners. The switch to bipedalism did not help us for endurance running, so we did it for other reasons."

To conduct his latest study, Carrier recruited eight to 12 men to participate in various punching analyses. He invited only participants with a background in boxing or marshal arts and ran them through tests designed to measure the power of their arm thrusts up, down, forward and sideways from standing and kneeling postures.

There was no match between the two positions. As Carrier expected, a standing man's blows in all directions were more dangerous.

According to Lieberman, sexual dimorphism, the physiological differences between males and females, is often associated with male aggression, which is rampant among the great-ape species. In advanced primates, males tend to be larger with stronger upper bodies than females.

"Males are beating up each other for access to females. The fossil record also shows canine [teeth] size in early hominids get smaller," he said. The flattening of the canine might have occurred because teeth became less important in fighting as forelimbs became more useful for subduing opponents.

Carrier "gives another line of evidence that body size really matters," Lieberman said. "Did we stand up to feed better, to walk better? We can now watch men punch each other."