19 May 2011
Two legs good, four legs bad - for beating the crap out of each other
by Kate Melville
Men can punch much harder when they stand on two legs and hit downward rather than when they are on all fours, giving tall, upright males a distinct fighting advantage, say University of Utah researchers who offer a controversial new theory on the origins of man's bipedal posture. Their paper, in the journal PLoS ONE, also suggests that the ability to hit harder is why women tend to prefer tall men.
"The results of this study are consistent with the hypothesis that our ancestors adopted bipedal posture so that males would be better at beating and killing each other when competing for females," said biology professor and study author David Carrier. "Standing up on their hind legs allowed our ancestors to fight with the strength of their forelimbs, making punching much more dangerous."
The idea that fighting played a role in making human ancestors shift from walking on all fours to walking on two legs is not new, but Carrier's study quantifiably demonstrates the advantage by comparing the force of punches by male boxers and martial arts practitioners as they hit in four different directions: forward, sideways, down and up.
Carrier used a punching bag fitted with a sensor that measured the force of forward and sideways punches. For strikes downward and upward, the men struck a heavy padded block on the end of a lever that swung up and down. In either case, the men struck the target as hard as they could both from a standing posture and on their hands and knees.
The findings revealed that for all punching angles, men hit with far more force when they were standing, and from both postures they could hit over twice as hard downward as upward.
The transition to two-legged posture is a defining point in human evolution, yet the reason for the shift is still under debate. Darwin thought that our ancestors stood up so they could handle tools and weapons. Other theories suggest that bipedalism evolved for carrying food, dissipating heat, efficient running or reaching distant branches while foraging in trees.
Locomotor advantages - increases in the efficiency of walking and running - have also been suggested, but Carrier argues that walking bipedally comes at a significant cost. "It's energetically more expensive, it's harder to speed up and slow down, and there are costs in terms of agility. In every way, going from four legs to two is a disadvantage for locomotion. So the selective advantage for becoming bipedal, whatever it was, must have been important."
Carrier believes that the usefulness of quadruped forelegs as weapons is a side-effect of how forelegs are used for walking and running. When an animal is running with its body positioned horizontally, the forelegs strike down at the ground. By lifting the body to a vertical posture, animals can direct that same force toward an opponent. He points out that all sorts of four-legged animals stand up and use their front legs to fight. They include anteaters, lions, wolves, bears, wolverines, horses, rabbits and many rodents and primates.
"My study provides a mechanistic explanation for why many species of mammals stand bipedally to fight," said Carrier, who added that many scientists are still reluctant to consider the notion that our ancestors were violent. "Among academics there often is resistance to the reality that humans are a violent species. It's an intrinsic desire to have us be more peaceful than we are," he says.
While the study primarily deals with the evolution of upright posture, it may also explain why women choose tall men as mates. Multiple studies have shown that women find tall men more attractive. Greater height is also associated with health, social dominance, symmetrical faces and intelligence in men and women. This has led some scientists to suggest that women prefer tall men because height indicates "good genes" that can be passed on to offspring, but Carrier believes there is more to it.
"If that were the whole story, I would expect the same to be true for men - that men would be attracted to tall women. But it turns out they're not. Men are attracted to women of average height or even shorter," he says.
Carrier's explanation is that tall males among our ancestors were better able to defend their resources, partners and offspring. An enhanced capacity to strike downward on an opponent may have given tall males a greater capacity to compete for mates and to defend their resources and offspring. "From the perspective of sexual selection theory, women are attracted to powerful males, because powerful males can protect them and their children from other males," he concluded.
Early Humans And Chimps Much More Than Just Good Friends
Missing Link A Tripping Chimp?
Cougar sex drive an evolutionary adaptation
Unnatural selection: Courtesy of The Pill
Experiments reveal aggression to be a "manhood-restoring tactic"
Source: University of Utah