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Human Ancestors Needed Short Legs to Fight for Mates, Study Says

Richard A. Lovett
for National Geographic News
March 19, 2007
 
We've all heard the line, "I'm a lover, not a fighter."

But four million years ago our early ancestors called australopiths often couldn't be one without being the other—and shorter legs might have given many males the upper hand.

Scientists have long been baffled as to why the human predecessors retained short legs for an unusually long time period: two million years.

Now a new study in this month's issue of the journal Evolution suggests that the hominids' squat physique gave males an advantage when battling for females.

Conventional theory says that australopiths used their short legs for keeping their balance in trees, where they spent most of their lives.

Over a couple million years, australopiths spent an increasing amount of their time on the ground. But even over a hundred thousand generations they didn't develop the long, loping legs of modern humans (explore an interactive time line of human evolution).

"That's plenty of time for evolution to happen, and it didn't happen," said study author David Carrier of the University of Utah.

Carrier instead thinks that australopiths kept their shorter legs because the early human ancestors were built for fighting, not running.

"Apes fight a number of ways, but usually they're standing on their hind legs," he said. "The idea is that short legs increase your ability to fight by increasing the stability of your stance."

Clues in the Teeth

Carrier compared nine primate species—including gorillas, chimpanzees, and humans (in this case, Australian Aborigines)—to determine whether there was a link between aggressiveness and leg length.

He looked at the animals' teeth and at the relative sizes of males and females, because it's difficult for field biologists to measure aggressiveness directly.

"It would take thousands of hours of observation of each species, and you would have different people making the reports, so there's no fair comparison," he said.

But aggressive species tend to have a large disparity between the weight of males and females, and males tend to have longer canine teeth.

"Canine teeth are the main weapon when an animal bites you," Carrier said.

What he found was that in general shorter-legged species scored high in both markers for aggression.

All primates display some level of aggression when fighting for access to females. If shorter-legged australopith males were better fighters, they would have had better access to reproductively active females and thus have been able to pass their genes to the next generation.

(Related news: "Feet, Not Fists, Cause More Severe Injuries, Report Says" [December 26, 2006].)

But Carrier cautions that just because modern humans have relatively long legs compared to apes doesn't mean we're not aggressive.

It's probably not coincidence, he said, that our ancestors started developing longer legs at about the same time that they started using tools and making weapons.

"Once they had those," he said, "the outcome of a fight would be less dependent on physical strength."

But so far nobody has asked if short-legged humans make for better fighters.

"It would make sense," Carrier said, "but nobody's done a study."

He intends to look into it, however, as he continues his research. Meanwhile, he noted, martial arts bloggers are already discussing his study.

And from looking at the comments, he said, "I don't think those people know, either."

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