for National Geographic News
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.
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