3 Oct 09

Conventional wisdom says our earliest ancestors first stood up on two legs when they moved out of the forest and into the open savannas. But this does not explain why Ardi's Ardi (short for Ardipithecus ramidus or 'root of the ground ape') species was bipedal (able to walk on two legs) while still living partly in the trees.

Owen Lovejoy from Kent State University said the answer could be as simple as food and sex. He pointed out that throughout evolution males have fought with other males for the right to mate with fertile females. Therefore you would expect dominant males with big fierce canines to pass their genes down the generations.

But say a lesser male, with small stubby teeth realised he could entice a fertile female into mating by bringing her some food? Males would be far more successful food-providers if they had their hands free to carry home items like fruit and roots if they walked on two legs.

Mr Lovejoy said this could explain why males from Ardi's species had small canines and stood upright - it was all in the pursuit of sex. He added that it could also suggest that monogamous relationships may be far older than was first thought.

Ardi stood 4ft tall and weighed 110lb. She lived a million years before the famous Lucy, the previous earliest skeleton of a hominid who was dug up in 1974. Experts believe Ardi is very, very close to the 'missing link' common ancestor of humans and chimps, thought to have lived five to seven million years ago.

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