According to a new research into the life of the recently confirmed world's oldest hominid 'Ardi' has suggested that the creature started walking on two legs for food and sex.
A hand-bone discovered in 1994 in Ethiopia by project scientist Yohannes Haile-Selassie, a paleontologist and curator at the Cleveland Museum of Natural History, eventually led a team of scientists to the partial skeleton known as Ardi, which they excavated during three subsequent field seasons.
The female skeleton is 4.4 million years old, 1.2 million years older than the skeleton of Lucy, or Australopithecus afarensis, the most famous and, until now, the earliest hominid skeleton ever found.
According to a report in National Geographic magazine, among the slew of research papers about the new find is one about the creature's sex life.
One of the defining attributes of Lucy and all other hominids is that they walk upright on two legs.
While Ardi also walked on two legs on the ground, the species also clambered about on four legs in the trees.
Ardi thus offers a fascinating glimpse of an ape caught in the act of becoming human.
At the time Ardi lived, her environment was a woodland, much cooler and wetter than the desert there today.
So why did her species become bipedal while it was still living partly in the trees, especially since walking on two legs is a much less efficient way of getting about?
According to Owen Lovejoy of Kent State University, it all comes down to food, and sex.
In apes-both modern apes and, presumably, the ancient ancestors of Ardipithecus-males find mates the good old-fashioned apish way: by fighting with other males for access to fertile females.
Success, measured in number of offspring, goes to macho males with big sharp canine teeth who try to mate with as many ovulating females as possible.
Males would be far more successful food-providers if they had their hands free to carry home loads of fruits and tubers-which would favor walking on two legs.
Females would come to prefer good, steady providers with smaller canines over the big fierce-toothed ones who left as soon as they spot another fertile female.
The results, according to Lovejoy, are visible in Ardipithecus, which had small canines even in males and walked upright.
Lovejoy's explanation for the origin of bipedalism thus comes down to the monogamous pair bond.
Far from being a recent evolutionary innovation, as many people assume, he believes the behavior goes back all the way to near he beginning of our lineage some six million years ago.