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?