Australopithecus afarensis is known to have walked upright, but it's been hotly disputed whether the famous Lucy and her kin spent a lot of time in the branches as well.
Now, though, a study of an A. afarensis child's shoulderblades has indicated that our ancestors were indeed living partly in trees as recently as three million years ago.
US anthropologists have examined both complete shoulder blades of 'Selam' - an exceptionally well-preserved fossil skeleton of an A. afarensis child from Dikika, Ethiopia, discovered in 2000. And their analysis shows these bones to be pretty apelike, suggesting that she was adapted to climbing trees, as well as walking bipedally on the ground.
"The question as to whether Australopithecus afarensis was strictly bipedal or if they also climbed trees has been intensely debated for more than thirty years," says Midwestern University Professor David Green.
"These remarkable fossils provide strong evidence that these individuals were still climbing at this stage in human evolution."
Selam was a three-year-old A. afarensis girl who lived about 3.3 million years ago, and represents the most complete skeleton of her kind to date. The team has compared her shoulderblades to those of other early human relatives - Homo ergaster, Homo floresiensis, A. africanus and two adult specimens of A. afarensis - as well as chimpanzees, gorillas, orangutans and human beings.
And the bones turn out to be pretty apelike, indicating a partially arboreal lifestyle. At the same time, though, most researchers agree that the A. afarensis's hip bone, lower limb, and foot are clearly adapted for upright walking.
"While bipedal like humans, A. afarensis was still a capable climber. Though not fully human, A. afarensis was clearly on its way," says curator of anthropology at the California Academy of Sciences Zeresenay Alemseged.
"This study moves us a step closer toward answering the question 'When did our ancestors abandon climbing behavior?' It appears that this happened much later than many researchers have previously suggested."