The long-standing theory that Australopithecus afarensis, one of humanity’s earliest hominid ancestors, was exclusively bipedal has been thrown into doubt after analysis of foot bones from an infant specimen found it likely that the child could climb trees in a manner reminiscent of modern chimpanzees.
As a species, A. afarensis was very successful, with the fossil record showing it lived in eastern Africa from at least 3.85 to 2.95 million years ago.
The first definitively identified remains of the hominid were uncovered in 1974 – although eventually bones collected as early as the 1930s would be included in the lineage – and so far more than 300 individuals have been discovered.
One of these is a juvenile, known from a partial skeleton unearthed in the Dikika region of Ethiopia, and formally described in a paper in the journal Nature in 2006.
“The foot and other evidence from the lower limb provide clear evidence for bipedal locomotion,” stated the researchers, led by Zeresenay Alemseged from Germany’s Max-Planck-Institute for Evolutionary Anthropology, “ but the gorilla-like scapula and long and curved manual phalanges raise new questions about the importance of arboreal behaviour in the A. afarensis locomotor repertoire.”
Those questions have now been at least partly answered following a fresh examination of the foot bones by a team again including Alemseged, now at the University of Chicago in the US.
The examination only became possible in 2013 following the slow and careful removal of the sediments in which the bones were encased. What the researchers found was surprising.
At the time of death 3.2 million years ago, the Dikika infant was likely three years old (and also likely female). Alemseged and colleagues found that at such a young age her foot bones already possessed many of the structures found in adult feet, all indicative of a life spent walking upright.
However, they also found that one particular bone, the medial cuneiform, which plays an important role in joint movement, was curved in such a way as to allow the big toe to move, permitting the foot to grasp.
This ability, more characteristic of non-hominid apes, the scientists suggest, indicates that the young A. afarensis was able cling on to her mother as she travelled, as well as efficiently climb trees.
The discovery, the scientists write in a paper published in the journal Science, was “unexpected”, based on earlier analyses of adult foot bones.