Published in The Hindu on March 29, 2012

If the ability to walk and run on two legs (bipedalism) sets humans apart from apes, could this transition from tree-climbing to terrestrial walking have been smooth? Scientists think not, and the evidence provided in a paper published today (March 29) in Nature , shows why.

A partially preserved skeleton of a single foot from a site (Worsanso-Mille) in the central Afar region of Ethiopia provides important evidence of the human ancestor’s ability to walk on land while still retaining the ability to climb trees. The taxonomic affinity of the new specimen remains undetermined.

The skeletal remains, dated to around 3.4 million years ago, do not match the Australopithecus afarensis early humans (hominins) found between 2.9 million years to 3.6 million years ago. In fact, the fossil shows close resemblance to the earlier Ardipithecus ramidus (about 4.4 million years ago) with a divergent and relatively short big toe, as in the case of apes.

By comparing the “functional morphology and proportions of several early hominin foot elements,” the researchers have built a strong case to test the diversity in bipedalism in early humans. The new species “indicates the presence of more than one hominin locomotor adaptation at the beginning of Late Pliocene epoch.”

The study also records the presence of more than one early human species in eastern Africa around 3.4 million years ago.

Anatomically, the fossil foot falls between modern humans and gorillas, the authors note. While certain features resemble As. Afarensis, it differs from chimpanzees and from African apes.

Immaterial of its affinity to any species, the fossil foot while still retaining certain anatomical characteristics of tree climbers, has features that clearly show an ability to walk on land. “The foot skeleton represents a hominin” despite retaining the grasping capacity, they write. “When on the ground it was at least facultatively bipedal, although it may have practised bipedalism in a novel fashion.”

As a News and Views piece in the same issue of the journal notes, the close resemblance of the fossil to Ar. Ramidus makes a strong case of tree-climbing bipedal early humans roaming eastern Africa from 3.4 million years to 4.4 millions years ago, the same time As. Afarensis walked firmly on the ground.

Though other hominins like As. Sediba and Homo habilis had many features that resembled the feet of modern humans, it was not until the arrival of Homo erectus that truly human-like feet finally evolved.