Like in humans, the fourth metatarsal bone of Australopithecus afarensis had become arched in adaptation to the stresses of walking on two feet.
Credit: Kimberly A. Congdon, Carol Ward, and Elizabeth Harman (deceased)
SYDNEY: The unearthing of a foot bone in Ethiopia belonging to the human ancestor Australopithecus afarensis has bolstered the long-disputed hypothesis that these hominids walked in an upright posture akin to modern humans.
While it is well known this species - which lived 3.7 million to 2.9 million years ago - was bipedal, researchers have long argued over the extent to which they had truly abandoned the tree climbing lifestyle of their predecessors.
“This new foot bone we present provides definitive evidence that the A. afarensis foot has stiff arches like humans, showing that by 3.2 million years ago, our ancestors had given up the ability to climb trees well, and had forfeited life in the trees for a committed life on the ground”, said lead author and palaeoanthropologist Carol Ward of the University of Missouri
Did Lucy walk like us?
The 1974 discovery of ‘Lucy’ - the famous partial skeleton of A. afarensis - revealed that the species was an upright walker. However, in the absence of a complete fossil record, and in particular crucial bones from the midfoot, researchers have been unsure of the extent of the species’ bipedalism.
Were they a dedicated terrestrial species? Or, did they alternate time on the ground with time spent clambering about in the trees as most modern apes do?
To the satisfaction of proponents of the former view, the discovery of this almost perfectly preserved fourth metatarsal appears to have finally laid the debate to rest.
Arches act as shock absorbers
The discovery - led by Ward’s co-author of the paper published in Science, William Kimbel of Arizona State University - was made at the famed site where ‘Lucy’ and the ‘first family’ of A. afarensis were found in Hadar, Ethiopia in the 1970s.
Sharing several features with the feet of modern humans, including a stiff, sloping arch - in contrast with the flexible feet of most apes - the newly discovered metatarsal points strongly to an upright lifestyle.
“We have very stiff feet that allow us to push off with our toes when walking on the ground, and we also have arches in our feet that act as shock absorbers”, Ward explains. “Seeing the presence of stiff feet with arches tells us that agile climbing was no longer important for [A. afarensis]”.
Insight into human evolution
For Peter Brown of the University of New England in New South Wales - the palaeoanthropologist who described the fossil remains of ‘the hobbit’ Homo floresiensis, the discovery provides valuable insight into human evolution.
“[The bone] is extremely similar to that in modern humans... suggesting that obligatory, human-like, bipedal locomotion was present in A. afarensis and may have evolved well before 3.2 million years”, said Brown.
Ward concurs, “not only does this fossil tell us that walking bipedally on the ground was the most significant mode of locomotion for A. afarensis, it also highlights the fact that tree climbing well, at least not much better than you or I could do, was no longer adaptively significant.”
