A Lovely Little Foot


April 23, 2012

“It’s a lovely little foot” declared Don Johanson, upon learning of a  new fossil find in Ethiopia. Johannes Haile-Selassie and others announced in the March 28 issue of the journal Nature the recovery of a partial foot from the Burtele area of  Ethiopia. Eight of the normally 17 bones of the hominin foot were found eroding from sandstone dated to 3.4 million years ago.

Johanson discovered the famous “Lucy” partial skeleton at Hadar in 1974, about 30 miles from Burtele. Lucy, Australopithecus afarensis, was dated to 3.1 ma and researchers have thought until now A. afarensis was the only hominin species in the period between three and four million years ago. Now there appear to have been two but with distinctively different modes of locomotion. Lucy and her species, living between 3.9 and 2.9 ma, were committed bipeds, with a pelvis indicative of upright posture and a foot with aligned toes and suspensory arch.  (Lucy’s distinctive pedal architecture is discussed in “Small Bone, Big Find elsewhere on this website.)

This new partial foot from Burtele (this is all that was found, no other post crania, crania or dentition) lacks an arch and two of the bones comprised an abducted toe, that is to say, an opposable toe, like the thumb on a human hand, capable of grasping tree limbs but less efficient than the human foot on the ground.

This find raises another question: is this foot from a member of the hominin clade or does it belong to an older, more ape like lineage? It has not been assigned to a taxon but its resemblance to the foot of Ardipithecus ramidus (“Ardi”) has been remarked by several scientists commenting on the find. On this question, hominin or not hominin, William H. Kimbel, successor to Don Johanson as Director of the Institute of Human Origins at Arizona State, says,

Ardipithecus is not an ape, but a hominin, and the [new] foot, which closely resembles that of Ardipithecus, demonstrates two divergent (apparently) stable locomotor adaptations middle Pliocene hominins.  It was already unlikely that Ardipithecus ramidus at 4.4 Ma was directly ancestral to earliest known Australopithecus at 4.2, given the fundamental anatomical changes across numerous regions of the body such an idea would necessitate, and this fossil drives that point home. It also puts an exclamation mark on the argument that A. afarensis was a fully committed terrestrial biped, given the radical restructuring of its foot that already had occurred relative to that of a contemporaneous partly arboreal hominin taxon.”

Much more information can be found by reading the Nature paper and The New York Times report of the announcement.