<em>Australopithecus afarensis</em>: a “committed terrestrial biped”

So much information from a single bone!  Paleontologists are expert in deducing a lot of stuff from a fragment of fossil.  Sometimes you have to wonder if they’re doing it rong, but when one of the team is Donald Johanson (the discoverer of the famous Australopithecus afarensis specimen named”Lucy“), then you’re pretty confident.

A new paper in Science by Carol Ward, William Kimbel, and Johanson (reference below) describes the discovery of a metatarsal bone (the fourth metatarsal, i.e., from the toe next to the little toe) from A. afarensis, a bone that pretty conclusively shows that the species was, as the authors affirm, “a committed terrestrial biped.”

You may remember that although the skeleton of Lucy was amazingly complete, it lacked the feet:

The inference that this 3.2 million year old specimen was bipedal was based on two things: the configurations of the pelvis and knee, and the discovery, nearby, of the famous Laetoli footprints, a long trail of footprints left about 3.6 mya by early hominins walking in volcanic ash.  The individuals who made these (probably three of them) certainly walked upright: there are no impressions of knuckles.

Because A. afarensis was the only hominin known from that era in that location (Hadar, Ethiopia), everyone concluded that the species was pretty bipedal.  But fragments of another Hadar foot were ambiguous: some workers suggested that they didn’t have the crucial longitudinal arch of modern humans.

A refresher—here’s where the metatarsals are:

The 4th metatarsal found by Ward et al. is fortuitous, for it  clearly comes from A. afarensis (it was found in the Hadar formation, where the only hominin is of that species), it’s the right age—about 3.2 my old—and it’s a crucial bone for determining how its owner walked.

Humans have a more rigid foot than modern apes, with pronounced arches, both longitudinal and transverse.  When our heel lifts off the ground, so does the rest of the foot up to the toes.  When the ape foot lifts off, there’s a break between the heel and the middle, which facilitates moving around in trees.  This is reflected in the shape of the metatarsals, which show much greater torsion (rotation) during walking than do ape toes.  The metatarsals in humans form a pronounced arch, one that’s much less distinct in apes (figure from Ward et al.):

The shape of the bone tells you how much torsion it underwent during walking, and the shape of the new A. afarensis metatarsal (“AL333-160″ in the figure below) clearly shows that its torsion is much closer to that of a modern human than to a modern chimp (P. troglodytes) or gorilla (Gorilla gorilla):

There are other morphometric data supporting a similarity in walking style between A. australopithecus and modern humans, but I’ll give just one more piece.  Below are the fourth metatarsals of a modern human, the new specimen, and of a modern chimp and gorilla.  In modern humans the metatarsal’s “diaphysis” (the long middle section of a bone) is at a sharp angle to the base (the short dark line at the right side of the following diagram), while in apes they’re roughly parallel. And the domed part of the metatarsal head (blue arrow in the diagram) is much higher (more dorsal) in humans than in apes.

The figure above shows that, in both respects, the new A. afarensis toe belonged to a creature that had a longitudinal arch in the foot, like humans but unlike modern apes.

Since the ancestor of humans and apes was likely a knuckle-walking ape (there is some dispute about this), these data all show that A. afarensis, at 3.2 mya, already had a stiff, doubly arched foot and therefore walked much like modern humans, confirming the earlier skeletal and footprint evidence.  Here is the authors’ conclusion:

By at least 3.2 million years ago, the fundamental attributes of human pedal anatomy and function were in place. This includes the transformation of the first toe and associated musculature from a grasping structure to one designed for propulsion and shock absorption [review in (1)]. Evidence from the Hadar fourth metatarsal adds to this human-like portrait of permanent longitudinal and transverse bony arches in the sole of the foot. The evolutionary trajectory suggested by these fossil remains makes it unlikely that selection continued to favor substantial arboreal behaviors by the time of A. afarensis.

In other words, we’d left the trees for good.

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Ward, C., W. H. Kimbel and D. C. Johanson. 2011.  Complete fourth metatarsal and arches in the foot of Australopithecus afarensis. Science 331:750-753.