Australopithecus afarensis: 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.

______

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.

28 thoughts on “Australopithecus afarensis: a “committed terrestrial biped”

  1. It’s funny: I saw the Lucy exhibit when it was in Houston, and it never clicked that the very human-looking feet on the reconstructed model were not implied by the Lucy remains, since they lacked foot bones. But it’s good to see that other evidence supports that reconstruction.

  2. Deputy Dawg – “Oooh!! My toe-bone!”

    Of related interest, this article in J Human Evolution by DA Raichlen, H Armstrong & DE Lieberman
    http://tinyurl.com/4at6ltl
    They have examined running ability in Neanderthals & think they were poorer at it than sapiens, due to the ‘Achilles tendon moment arm’ putting them at a comparative selective disadvantage in distance pursuit of game (they would appear to have used ambush techniques).

  3. “Since the ancestor of humans and apes was clearly a knuckle-walking ape”

    Clearly? I thought this was still up for debate with Ardi giving more ammunition to the idea that we were never knuckle walkers.

      1. If the chimp-chump problem is resolved to be ((Human, Chimp)Gorilla), as most evidence suggests, and since both chimps and gorillas knuckle walk, then the common ancestor of humans and chimps should be a knuckle walker.

        1. You’re assuming all knuckle-walking is equal, whereas (AIUI) the way chimps and gorillas do knuckle-walking is actually quite different. I believe that their wrist bones are quite different, meaning that parallel evolution is quite possible.

          Orangs walk in yet another way entirely.

        2. I thought a simple comparison clearly showed that knuckle-walking evolved independently two times:

          “Contrary to previous expectations, features
          long-assumed to indicate knuckle-walking behavior are not found
          in all African apes, show different developmental patterns across
          species, and are found in nonknuckle-walking primates as well.

          However, variation among African ape wrist morphology can be
          clearly explained if we accept the likely independent evolution of
          2 fundamentally different biomechanical modes of knuckle-walking: an extended wrist posture in an arboreal environment (Pan) versus a neutral, columnar hand posture in a terrestrial environment
          (Gorilla). The presence of purported knuckle-walking features in the hominin wrist can thus be viewed as evidence of arboreality, not terrestriality, and provide evidence that human bipedalism evolved from a more arboreal ancestor occupying the ecological niche common to all living apes.”

          What would be the trouble with that research, or at least the “2 fundamentally different biomechanical modes of knuckle-walking”? AFAIU you can’t go from one to the other, because they have different support structures. But you can go from the purported general hand mechanism that we and our ancestors seem to have (retained). [Layman here, I haven’t seen previous or later research.]

          1. The Kivell & Schmitt PNAS independent evolution of knuckle-walking paper quoted above has not gone without criticism – a Journal of Human Evolution paper from last year provides a nice rebuttal to it: doi:10.1016/j.jhevol.2010.03.005

  4. Please excuse the length of this post. I’m confused about modern ideas on the evolutionary relationships between all known Primates [both extinct & extant]…

    Reading this thread I’ve just noticed that the term ‘Hominid’ is not used so I did a google to find out why & I came across this ~ I therefore assume ‘Hominid’ is a defunct term & ‘Hominin’ is preferred:

    http://archaeology.about.com/od/hterms/g/hominin.htm
    “Up until the 1980s, palaeoanthropologists generally followed the taxonomic system followed by the 18th century scientist Carl Linnaeus, when they spoke of the various species of humans. The family of Hominoids included the subfamily of Hominids (humans and their ancestors) and Anthropoids (chimps, gorillas, and orang-utans). The problem is, recent molecular studies show that humans, chimps and gorillas are closer to one another than orang-utans. So, scientists split the Hominoids into two subfamilies: Ponginae (orang-utans) and Homininae (humans and their ancestors, and chimps and gorillas). But, we still need a way to discuss humans and their ancestors as a separate group, so researchers have proposed a further breakdown of the Homininae subfamily, to include Hominini (humans and their ancestors), Panini (chimps), and Gorillini (gorillas).

    So, roughly speaking, a Hominin is what we used to call a Hominid; a creature that palaeoanthropologist have agreed is human or a human ancestor. These include all of the Homo species (Homo sapiens, H. ergaster, H. rudolfensis), all of the Australopithecines (Australopithicus africanus, A. boisei, etc.) and other ancient forms like Paranthropus and Ardipithecus.”

    To understand the above quote I drew a tree diagram

    QUESTION: Where do I put Neanderthal ~ (I presume inside ‘Hominin’ somewhere) ? I’ve looked here, but it’s not helped me:
    http://archaeology.about.com/od/hominidancestors/a/neander.htm

    Michael [interested non-scientist]

    1. AFAIK, Neanderthals are now considered an H. sapiens subspecies (Homo sapiens neanderthalensis).

      Related question–is there no longer a push to change Neanderthal to Neandertal?

      1. thnx Diane & Torbjorn

        I’ll have to do more googling & get a book on the subject

        Any suggestions for an up-to-date book ?

        Michael

          1. Thnx Diane

            I haven’t found anything so I’ll keep digging
            [as a fossil person might say 🙂 ]

            Michael

  5. And again no directed evolution,affrimng the teleonomic argument! As the atelic argument notes theistic evolutionists beg the question of that direction.
    Yes, I do emphasize this point!This is the Achilles heel of theism!
    Science indeed does slay theism!

  6. Thanks, the data was more interesting than I thought! The toe mechanics confirm Laetoli toe vs heel print depths close to human range, see fig.2:

    “Note that the values for Laetoli fall within the range of human normal prints but outside of the range of human BKBH [human using ape-like bent knee-bent hip walk] prints.”

    Since the ancestor of humans and apes was likely a knuckle-walking ape (there is some dispute about this),

    And the authors seems to stand at the other end of that dispute. The sentence just before the quote concludes:

    “The 4.4-million-year-old skeleton of Ardipithecus ramidus suggests that the transition to terrestrial bipedality occurred in the earliest hominins, while selection maintained adaptations in the foot for arboreal climbing and grasping (25).

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