Lucy's Feet: Was Australopithicus Afarensis an Obligate Biped?
Lovejoy says that Australopithecine afarensis is fully adapted to bipedal locomotion. He bases this assertion on features associated with the pelvis from the specimen known as Lucy (Stern, 2000). When Donald Johanson first found Lucy in Hadar, Ethiopia in 1974 his initial interpretation was also of an obligate biped. One thing I have learned about this field of scientific inquiry is that every scientist wants to make a name for their selves by finding something new and / or extraordinary. Lucy is a fantastic find, it is very old (3 million years) and if not related to humans directly, it is certainly in the ball park. Whether or not it is obligate biped is the question.
Near to the same time that Lucy was found, Mary Leakey uncovered the Laetoli footprints which were also associated with A. afarensis. These footprints in a Pliocene landscape were of hominids walking in a bipedal fashion. Arched foot, forward facing big toe, and heel toe strike pattern all point to bipedalism (Lewin, 1988).
Figure 1. Topographic Footprint Analysis, Lucy on the left , modern human on the right
Studies of the footprints by Sterns show many traits that support the obligate bipedalism theory that Lovejoy supports. A little rudimentary content analysis of Sterns studies show 19 total items that the Lovejoy position uses to maintain the obligate biped position. Sterns concludes that “A combination of traits that are interpreted as either; showing insignificance of arboreal behavior or showing human like bipedalism or signs of both” (Stern, 2000). One of the most distinctive and telling features is the distal articular surface of A. afarensis’s medial cuneiform faces more directly distally than either chimps or gorillas (Stern, 2000). This demonstrates graphically, that bipedalism as we know it was present in A. afarensis. I am also impressed by the theromographic scans of the Laetoli footprints. The arch is present as is the heel toe strike pattern, but there is a toe that is at least partially divergent. It is certainly not in line with the rest of the foot the way ours is. Lovejoy uses this to strengthen his case, but it can also be interpreted as an intermediary form between arborealism and terrestrialism.
Figure 2. Thermographic scans of foot prints from laetoli and a modern human
Stern and Susman contend that Lucy is not an obligate biped but rather has traits that show it to be both able to utilize bipedal locomotion and still retain arboreal functionality (Stern, 2000). Furthermore Stern and Susman contend that A. afarensis used what they are terming a novel form of bipedalism. This means that, yes A. afarensis was walking around on two legs at least some of the time but with different mechanics than modern humans use to walk around. Content analysis of Sterns studies show 36 different features or traits that , for Stern and Susman, can be interpreted as supporting their theories. 14 items on the list are related to an arboreal component of behavior, 18 items are interpreted as being related to a novel form of bipedalism and 4 items show traits of both (Stern, 2000).
Figure 3. Skeleton showing adaptations for bipedal locomotion
The most telling features of the footprint / foot bone reconstruction shows a comparison of pisifrom bones showing the rod like nature of this bone, so very much like a chimpanzees, in A. afarensis. It is slightly diminished but the comparison is a stark contrast unlike some of the others that Stern makes.
It can be agreed upon by most evolutionary anthropologists that A. afarensis seems to be an intermediary form between the apes that once existed and the humans that now exist. The ongoing argument about the status of A. afarensis seems a bit superfluous. Both the Lovejoy / Johanson view as well as the Stern / Susman position have merit. A. afarensis does indeed show features related to both modern human style bipedalism and ape like arboreal locomotor capability. It seems not to be a matter of which one is right, in their own ways they are both right. It is the matter of this novel form of bipedalism that gives one pause. Stern produces enough evidence to support his idea and it does seem intuitive that A. afarensis would have a different type of bipedalism than we have now but to say that its bipedalism was different enough to constitute a whole different kind of bipedalism that then went extinct is unprecedented. As we are the only organism with our type of bipedalism, all of our ancestors have related morphology. Some adjustments might be dead ends in an evolutionary sence, but a definite progression can be shown.
A. afarensis was not an obligate biped. But there can be no doubt that bipedalism was a part of life for Lucy. There also seems to be some question as to whether or not Homo habilis was an obligate biped for many of the same reasons. Definitively, the first obligate biped was Homo erectus. Built to run, H. erectus has heel toe alignment and all the rest of the morphology to support the idea that H. erectus is a dyed in the wool, no question full time biped. H. erectus is the first to leave Africa and they walk all the way to Java and Europe.
The Time Life Books called The Emergence of Man have a volume titled The Missing Link which is all about Australopithecines. The term “Missing Link” is a bit antiquated as we now know that we are not looking for a single species that is transitional between humans and apes but the term is correctly applied to Australopithecines as they are very transitional morphologically.
For more about Homo Hablis check out my article about whether or not Lucy is indeed a Handy Man: