Afarensis is a 3.5-2.8 million year old hominin from the Kada Hadar member of the Hadar formation in the Middle Awash, Ethiopia. He is approximately 41 inches tall, weighs approximately 60 pounds and has a cranial capacity of a whopping 410 cc (approximately). Afarensis is currently considered to be transitional between apes and humans and displays some traits of both. Since he spends a lot of time on the couch watching monster movies, some observers question whether he is an obligate biped (although no one has observed him climbing a tree). He also has a blog called 
Via Anthropology in the News comes this interesting article on the ankle of robust australopiths. ASU anthropologist Gary Schwartz and Northern Illinois University anthropologist Dan Gebo studied the ankle of robust australopiths and came to two conclusions:
First, certain ancestral anklebones that were thought by some to be "half ape, half human" were found to be much more similar to humans, confirming these specimens were obligate bipeds--in other words, they most likely walked on two feet in a manner similar to how we walk today.The second discovery was that although the samples were certainly from bipeds, there were structural differences in some of the anklebones that indicated they would have walked a little differently than modern humans. Specifically, an ancestral species commonly referred to as robust australopithecines appear to have been a little knock-kneed.
The research is reported in the April edition of the AJPA (which is linked to Eureka Alert but the link seems to be broken). At any rate, Schwartz and Gebo say that:
"We noticed that in the specimens of robust australopithecines, there were characteristics of the anklebone that would have affected its bipedal locomotion," Schwartz said. "By looking at the location where the shin bone rides across the anklebone, we found that the shin bones would have been angled inward."
It is this later that I would like to look at. The human ankle is composed of several bones all working toghether. The medial malleolus of the tibia and lateral malleolus of the fibula form part.
The tibia rests on the talus (called the astragalus in most other vertebrates). The talus also articulates, medially with the medial malleolus of the tibia and laterally with the lateral malleolus of the fibula. It also articulates inferiorly with the calcaneus (the large bone that forms part of your heel).
Together, they look like this:
Since I have not read the article I can not say what specific anatomy of the talus they claim would cause the tibia and fibia to angle in towards the knee, although I suspect that if this is true it should be reflected in the tibial plateau and the proximal articular surface of the fibula (possibly).
This has some interesting implications for the evolution of bipedalism:
"The skeletal modifications associated with bipedalism represent a phenomenal reorganization of one's anatomy," Schwartz said. "It is unlikely that it could have evolved independently in multiple hominin lineages."Still, even if it only evolved once, the new research suggests there was a lot of tinkering within subsequent lineages.
"Think of the robust australopithecines as having developed a variation on the theme of bipedalism," Schwartz said. "Undoubtedly, it was not as efficient as the way we walk today, but it might have conferred some other evolutionary advantages."
Comments
Huh. Relatedly, do you know if anyone's looked at the morphology of the astragalus in Oreopithecus? This paper suggests that its navicular reflects arboreal locomotor behavior, but also claims a degree of opposability for the hallux of A. afarensis that seems at odds with other sources I've read.
Posted by: Sean Foley | February 20, 2006 12:00 AM
I am not aware of any studies of Oreopithecus other than Harrisons' 1987 article in the Journal of Human Evolution and Jungers 1987 article also in J. Hum. Evol. - but then I'm not real current on Miocene material. Incidently, if you don't want to go through BioOne you can find the article here. The AMNH has all their stuff on the web for free... I think the original article on the A. afarensis metatarsal also mentioned some traits that indicated opposability (which, if memory serves, was in vol 57? of the AJPA), but I wouldn't swear to it since it's been awhile since I have read them. Stern and Susman's stuff springs to mind as well. On the other hand, Conroy, in the second ed of Reconstructing Human Origins, is pretty definite about A. afarensis not having an opposabble big toe...
I'm hoping to track down a copy of Schwartz and Gebo's article because as phrased (bearing in mind it was for a an audience not familiar with osteology) doesn't make sense...they seem to be arguing about how the tibia articulates with the talus but it is hard to evaluate without knowing more...
Posted by: afarensis | February 20, 2006 12:40 AM
Afarensis, the article isn't up yet. I just checked this morning. I put my own thoughts up at my blog but refuse to get into it until I can read the full article.
This slots in nicely with the theory that bipedalism originally evolved as a postural/feeding adaptation rather than a locomotor adaptation, but only provided that the ankle morphology mentioned is also evident in gracile australopithecines.
Otherwise, this would mean that Paranthropus spp. put more emphasis on the postural side of things than either graciles or we do. And yes, I know I used horrible teleological grammar. I'm lazy.
Posted by: IndianCowboy
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February 20, 2006 8:01 AM