Were Australopithecines Obligate Bipeds?

In a recent paper published in the journal Nature, Jeremy DeSilva demonstrates that early hominins did not climb like chimpanzees.

By studying the way chimpanzees climb, DeSilva was able to get a detailed understanding of the role their ankles play. While climbing a tree trunk, a chimpanzee's ankles flex and rotate in ways that would be impossible for a human to replicate.

DeSilva compared the ankle anatomy of chimps and humans, and then compared these to fossil tibia and tali (the tibia is the weight-bearing bone of the lower leg, and the talus is the upper foot bone which, along with the tibia and fibula, forms the ankle joint) from over a dozen hominins from 4.12 to 1.53 million years ago.

What he found is that the anatomy of early hominin ankles shows that they were as poorly adapted as humans to the kind of climbing done by chimps. John Hawks has a really good summary on his website, and brings other recent papers and findings into the discussion-- you should read it.

Here's the rub for me: Was Astralopithecus afarensis a facultative or obligate biped? The thinking up until now has been that they were climbers and facultative walkers. The problem here is that many of the adaptations present in the A. afarensis post-cranial anatomy show that a life in the trees is likely far in their distant past (although clearly it it further in our past-- our body mass relative to arm length/strength is all wrong, and we do not exhibit curved finger bones as the australopithecines did).

For climbing, they no longer have an opposable phallux to allow them to grasp branches with their feet, Their arms are not long enough to wrap around a tree trunk (being closer in proportion to humans than chimps), and DeSilva has shown their ankles are no longer adapted to climbing. Take a look at the figure below, which compares the skeletal anatomies of Homo sapiens (a), H. erectus (c), Pan troglodytes, aka chimps (b), and A. afarensis (f) (from Endurance running and the evolution of Homo).

This leaves us with terrestrial locomotion. Their pelvis, knee, ankle, and big toe are all well-adapted to an upright, striding gait, and they could not have moved about as a quadruped, since their arms are simply not long enough. All of this seems to point to A. afarensis being an obligate biped.

I'm not a biological anthropologist, so don't take my word for it... but it's something to think about.




References:

2009 DeSilva JM. Functional morphology of the ankle and the likelihood of climbing in early hominins. Proc Nat Acad Sci USA 106:6567-6572.

2004 Bramble DM, Lieberman DE Endurance running and the evolution of Homo. Nature 432:345-352.

Another Casualty of the Bad Economy

I've always given my employer a lot of credit for being so supportive of my efforts to go back to school, even allowing me to take courses during the day and flexing my schedule to accommodate.

Well, the good times are over here too, it seems. I was notified that full-time employees would no longer be allowed to take college courses during regular business hours. In these tough times, they want to get everything they can out of their existing workforce, and I get that.

When I was taking lower division anthropology courses at the community colleges here in Phoenix, that was no big deal-- most were offered at night. Graduate courses are another issue entirely. This severely restricts what I can do, and certainly eliminates any possibility of attending NAU any time soon.

Lots of thinking to be done...

Back to NAU?

Sometimes a little thing can make a big difference.

I've been feeling as though my educational plan was stalling-- I'm about to finish the last anthropology course of my B.A., and had no real prospects for next year. ASU won't accept me (since my BA is from there), and I wasn't excited by my UofA experience in Tucson.

I had looked at the Fall 2009 schedule at NAU, but all of the graduate classes I would consider taking were at highly inconvenient times, with any one of them causing me to basically miss a day of work every week. Given the current job market, the last thing I need to do is draw attention to myself by asking for exceptional treatment.

I have no idea why I did it, but I looked at NAU's online schedule for the Fall again... and noticed that Dr. Smiley's Lithic Analysis course had been moved to Tuesday afternoons at 4:00pm!

I applied to NAU as a non-degree-seeking grad student, and e-mailed the professor to make sure he was OK with me taking the class. He replied almost immediately that I was welcome to take the class, so I guess I'm heading North next semester.

How does this fit? Well, if I apply and get accepted into the NAU Masters program for Fall 2010, I can already have 6 graduate hours to apply to the program (plus 3 more transfer credits for the graduate seminar on the Preclassic Maya I took in Tucson).