A new fossil discovered by Yohannes Haile-Selassie has been announced this week in the PNAS. The partial skeleton, nick-named Kadanuumuu, or “Big Man,” is taxonomically consistent with other postcranial fossils belonging to Australopithecus afarensis. But, there are a few interesting and notable bones represented in this fossil which amend our understanding of how early Australopithecus moved around.
First- how do we know that Kadanuumuu is a male? One clue that the authors used was to look at the greater sciatic notch. This feature is much narrower in males than in females. When compared with other specimens belonging to A. afarensis, like Lucy and STS-14, Kadanuumuu has a much narrower sciatic notch.
Now, on to the locomotry bits. Kadanuumuu preserves a tibia, which forms part of the ankle joint. The talus forms the other main component of the joint. In humans, these bones articulate in a rounded formation, so that the talus is domed, and the end of the tibia- called the plafond (apparently)- is more convex in order to accept the talus. Apes, on the other hand, have a much flatter articular surface, which is a reflection of the vertical climbing that they do. Such climbing requires the ankle joint to dorsiflex, and having a flatter ankle joint permits this. Kadanuumuu has a convex tibial plafond, which groups it more with humans.
What gets me excited about this fossil is the elements preserved in the thorax. Ribs, clavicles, and scapulae! Oh, my!
What’s special about these ribs? They are curved in a manner that is much more human-like than ape-like. Ape torsos are sort of funnel-shaped, so that they’re skinny up toward the neck, and get wider toward the bottom. Humans have a wide, broad thorax. Kadanuumuu preserves a couple of ribs, and the second one is particularly informative in this case. If Kadanuumuu had an ape-like, funnel-shaped thorax, this second rib would reflect that by being more dramatically curved. Instead, Kadanuumuu falls well within the ranges of curvature displayed by modern humans.
Ribs from a Chimpanzee, Kadanuumuu, and a Human. From Haile-Salassie 2010.
And now, on to the scapula! When we are studying the scapula, the most important parts to look at are the orientation of the glenoid fossa, the orientation of the scapular spine, and the overall shape. The orientation of the glenoid fossa can tell you about mobility in the shoulder, as this is where the humerus articulates with the shoulder. The orientation of the scauplar spine can tell you the ratio of different muscle groups involved in shoulder movement.
The most complete scapula that we’ve known about before now belonged to the Dikika baby. There are problems when trying to draw conclusions about past species from the bones of infants, the least of which involve issues of scaling. Since Kadanuumuu is adult-sized, and falls within the size range of adult humans and apes, these issues should not exist.
Principal components analysis showing the distribution of scapular measurements. PC1 is the best discriminator between groups and best reflects the angle of the scauplar spine. From Haile-Salassie 2010.
The authors took a series of measurements which captured the orientation of the different elements of the scapula discussed above and analyzed them using a principal components analysis. This PCA very nicely distinguishes humans from apes, and especially from chimpanzees. The main discriminator between these groups is the angle of the scapular spine (PC1 on the graph). Kadanuumuu falls well within the human distribution for these componenets.
Overall, the shape of the scapula is similar to gorillas in that the area above the scapular spine is rather large and the glenoid fossa points toward the head, but the angle that the scapular spine forms with the borders of the scaupla and the overall shape are more human-like. Basically, what this means is that the torso in A. afarensis is unique when compared to the other large-bodied apes. The authors suggest that this uniqueness means that A. afarensis had been on the ground for a long time and had been evolving away from whatever its arboreal ancestor had looked like, or that this unique morphology was a “holdover” from an ancestor whose locomotion is not represented by any living apes today (like, perhaps, a quadrupeal clamberer like Ardi?). In other words, A. afarensis was NOT a knuckle-walker.
Haile-Selassie, Y., Latimer, B., Alene, M., Deino, A., Gibert, L., Melillo, S., Saylor, B., Scott, G., & Lovejoy, C. (2010). An early Australopithecus afarensis postcranium from Woranso-Mille, Ethiopia Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1004527107
I agree, the scapula is the newest and most interesting part. It is a mosaic. The authors’ inference of “unique” functional affinities isn’t very helpful, however. Must have been really good at jumping jacks! Or maybe walking around with his hands on top of his head…
The rib cage reconstruction is just that, a reconstruction from battered ribs. How does this reconstruction invalidate the one done by Peter Schmid in Zurich of Lucy? Hard to know who’s right on this one.
Au. afarensis wasn’t a knuckle-walker? Wow! Who has ever claimed such??
Oh, come now. The authors meant, and clearly stated, unique among the living apes. It would probably have been useful to compare it with some monkey scapulae, though.
I’m thinking in particular of Richmond and Strait (2000), though I suppose they didn’t claim that afarensis was a knuckle-walker, but rather retained kncukle-walking morphology from a recent ancestor. Either way, I bet that that’s what the authors were getting at by mentioning
Maybe I’m wrong, though. Maybe they were trying to exclude vertical climbing or suspension. Or maybe they were just trying to get us thinking about Ardi again.
I don’t know, what kind of functional inference do you think they could’ve made here that they didn’t make? I know that there are certain “camps” in paleoanthropology that are unlikely to agree with each other, but I’m a young up-and-comer and I just want to learn how to do good science. So, what does the audience want from this fossil that they didn’t get?
I just love to think out loud about this stuff. No offense, but I beg to differ in how I interpret the way they dodge speculating re: “unique” functional affinities of the scapula. It’s their default position — if it’s not just like a human, then it’s a heritage feature of uncertain function. They have ape-ophobia.
The debate over the locomotor repertoire of afarensis is MUCH more nuanced than you would ever gather from their discussion and their media talking points.
What have we learned from this new skeleton that is really novel?
Males are big. Check.
Afarensis was a biped. Check.
I thought that was old news.
And you are very perceptive about the Ardi subtext!
No offense taken- disagreements are allowed! To me, the data present a fundamentally human scapula. Gorilla and human are more similar than chimp and human, but I’m not sure how I would use that to assign functional significance.
We really are damned if we do, damned if we don’t. Attribute a fossil to a certain species based on similarities of the postcranium? Pure speculation. Exhibit caution when attributing function to a certain bone? Not speculative enough. And that’s actually pretty fair, I think, because at least for me it makes me double-check my assumptions.
Fair enough. Anyway, I’m more interested in what they did (function) than what the taxonomy du jour is.
Back to the torso. Lucy has a complete first rib (you can see it here on eLucy: http://elucy.org/about.html), so Peter Schmid’s reconstruction had excellent information on the upper part of the rib cage in his decidedly ape-like rendering. Unfortunately, the details of his effort were published in German in Folia primatologica, and few people seem to have read or fully appreciated it.
Big Man’s LOWER ribs are what could well distinguish him from the Lucy reconstruction. The reported declination of the one well-preserved rib is almost certainly correlated with increased torsion as the rib comes around to the front (the chapter on the rib cage in the Nariokotome Boy book in an excellent ref on this stuff), and that also implies a substantial distance between the last ribs and the top of the iliac crest. If accurate, that would make Big Man’s torso less ape-like that Schmid’s reconstruction of Lucy.