Last Friday, human paleontologists working in Ethiopia unveiled a partial skeleton and additional elements of Ardipithecus ramidus. Most of the material dates to around 4.4 million years ago. The discovery of the skeleton was announced in 1994, and for the past few years I’ve been pretty irked that it’s taken so long to be published. But given the state of preservation of the fossils and the fact that the technology to carry out the studies’ analyses just wasn’t available until recently, I suppose the long prep time is alright.

I’ve only had a chance so far to read the papers on the skull, dentition (and peruse the monstrous supporting online material), and wrist. Let’s start with the skull. If I could summarize the paper with a question, this would be it: If Ardipithecus ramidus so typifies an ancestral condition (primitive compared to later australopithecines), and Pan species are variously derived relative to this condition, what’s keeping Ardi from being a Pan-Homo common ancestor instead of a hominin?

The skull was reconstructed using CT-scanned images of the fossils, much as was done for Sahelanthropus a few years ago. One cool thing they did was make a composite cranium from the ARA-VP-6/500 face and vault and VP-1/500 temporal-occipital fragment. I don’t see any reasons to distrust the reconstruction. What does it look like? To me, the first fossil that came to mind was the AL-333 composite cranium (Australopithecus afarensis from Hadar, Ethiopia ~3 million years old). However, the lower face of Ardi is surprisingly short compared to what we have for later hominins, or really anything else I’ve seen for that matter. Also, the orbits are surprisingly large. Honestly I do not really see a strong similarity to the Sahelanthropus TM 266-1 cranium, even though the authors go to pains to point out similarities between the two (mostly it’s in the basicranium). One thing Ardi certainly lacks is Sahelanthropus’s massive supraorbital torus—Ardi’s appear more similar to Australopithecus afarensis frontal bones.

From the reconstruction, the brain was probably around 300 cubic centimeters (cc), with an estimated range of from 280-350 cc. This is about the size of a small African ape. We’ve known for a while now that increased brain size was not a hallmark of human origins. But what does seem different is that the cranial base is fairly flexed (the bottom of the brain was somewhat ‘tucked under’); the authors argue that some kind of neural reorganization, different from other African apes, must have occurred early in hominin evolution. Sahelanthropus apparently shares with Ardi a relatively short basicranium, though I’m not sure about the flexion. While the authors argue this confirms Sahelanthropus’s hominid status, there’s no major reason why this can’t be an ancestral condition from which later apes are derived; I’ve never been convinced that Sahelanthropus is not just an ape.

While the canine teeth are not as projecting as they are in African apes, they project further above the other teeth than in Australopithecus. However, they lack the C/P₃ honing complex that is expressed in apes and most monkeys. This arguably links Ardi with Sahelanthropus, although it was never clear to me that Sahelanthropus’s lacked some sort of a honing complex. Also like Sahelanthropus, the teeth and skull of Ardi do not display the heavy-chewing adaptations of the later australopithecines.

The authors tend to reach two conclusions about Ardi, which are not unequivocal. First, a common conclusion the authors reach based on comparative anatomy is that for most features, the probable morphology of the chimp-human common ancestor is represented in Ardipithecus and Sahelanthropus, among others. As a result, the common chimpanzee appears to be quite derived, both in terms of its large canine dimorphism and lower-facial prognathism. In fact, the authors attribute the latter trait to the former; Pan troglodytes is argued to be morphologically derived because of its high levels of male aggression. The problem that arises with this is that if so much of Ardi’s morphology represents the ancestral condition, these traits are symplesiomorphic, and not necessarily informative about its relationship to later hominins. That is to say, if Ardi so typifies the ancestral condition, there’s not a lot making it, say, a chimp-human common ancestor rather than a hominin.

A second common conclusion is that Ardipithecus was probably not very sexually dimorphic in terms of canine or body size. Recall from above that the authors posit that the chimpanzee is actually unique/derived relative to the chimp-human common ancestor, and this may be due to canine size, which is related to male aggression. That Ardi lacks such canine honing and dimorphism argues for low levels of male aggression. Then there’s this quote:

“…our scaling analysis shows that postcranially dimorphic species tend to exhibit a large cranial size relative to that of the endocranium, as well as a large degree of cranial size dimorphism. In this context, it is instructive that Ar. ramidus shares its relatively small cranial size with taxa that are weakly dimorphic both cranially and postcranially” (Lovejoy et al. 2009: 68e6).

I don’t know if this is what their scaling analysis shows. They regress log-transformed cranial length on log-transformed cranial capacity for several catarrhine taxa. There is a clear separation between great apes, on the one hand, and other anthropoids and Hylobates (the gibbon, the smallest living ape) on the other. This difference is due to great apes’ relatively larger brains, which in turn is probably due to their relatively larger body size. Ardi does fall below both male and female regression lines, indicating a relatively short (but not necessarily small) cranium compared to its cranial capacity. But then, so do two “African Apes” on the plot—this could be the highly sexually dimorphic gorilla or the less dimorphic chimpanzee. And I believe that both these apes display fairly high levels of male-male aggression. Furthermore, if separate regressions were made for the small-bodied anthropoids on the one hand, and large-bodied hominoids on the other, it looks like Ardi may actually fall above the regression line, indicating a fairly long cranium.

The point is that there are persistent assertions of low male aggression in Ardi. Some may recall Lovejoy’s 1981 paper in which he argues that low sexual dimorphism and a more monogamous reproductive behavior and male provisioning of female and offspring were responsible for hominin origins and bipedalism. While the Ardi material makes it unlikely that this reproductive behavior an unlikely cause of terrestrial bipedalism, it is interesting that this theme of reduced male aggression/sexual dimorphism and hominin origins emerges once again. Not that it’s incorrect or silly, just interesting. Of course, if this is the case, one should note that later hominins appear very sexually dimorphic.

References

Lovejoy CO. 1981. The Origin of Man. Science 211: 341-350

Suwa G, Asfaw B, Kono RT, Kubo D, Lovejoy CO, and White TD. 2009. The Ardipithecus ramidus Skull and Its Iimplications for Hominid Origins. Science 326: 68.