Monday, December 20, 2010

Iliac spines and cannibal hominids: together at last?

For various reasons I'm reviewing last year's publications on Ardipithecus ramidus, namely the pelvis paper (Lovejoy et al. 2009). I also found a recent review of the hominid spine & pelvis by Lovejoy and McCollum (2010). Both papers discuss the anterior inferior iliac spine (AIIS) - a bump on your pelvis just above your acetabulum (hip socket), to which attaches rectus femoris, a muscle that both flexes the hip and extends the knee. In apes and monkeys (quadrupeds), this 'spine' is generally negligible or absent, whereas in humans and other (extinct) hominids it is quite protuberant. This is because our upright posture necessitates this muscle insertion to be large and protuberant in order to be effective.
Pelvic comparison, yellow arrow points to AIIS. Lovejoy et al. 2009: 71.
Let's see if this discrepancy between these two papers won't drive you nuts:

The form and size of the AIIS in [Ardipithecus ramidus], as well as its projection anterior to the acetabular margin, indicate that this structure had already begun to appear and mature via a novel physis (from Lovejoy et al. 2009: 71e3, emphasis mine)

What distinguishes the AIIS in hominids from those in apes is not its protuberance (those of Gorilla are often very prominent), but rather its emergence from a novel, separate physis, a hominid adaptation that is almost certainly associated with dramatic expansion of iliac isthmus breadth (Lovejoy et al. 2009b) (from Lovejoy & McCollum 2010: 3295, emphasis original)

Now, the second quote tells us we can't use the size and protuberance of the AIIS to completely infer its function (biped-like or ape-like), but rather that its origin of growth tells us whether it is hominid. But the first quote tells us that the size and projection of Ardi's pelvis tells us it did come from a unique growth center. So... I'm confused. Do the prominent Gorilla AIISes arise from a unique growth center as well? (another thing that drives you nuts is that the reference in the second quote, to "Lovejoy et al. 2009b," is to the Ardipithecus hand paper, probably an accident, but not helpful nonetheless)

Scheuer and Black (2000: 357-361) state that "accessory [growth] centers" including the AIIS tend to be variable. That is, the AIIS may have its own center, but often the epiphysis of the upper acetabulum 'reaches' up to form the AIIS. The subadult skeleton in our lab here has billowy-looking bone int he region of the AIIS, but this seems to connect with the rest of the unfused acetabulum. So far, then, the jury's (my jury, at least) out on whether the AIIS is its own growth center in hominids; the uniqueness of it we'll have to wait and see (such as those gorillas described above, or possibly Oreopithecus...).

Lovejoy et al. 2009 (the pelvis paper, not the hand) also reference this novel growth center, citing the great Raymond Dart's description (1953) of the MLD 25 ilium, attributed to Australopithecus africanus from Makapansgat in South Africa. Now, I knew Dart was a clever man, indeed it was he who coined terms like Australopithecus and 'osteodontokeratic.' Very smart guy, Dart, but I'd also read something lo these past several years of graduate school, in which he was described as 'bloodthirsty.' An odd descriptor, I'd thought. But, reviewing Dart's 1953 paper, I came across the following passage, wherein he discusses how unlikely it is that this australopithecine ilium should be so closely associated with another at the site (MLD 7):
Further it is an extraordinary occurrence that two adolescent ilia of apparently opposite sex but virtually identical age should be found in close company in this deposit. The boy [MLD 7] was killed by a bone-smashing blow on the chin from a club or fist. Were they brother and sister [MLD 25] twins, that shared in death the same cannibalistic fate? (Dart 1953: 75)
Possibly. I'll let you know what I uncover about the ontogeny of the AIIS in other animals. NB: this post isn't meant to disparage any of the references. In fact I rather respect these authors, and their work (McCollum's especially) has been quite influential to me.

Dart RA. 1953. The second adolescent (female) ilium of Australopithecus prometheus. Journal of the Palaeontological Society of India 2: 73-83.

Lovejoy CO, Suwa G, Spurlock L, Asfaw B, White TD. 2009. The pelvis and femur of Ardipithecus ramidus: The emergence of upright walking. Science 326: 71.

Lovejoy CO and McCollum MA. 2010. Spinopelvic pathways to bipedality: Why no hominids ever relied on a bent-hip - bent-knee gait. Philosophical Transactions of the Royal Society B: 365: 3289-3299.

Scheuer L and Black S. 2000. Developmental Juvenile Osteology. New York: Elsevier Academic Press.


Eric said...

Doesn't a "new physis" means that the "Gorilla/Chimpanzee condition" (since Lovejoy et al. compared Ardi's Pelvis to the Pelvis of a Chimpanzee) of the AIIS actually represents the ancestral condition of the african Ape/human clade?

I'm just asking because a few weeks ago I was wondering that the morphology of the Pelvis of the early hominids might represent the ancestral condition of at least the Chimpanzee/human clade.

Right now the assumptions where Ardipithecus resembled the Human/Chimp LCA and where not seems a little bit arbitrary for me. But maybe I just missed something important which lead to this conclusion. Are there any other pelvises from miocene Apes besides the one from Oreopithecus and Ardipithecus which could help us (me) on this problem?

Aidan said...
This comment has been removed by the author.
Zinjanthropus said...

It's my understanding that the AIIS growth plate is derived from the one in the upper acetabulum, in the same manner that the greater trochanter and femoral head are derived from the same growth plate. Accelerated growth causes them to grow apart spatially before they are able to ossify, and hence, you get two different growth plates.

And of course the AIIS isn't the primary target of selection (or even the attachment of rectus femoris), but rather the repositioning of the anterior gluteals. So no, I don't think the gorilla AIIS would form from its own growth plate because there's no lateral flaring to push it away.

Whaddya think?

Zinjanthropus said...

Just re-read my comment and thought the part of the rectus femoris might be confusing: the AIIS is the attachment of the rectus femoris, but the positioning of the rectus femoris is not the primary target of selection. At least, I don't think it is...

zacharoo said...

I'd never actually looked into the topic that much before. I doubt that the australopith morphology is primitive for chimps and peeps, but the idea of 'arboreal bipeds' being the ancestral condition has been around a while. What that might look like in a pelvis is unclear to me so far. The next most ancient pelvis I know of aside from Ardi is Oreopithecus, which also has this pesky AIIS. There's a good skeleton for Nacholapithecus from the Middle Miocene, but I don't know about its pelvis.

The main reason I bring up the AIIS and its 'new physis' is that its become a major part of the Ardi story of bipedalism. I really hate Ardi, just because they crammed way too much stuff into too few papers in too little time. It's not fair, they had 15 years to figure it out for themselves, I've only had 1/15th!