Ardipithecus ramidus: mystery hips and missing gams

Let’s talk about the Ardipithecus ramidus pelvis from the partial skeleton ARA-VP-6/500. Variously preserved are a left ilium, a small part of the right ilium, and the caudal (bottom) portions of a sacrum. The fossil on which most of the reconstructions are based, the left ilium, is quite distorted and fragmented, the cracks in the bone filled in with matrix which subsequently expanded and contracted over time. The authors used CT-scans of the fossils to virtually remove adherent matrix, readjust bone to its (presumably) accurate position, and fill in cracks. The result:

(Lovejoy et al. 2009, fig. 1, p. 71e2)

Quite an odd mix of hominin and ape features, some of which are clear prior to reconstruction, others only after the reconstruction. The main things to note are the antero-lateral flare of the iliac blades, and the fairly wide and short sacrum (not clear from the photos; the sacrum is almost entirely imaginary), which are features also seen in bipedal hominins. Clear from the pictures, however, is the anterior inferior iliac spine (see bottom left inset), which is pronounced in bipeds like us, but weak/absent in apes. Finally, a bit more ape-like, is the relatively tall ischial and pubic region.

Listing individual features is all well and good if you’re into cladistics, but more interesting is the functional interpretation of the fossil and reconstruction. Here’s what the authors have to say about the ilium:

[The] exceptionally derived ilium is striking. It implies an early adaptation to habitual terrestrial bepedality before any increase in the lumbar entrapment seen in the African apes. (Lovejoy et al. 2009, p. 71e3).

Hold the phone! Why are the lateral flare and low height of the ilium necessarily adaptations to terrestrial bipedalism? One key word that follows from their reconstructions, but is not actually manifest in any of the preserved fossils, is lordosis. Lumbar lordosis refers to the frontward concavity of the lower spine, seen only in humans, fossil hominins, and bipedally-trained macaques (it’s also why we’re prone to lower back inuries).

Lordosis in Ardipithecus is entirely inferred. The (reconstructed, probably realistically) broad sacrum and shortened superior iliac blades suggest that Ardipithecus was capable of lumbar lordosis, because the lower lumbar vertebrae were not closely flanked by the adjacent ilia, as in apes. But to the best of my knowledge, they don’t mean there was lordosis. Nevertheless, Ardi’s lordosis is constantly referred to in the paper. The final word on the pelvis is that the ancestral condition of hominins (like the skull and other features, claimed to be manifest in the Ardipithecus ramidus fossils) “involved situationally dependent lordosis (during terrestrial upright walking)” (Lovejoy et al. 2009, p. 71e3). I’m not sure what exactly they mean by this, because animals can’t just adjust the relative front and back heights of their vertebrae willy-nilly. If that were the case I’d have a straight spinal column when I sleep, and overly-lordotic when I’m pregnant (which is often; Whitcome et al. 2008), and random on Halloween.

It is an interesting pelvis, though I wouldn’t be as cavalier about asserting that it belonged to an adroit terrestrial biped. Unfortunately, the partial proximal femur described with the pelvis mostly lacks any diagnostic morphology. It would be great to see what the thickness of the cortical bone in the femoral neck was like in Ardi, or the extent of articular surface on the femoral head, because these have been shown to have characteristic forms in bipeds. Hopefully future fossil discoveries will shed light on these in Ardipithecus, as well as pelvic morphology in the earliest Australopithecus.

Until then, I’m content to conclude that Ardi had a unique form of locomotion (arboreal bipedalism?), but I’m hesitant to call it a terrestrial biped.

References
Lovejoy CO, Suwa G, Spurlock L, Asfaw B, and White TD. 2009. The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking. Science 326: 71e1-71e6.

Whitcome KK, Shapiro L, and Lieberman DE. 2007. Fetal load and the evolution of lumbar lordosis in bipedal hominins. Nature 450: 1075-1080.

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