Another take on Ardi’s pelvis

First, let me say that I agree with Zach’s assessment: the lordosis is completely imaginary, and I’m not at all convinced you can extrapolate it from a complete pelvis, let alone one with an imaginary sacral width.

Now, on to some more discussion of what the pelvis does (and does not) tell us:

Four questions that occurred to me while I read this article (in addition to the lordosis problems presented already by Zach):
1. Was Ardi bipedal based on the pelvis?
2. Wait, wasn’t Ardi a female? What about her birthing process?!
3. Was platypelloid the ancestral condition for hominid pelves?
4. How come there’s only four pages about the pelvis if it’s so darn complete? Have you seen how many pages were written on KNM-WT 15000? That pelvis was way less complete than this one!

Here’s my take:

The first question is by far the hardest (for me) to answer. Maybe? Sort of sometimes? I don’t even want to get into terrestrial vs arboreal bipedalism. I’m satisfied with saying she was capable of being bipedal but also of climbing (what type of climbing, I don’t dare try and suggest, I’ll just embarrass myself). Problems with saying Ardi is bipedal: the lower pelvis is totally chimp-like! How much could this affect locomotion? I don’t know. Most of the bipedal/not bipedal muscles would be attaching to the iliac blades, which are certainly more australopithecine-like than the lower pelvis. But that crazy curved medial portion of the ischio-pubic ramus has got to serve some functional purpose (unfortunately, for me to hazard a guess at this purpose, I would have to go study an anatomy text – something I’m just not going to do right now, so you’ll have to keep wondering). So maybe I can’t answer this question satisfactorily at this point (sadly the femur wasn’t a great help, and I don’t believe in the mythical lordosis).

I was told Ardi was a female (I believe this is mainly based on the skull, which I’ll admit I haven’t read about yet). HOW can you write a paper about a female primate’s pelvis and NOT discuss how she was having babies??? The only mention of this is at the very end of the pelvis section, where it’s pointed out that the differences we see between Ardi and Homo are probably due to “optimization of birth-canal geometry.” That’s lovely, but I want to know how Ardi gave birth – was it like a chimp or an australopithecine? How did that work with a mosaic pelvis? The birth canal seems awfully circular compared to Lucy, despite the flaring iliac blades that make it platypelloid. What does that mean? The lateral image of Lucy and Ardi shows that Ardi’s outlet was either gigantic (doubtful) or, because her sacrum would be positioned differently than Lucy’s, positioned antero-inferiorly compared to the australopithecine. Not to mention that the ischio-pubic region on Ardi would make for a much longer birth canal than what Lucy’s progeny had to deal with.

The platypelloid pelvis: Ardi is said to have one based on how the iliac blades were straightened out (something that could have been done differently, I’m sure). It was based on Lucy and Busidima (a Homo erectus), both of which were female hominids with platypelloid pelves. This is a great set up for this being the ancestral condition, and makes sense when we consider that a wider pelvis will biomechanically make bipedalism easier (human females have narrower pelves only because of obstetric constraints). So this is a great story. A word of caution though: we can’t assume Ardi’s pelvis is wide because she’s bipedal, AND that she’s bipedal because her pelvis is wide – that’s called circular logic! Assuming that her pelvis should be morphologically similar to other bipedal hominids in ways that make them bipedal, and then saying “ta da, Ardi is also bipedal!” doesn’t work.

This should have been a much longer paper. It’s ridiculous that the paper overall was so short, and that the pelvis had to share space with the (albeit almost non-existent) vertebrae and the femur. Get your own paper! Write more about the pelvis!! Address topics like obstetrics!!

This post refers to the same paper cited by Zach in the previous post – Lovejoy et al., 2009

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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.

Tabūn Pelvis Reconstruction

News: Weaver and Hublin (2009) virtually reconstructed the Tabūn C1 female Neandertal pelvis using CT scans.

Background: This is the closest we have to a complete female Neandertal pelvis, so a lot of the discussion centers around obstetrics. When a modern human woman gives birth, the infant enters the birth canal facing sideways so that the head will fit through the transversely oval inlet, then turns 90 degrees so it is facing the back so that the head will fit through the AP oval midplane and outlet, and finally turns another 90 degrees after the head passes through the outlet so that the shoulders can also fit through the outlet.

Tabūn conclusions: Neandertal infants (based on Tabūn’s inlet, midplane, and outlet diameters) only required two rotations: the initial turn so the head faces laterally, and the last turn so the shoulders fit through the transversely oval outlet. This means the infant comes out with the head facing sideways and the shoulders facing front (this is also how australopithecines are thought to give birth). This, the authors suggest, means that Neandertals were more primitive than modern humans. Furthermore, the transversely oval birth canal reflects the cold-adapted wide pelvis associated with Neandertals.

Problems:

  1. Methods: There is no sacrum for Tabun. None. It is possible to predict sacral width and thus reconstruct the inlet, but it is improbable for the outlet to be reconstructed accurately without knowing the sacrum’s length, curvature, and orientation.
  2. Sexual dimorphism: They confuse this throughout the paper. First, they female-ize Kebara (a complete male Neandertal pelvis) by assuming that Neandertals were as sexually dimorphic as modern humans. This has been shown to be wrong previously, so it was a dumb assumption. They also find that this is not the case, making me wonder why they bothered with it in the first place. Then, they claim that the difference between Neandertals and modern humans is that Neandertals are like modern males (who have short pubic rami) when really they’re like modern females (who have long pubic rami). See Rosenberg (2007) for more discussion of this.
  3. Cold- and warm-adaptations: They say that Neandertals were cold-adapted because of the wide birth canal, in contrast to warm-adapted modern humans from Africa. First, wide birth canals do not go hand-in-hand with wide pelves. Second, Tabūn lived in the Levant and thus did not need to be cold-adapted. Third, the Busidima female Homo erectus pelvis from Gona is also wide and also not cold-adapted. Fourth, modern humans in Africa evolved a narrow pelvis to be better adapted to the warm environment is based on… uh… KNM-WT 15000? No, wait, that’s also a H erectus, and Gona has already shown that they have wide pelves despite their climate. But what else is there to support this long-held idea? Answer: Not much.

Citations:

  • Weaver, TD and JJ Hublin (2009) Neandertal birth canal shape and the evolution of human childbirth. PNAS Early Edition: 1-6.
  • Rosenberg, KR (2007) Neandertal Pelvic Remains From Krapina: Peculiar or Primitive? Periodicum Biologorum 109(4).

Busidima female pelvis

Yay! A female, mostly complete, Homo erectus pelvis has been found (Simpson et al 2008)! I say “yay” because female pelves are the best way to learn about the effects of birthing on pelvic morphology, despite the numerous studies that claim to glean this knowledge from male pelves (I’m basing this mainly on Neandertal examples as I am admittedly not as well versed with H erectus papers).

Quick summary of the facts: Found in Gona, Ethiopia, BSN49/P27a-d (a.k.a. the Busidima pelvis) consists of both os coxae (hipbones) and the upper part of the sacrum of what has been identified as an adult female Homo erectus dated to 0.9 to 1.4 Ma. The hipbones are mostly complete, except for portions of the ilium missing on both sides and portions of the ischial tuberosities. Overall, the pelvis has been reconstructed so that measurements can be taken (see image below, taken from the Simpson et al 2008 article).


Why I’m excited: Since this fossil is female and has the first complete early Pleistocene pubis, and thus the first complete pelvic inlet, it means legitimate inferences about birthing can be made. The initial paper addresses this by exploring neonate (i.e. infant at birth) head sizes compared to inlet dimensions. The authors found that Busidima would have been capable of birthing infants 30% larger than predicted based on the Homo erectus pelvis KNM-WT 15000 (male subadult). Already this find is showing why making assumptions about birth based on male specimens is flawed! Yay!!

Why I’m concerned: The authors draw conclusions about the width of the trunk based on the bi-iliac breadth. As the picture shows, the ilia are largely reconstructed, making any measurement between the widest points on the iliac crest dubious. I’m not saying it’s a bad reconstruction; in fact it looks reasonable to me. I just believe caution must be used when drawing conclusions based on reconstructed measurements. Especially since I cannot find details on how the ilia were reconstructed (if I’ve missed something in my reading of the online supporting material, please, someone correct me). Given that this is the case, it seems extreme to use incomplete ilia as evidence against an endurance running hypothesis and as evidence for what type of environment this specimen lived in.

What others are saying: Hawks goes into more anatomical detail in his blog post about the article. He discusses questions raised by the study and points out that you really have to read the supporting stuff to get the full picture as the Science article is “superficial”. Beast Ape’s blog post discusses the bi-iliac interpretation and what it means. For a multilingual look, Mundo Neandertal also discusses the article, and based on my imperfect Spanish capabilities, I think they discuss the infant head size findings. Finally, New Scientist describes the findings in a newsy way, detailing the bigger birth canal (compared to WT 15000) and the squat proportions of Busidima. Enjoy!

References:
Simpson et al. 2008 A Female Homo erectus Pelvis from Gona, Ethiopia. Science 322: 1089-1092.