Pages

Sunday, November 16, 2008

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.

2 comments:

zacharoo said...

I think this pelvis is quite interesting, and has important implications like you point out. But comparing it to WT 15000 (AKA the "Nariokotome boy," the "Stripling Youth," the "Turkana Boy," and finally I call him "Gimlet" for absolutely no reason at all) is kind of like comparing apples to oranges:

-Busidima is female, Nariokotome is a male

-Busidima is poorly constrained at ~1.4-0.9 million years ago, while Nariokotome is ~1.5 million years old--so she's AT LEAST 100 ky younger

-Busidima is apparently an adult, while Nariokotome is a subadult and many of his bones still had more growing/developing to do.

Throughout the paper the authors generalize Homo erectus and describe how Busidima is different. This is somewhat problematic: to conflate the record of a 'species' that is quite widespread both geographically and temporally. Given the fact that H. erectus is found in Africa and Eurasia from ~1.9 until ~1 million years ago, I think this kind of diversity is to be expected.

I don't know how well the "short-statured, broad-hipped" morphology of the torso necessarily undermines previous hypotheses about the selective importance of environment and endurance running. Earlier H. erectus (i.e. Nariokotome) fit this bill, and it could be that there was not selection for EVERYONE to be tall and lean, adapted to running and/or living in a tropical/semi-arid environment. Maybe as more pelves emerge we'll find that lateral iliac flare was more pronounced in hominin females because of pregnancy, who knows? Just as we should be cautious in applying certain evolutionary models to the fossil record, we should also be careful not to reject hypotheses with the addition of a single specimen.

So what's cool about this pelvis, then, is that it reveals Homo erectus to be much more diverse than was once thought. They point out that sexual size dimorphism was thought to have been lower in H. erectus, but recent finds are showing this not to be the case, at least not in East Africa. Hawks does raise the point about whether Busidima might be an Australopithcus (i.e. boisei). Aside from what he points out to suggest it is in fact Homo, I could also point out that 1.4 Ma is about when robusts 'disappear' in E. Africa, and the presence of Acheulean stone tools (associated with H. erectus), also argue against Busidima being australopithecine. But these are fairly circular arguments...

Anyway, cool new find--hopefully there will be more to come.

Caroline said...

I think the comparison to "Gimlet" is due to it also having a fairly complete pelvis. I'm not sure how many Homo erectus pelves we have, but from my brief readings I can't think of many. But the problems you bring up do exist and need to be addressed.

I like the idea of this specimen showing H erectus diversity! And I think Hawks is right to ask the question "could this be australopithecine?" but also right to conclude the answer is "no".