Anthropology.net

What you may call the hip or pelvis is actually formed by the joining of ilia, ischia, pubis bones to the sacrum and the coccyx. The shape of the human pelvis is unique amongst primates and part of the complex of anatomical changes which allow us for bipedal motility.

The Male Pelvic Girdle

Between males and females, the pelvis is significantly different. I’ll review some of the features that should be common knowledge to anyone with a forensic or physical anthropology background. For starters, where the two pubis bones meet, at the lower edges of the two inferior public rami, there is a feature called the subpubic angle. Males have an angle smaller than 90 degrees, while females have a larger one. You need not take a protractor to make this observation. If you got a pelvic girdle with subpubic angle wider than a right angle then you most likely have a female… anything smaller and you have a dude.

But there is more, under the posterior inferior spine of the illium, and above the ischial spine, exists a feature called the greater sciatic notch, a sort of passage way for the piriformis muscle and the sciatic and posterior femoral cutaneous nerves. In females the notch is broader than males. Another feature, the biiliac width, a metric measurement made from the widest point between the two ilia is also a sex determining feature. Relative to their overall body sizes, females have large biiliac widths. The two ilia seem to flare out wider in a female. With a wider biiliac width, comes a wider pelvic inlet or the circumference of the lesser pelvis forms.

The Female Pelvic Girdle

So why do females have larger pelvic inlet, width between the two ilia, and a larger subpubic angle? That’s because they give birth. A wider pelvis allows for better distribution of the added weight that comes during pregnancy. A wide pelvic inlet allows for more space to hold the baby in utero. Furthermore, passing the largest brain to body size mammal through a narrow pelvis would not only be painful, but poses a serious danger to both the mother and baby during childbirth.

The average female adult has a biiliac width of 28 cm. Certain populations, such as Greek women have biiliac widths of 27.5 +/- 2.29 cm, falling within the average (Steyn et al., 2008). Inupiat women have widths averaging 28.6 +/-  0.2 cm, Finns at 27.9 +/- 0.2 cm (Ruff et al., 2004). But, east Asian populations, such as the Japanese have smaller pelvises, with less variation. The average billiac width of women from Japan is around 27.2 +/- .02 cm (Ikoma et al., 1988).

This all makes sense, east Asian people are on average smaller than white people or people from Africa. In fact, anthropologists have regularly relied on estimating body size and mass from biiliac measurements. The average Japanese woman is 153 cm tall, while European women from Germany or the Netherlands average 166 cm in height. You can see such a distinction when comparing Finnish and Japanese pelvic girdles. Asian newborns babies are also have smaller weight at birth (3.2 kg) compared to white babies (3.4kg). A  white woman with a wider pelvis can give birth to larger white babies.

So what happens when a white man, with big white genes, reproduces with a small Asian woman? Well, Razib pointed out a new study in the American Journal of Obstetrics and Gynecology which reviews the impact of such couplings. The paper, “Perinatal outcomes among Asian–white interracial couples,” documented that 33% of such couples surveyed had caesarean deliveries. The latest NIH data on the caesarean rate in the United States is 30%.

The authors suggest that the reason why such couples have 3% more C-section deliveries is that the smaller Asian pelvis is less able to accommodate babies of a certain size. The Asian-white couples had larger babies, with a median 3.36 kg for Asian-mother/white-father versus 3.21 kg for babies from Asian-Asian couples.

There’s a much larger discussion to be had than just reviewing a review of the anatomy and evolutionary history of such a study. Ever so recently, we hosted yet another post on the anthropology of race, which summarized that, “race does not exist in the world in any ontologically objective way.” If you’ve been a regular reader, you would know I’ve tackled this mantra many times. What could be anymore ontologically objective than such a study?

The nature of an Asian is on average smaller in body size than other humans. Of course there is variation. There are some large Asians, but the are very few. The majority are smaller in comparison to other humans. Studies like this show that Asian-mother and white-fathers produce larger babies and have increased rates of C-sectiond deliveries.

There are serious health issues with C-section deliveries, and thus serious, tangible biologically race related issues when people from two different populations mate and increase their chance of having a C-section delivery. The health issues I mention are the increased childbirth mortality rate. On average 1 in every 10,000 women who gives a natural birth will die during childbirth, while 1 in every 2,500 women who undergo C-sections will die during childbirth. In otherwords, women who give birth via C-section are 4x as likely to die. Furthermore, caesarean deliveries increase the risks for malpresentation, placenta previa, antepartum hemorrhage, placenta accreta, prolonged labor, uterine rupture, preterm birth, low birth weight, and stillbirth in their second delivery. The paper also outlines the higher prevalence of gestational diabetes for such interracial couplings.

That all being said, race is not just a social construct. How we interpret biological differences, such as pelvis size, skin color, etc. are not socially constructed but real observations, made from quantiative analysis. There are distinct anatomical, genetic, even behavioral differences that are not derived soley from stereotypes.

Michael J. Nystrom, Aaron B. Caughey, Deirdre J. Lyell, Maurice L. Druzin,Yasser Y. El-Sayed (2008). Perinatal outcomes among Asian–white interracial couples in American Journal of Obstetrics & Gynecology 199 (4), (385.e1-385.e5) DOI: 10.1016/j.ajog.2008.06.065

The other day Dienekes pointed out a paper on ancestral human population dynamics within Africa before the out of Africa migrations. The paper is very similar to one I reviewed in April, which also focuses on the diversity of the mitochondrial haplogroup L — one of the oldest mtDNA haplogroups out there.

The new paper, “Bayesian coalescent inference of major human mitochondrial DNA haplogroup expansions in Africa,” published in the Proceedings of the Royal Society B, uses coalescent theory to investigate past population sizes of each of the four major African mtDNA haplogroups (L0-L3). 224 different mitochondrial genomes were analyzed and the comparison yielded some similar results to the previous paper I mentioned. But remember, the last paper investigated the time of the emergence of each haplogroup. This paper focuses on effective population sizes.

Anyways, for starters, the results show that three distinct demographic histories can be seen from the underlying the four haplogroups. Two of the oldest haplogroups, L0 and L1, show exponential growth from 213,000 to 156,000 years ago. The previous paper suggested that the L0 and L1 split about 200,000 years ago. Soon after this split, one of the the paleoafrican branches L0 established what we now consider sub-Saharan Khoisan peoples.

L1 split up into the L2 and L3 branches sometime around 127,000 to 72,000 years ago, again consistent with the previous paper. The L2 and L3 branches show two exponential growth periods, one around 86,000 to 61,000 years ago and another around 20,000 to 12,000 years ago. The authors observed a distinct expansion of the L3 branch around 12,000 to 8,000 years ago. They suggest that,

“L3 did not simply spill over into Eurasia, but was driven as part of an expansion that had begun in sub-Saharan Africa thousands of years earlier.”

While this date is a bit later than the one suggested in the previous paper, both indicate that there were deep African migrations within Africa. The later expansions of L2 and L3, coincide with environmental and cultural changes, such as the greening of the Sahara and emergence of pastoralism. The authors write that,

“The timing of the L3 expansion-8-12kyr prior to the emergence of the first non-African mtDNA lineages-together with high L3 diversity in eastern Africa, strongly supports the proposal that the human exodus from Africa and subsequent colonization of the globe was prefaced by a major expansion within Africa, perhaps driven by some form of cultural innovation.”

Quentin D. Atkinson, Russell D. Gray, Alexei J. Drummond (2008). Bayesian coalescent inference of major human mitochondrial DNA haplogroup expansions in Africa Proceedings of the Royal Society B: Biological Sciences, -1 (-1), -1–1 DOI: 10.1098/rspb.2008.0785