The female pelvis and its significance in human evolution
A post about the female pelvis may appear a bit strange on a blog like this. However, considering the crucial role of the pelvis in (human) evolution it certainly does deserve more than an honourable mention. When I discovered a connection between the four evolutionary temperaments (hunter-gatherers, farmers and pastoralists) and some anatomical features, like facial structure and body shape, the pelvis was one of the first anatomical structures I checked out. The female pelvis has two crucial functions:
- Locomotion (survival, subsistence)
- Childbirth (reproduction)
If there are structural differences in female pelvises, the obvious thing is to assume that those are adaptations to differences in locomotion for different subsistence strategies (hunters, gatherers, farmers and pastoralists). However, different reproductive strategies may also be at play as farmers and herders have much higher reproductive rates than hunter-gatherers (every 2 years vs every 4 years).
To my great joy, I really did find four differently shaped types of the pelvis:
- The gynaecoid pelvis is the so-called normal female pelvis. Its inlet is either slightly oval, with a greater transverse diameter, or round. The interior walls are straight, the subpubic arch wide, the sacrum shows an average to backward inclination, and the greater sciatic notch is well rounded. Because this type is spacious and well proportioned there is little or no difficulty in the birth process. Caldwell and his co-workers found gynaecoid pelves in about 50 per cent of specimens.
- The platypelloid pelvis has a transversally wide, flattened shape, is wide anteriorly, greater sciatic notches of male type, and has a short sacrum that curves inwards reducing the diameters of the lower pelvis. This is similar to the pelvis where the softened bones widen laterally because of the weight from the upper body resulting in a reduced anteroposterior diameter. Giving birth with this type of pelvis is associated with problems, such as transverse arrest. Less than 3 per cent of women have this pelvis type.
- The android pelvis is a female pelvis with masculine features, including a wedge or heart shaped inlet caused by a prominent sacrum and a triangular anterior segment. The reduced pelvis outlet often causes problems during childbirth. In 1939 Caldwell found this type in one third of white women and in one sixth of non-white women.
- The anthropoid pelvis is characterized by an oval shape with a greater anteroposterior diameter. It has straight walls, a small subpubic arch, and large sacrosciatic notches. The sciatic spines are placed widely apart and the sacrum is usually straight resulting in deep non-obstructed pelvis. Caldwell found this type in one quarter of white women and almost half of non-white women. (from Wikipedia)
My joy became more moderate when I tried to match the four pelvis shapes to the four temperaments. The percentages simply did not line up. Farmer types make up about 50% and so does the distribution of the gynaecoid pelvis. However, the percentage of the other types didn’t work out. So, I gave up my attempt to match them. That is until recently an online friend (thanks Jari) contacted me telling me about this pattern. I told him that the percentage didn’t align and he encouraged me to continue my research, as a perfect match wouldn’t be necessary.
A tentative match produces the following pictures:
The gynaecoid type matches with farmer types not only in percentages but also regarding the fact that this is the shape that produces the least complications during childbirth, i.e. it is adapted to high fertility rates. Even though pastoralist types may be more r-selected, farmer types probably had the higher birth rates due to sedentism throughout history.
On the flip side, female hunter types would have had the lowest fertility rates, slightly lower than the caregiving gatherer types as they would have had less time for nursing, having an evolutionary providing profile. The fertility rates in foragers are furthermore limited by later menarche as well as earlier menopause.
Menarche in different forager groups
The platypelloid type is the one that has the most complications for childbirth. My wife has a hunter-type friend who had huge difficulties giving birth to all of her four children (all cesarean section). Unfortunately, I haven’t been able to find data from forager societies, however, the following piece of research gives us some clues:
In our earliest upright ancestors, fundamental alterations of the pelvis compared with non-human primates facilitated bipedal walking. Further changes early in hominin evolution produced a platypelloid birth canal in a pelvis that was wide overall, with flaring ilia. This pelvic form was maintained over 3–4 Myr with only moderate changes in response to greater habitat diversity, changes in locomotor behaviour and increases in brain size. It was not until Homo sapiens evolved in Africa and the Middle East 200 000 years ago that the narrow anatomically modern pelvis with a more circular birth canal emerged. ( )
This means the platypelloid type is evolutionarily the oldest type of pelvis and can therefore be attributed to hunter-gatherer types. A more circular shape evolved in gatherer women around the time of the emergence of modern Homo Sapiens. Empirical research could easily show if the remaining two types of pelvis are correlated with farmer and herder types.
The four types of pelvis should also be somewhat correlated with body shape and body fat distribution, which may be very different in hunter-gatherers, farmers and herders. Future research may uncover some more interesting connections.
This post is dedicated to Jari for encouraging my research
Originally published at http://the-big-ger-picture.blogspot.com on November 27, 2021.