It is general knowledge that an increase in brain size runs parallel with an increase in head size and with a change from dolichocephaly to brachycephaly as we ascend the scale from the anthropoids to modern man. This appears primarily to be a volume phenomenon.A. Thomson (quoted in the above paper) notes that:
Given a cavity of oval or ellptical form with elastic walls, the more its contents are increased the greater will be the tendency to assume a spherical shape.Pickering studied cadavers and their weight and volume directly, rather than relying on estimations from cephalic measurements:
The specimens used in this work gave an average capacity for dolichocephalic skulls of 1402.8 cc.; for mesocephalic, 1474.8 and for brachycephalic, 1520.6 cc.Interestingly, according to K. Beals et al. (Brain size, cranial morphology, climate, and time machines. Current Anthropology, 25, 301-330. ) there is a +0.37 correlation between cranial capacity and the cephalic index, i.e., broader-headed, more brachycephalic populations also have bigger cranial capacities.
The same tendency was shown in the size of the brains. The average size of the brains in this series was for the dolichocephalic heads, 1135.4 cc.; for the mesocephalic, 1144.2 cc., and for the brachycephalic, 1180.7 cc.
The figures show a marked increase from the dolichocephalic to the brachycephalic type. Though the number of specimens of the dolichocephalic type is not sufficient to be conclusive proof, the fact that brachycephaly runs parallel with an increase in brain and head size is very evident.
Why are brachycephalic heads more capacious? I don't think that anyone has discovered the exact reasons why this is the case, but here is my theory:
In general, human heads are longer than they are broad, and broader than they are high. Heads of course, like any other organ, are expensive things (developmentally), and this is particularly the case for humans where the head forms a substantially larger part of the body than in other mammals.
During human evolution, cranial capacity has increased. Cranial capacity is of course correlated with gross size: bigger people have a bigger cranial cavity, and a bigger brain.
However, things are not that simple. It can be easily proven that different shapes with the same surface area have a different capacity. In two dimensions, ellipses and ovals have a smaller capacity than round shapes with the same perimeter.
Thus, a broad head achieves a greater volume than a long one of equal surface. Since surface corresponds to bone mass, which corresponds to developmental cost, it is possible that a rounder head shape is more economical: it achieves the same volume with smaller cost.
The same is also true for the height of the skull. Since skulls tend to be broader than they are high, a skull that expands on the vertical plane will tend to approach a more spherical shape.
It is thus not surprising that according to Lahr and Wright's useful definition:
...it is generally agreed that a modern skull should present a relatively small face tucked under the vault that is relatively short and high, a relatively vertical forehead, parietal enlargement, a relatively rounded occiput, a flexed cranial base, a canine fossa, an occipital protuberance in the occipital bone and mental eminence or chin. Most descriptions of modern H. sapiens would also include skeletal gracility as characterizing the group.or that according to K. L. Beals (Climate and the evolution of brachycephalization, American Journal of Physical Anthropology, Volume 62, Issue 4, Date: December 1983, Pages: 425-437):
During the Holocene, the [cranial] index increases under all climatic conditions.Hopefully, a new generation of scientists will overcome the knee-jerk reaction against studying cranial variation and cognitive function, perhaps by relying on cranial models built with 3D digitizers rather than the older methods based on lengths, arcs, and indices.
Turning to the Pleistocene, the mean hominid cranial index has increased 9 units.