Re Baby head size and pelvic floor circumference…
It seems that may have something to do with a curious fact. The late Neanderthal, as well as Cromagnon (including late Strandlopers) had larger heads than we have. An adult average was 1600 ccm, while we have 1450 ccm (there is some variability, smaller skulls at 1100 ccm and large skulls at 2100 ccm boundary, the later which are a result of a caesarian as Teresita hints).
It may be possible that Neanderthal demise is related to the increase in skull size, not by HSS hunting them out of existence. In comparison of physique, Neanderthals would be terminators–their bones were twice as thick and apparently they had a larger muscular mass. A heartfelt hug by a Neanderthal would result in a permanently altered thorax of the recipient HSS member.
As Cromagnon is concerned, at about 12kya boundary (the beginning of the current interglacial) we find only the small skulls. It is possible that the same factor was responsible as with HSN. We had a greater variability and greater numbers–the larger skulls simply were not viable.
Not always the case, though. somewhere around 70kya, we were on the brink of extinction, with some 2000 individuals representing our entire species (Y chromosomal and mitochondrial DNA bottleneck both indicate that same period. Another happened at 12kya, with an estimated figure of 15,000 individuals, and another at 3.5kya with an estimated figure of 400,000).
One way out of the skull size conundrum would be to become coneheads. Judging from Ica (Peru) skulls displayed in the local museum, that avenue has been also tried. The skulls are explained as a result of a head-boarding… the problem is that the skulls are larger than normal, at least based on some reconstructions that looked at the geometry and projected possible cranial capacity–somewhere about 1750 ccm and larger. So, that apparently did not work either.
If we ever started establishing space based colonies (low gravity), our heads would start to expand and bodies become frailish; possibly our eyeballs would enlarge too, because the wider cranium may accommodate them as an adaptation to lower light conditions. We would look much like these, ahm… grays.
