In hominids, upright walking evolved 4-5 million years ago. The human pelvis was affected by these changes and evolved accordingly to better serve the new pattern of movement. Later, after bipedal movement had long become obligatory, brains evolved to increasingly larger volumes. With that change, the head size of neonates also increased. The growing heads, however, had to be delivered through pelvises that were earlier adapted to upright walking. This is the reason for the lack of space in the modern human birth canal during childbirth, which we have to live with today. The evolution of these patterns remains to have severe consequences. Women in developing countries, who do not have access to modern medical care or cesarean sections during birth, still suffer from high mortality due to childbirth.
The size of the human head is to a large extent genetically determined. This is why women with large heads tend to give birth to neonates with large heads. “We found out that women with large heads, compared to women with small heads, possess a birth canal that is shaped in a way that neonates with large heads can pass it easier,” explains Barbara Fischer: The sacrum is shorter in these women and it leaves more space in the outlet of the birth canal, which is beneficial for birth.
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