Cephalo-Pelvic Proportions

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Human Uniqueness Compared to "Great Apes": 
Absolute Difference
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The upright posture of humans that arose ~5-6 million years ago had a much delayed consequence, due to the fact that the pelvis became narrower. This was not a major issue as long as the size of the fetal head was small. With increasing brain expansion over the last ~2 million years this fetal-pelvic disproportion became an increasing problem. It is unclear exactly when the problem became as severe as it currently is.  Regardless, it is clearly a great disadvantage to the human birth process, resulting often in injury to both the mother and the baby, or even death. Also, because of this disproportion there is a need for the human fetus to rotate before the head can pass through the pelvis and this results in the baby coming out face-down instead of face-up, as it occurs in apes. This also makes it harder for the mother to visualize the baby and or assist the delivery, something which occurs in the apes. Given how maladaptive this feature is in terms of survival of both the young mothers and babies it is surprising that increasing size of the head continued, in the face of this literal "bottleneck". Meanwhile, many of the features of modern human behavior appear much later in the archeaological record.  Thus the earlier increase in brain size had some other unknown advantage that outweighed the big disadvantage caused by the fetal pelvic disproportion.

 

References

  1. Cliff-edge model of obstetric selection in humans, Mitteroecker, Philipp, Huttegger Simon M., Fischer Barbara, and Pavlicev Mihaela , PNAS, 2016/12/05, (2016)
  2. Developmental evidence for obstetric adaptation of the human female pelvis, Huseynov, Alik, Zollikofer Christoph P. E., Coudyzer Walter, Gascho Dominic, Kellenberger Christian, Hinzpeter Ricarda, and de León Marcia S. Ponce , Proceedings of the National Academy of Sciences, Volume 113, p.5227-5232, (2016)