Birth in Australopithecus, apes, & man: a passage over time (part 2)

Walking into trouble: birth in higher primates.

Birth of their baby is usually fast & uncomplicated in higher primates such as gorillas, chimpanzees, & orangutans. However in man the birth is usually prolonged and obstructed labor a common problem, with potentially serious consequences for mom & her baby e.g. ruptured uterus, birth asphyxia.

The “Obstetric dilemma” :- This term refers to problem evolution of man faced in balancing the need of a larger brain with the difficulties that would create when the baby was born via a narrow pelvis.

Fossil records indicate that our ancestors began walking on two legs (bipedal locomotion) 4 million years ago. Standing upright, maintaining balance & supporting body weight, required the evolution of major anatomical changes  in the skeleton. For example the relation of the position of the skull to the spine, & the proportions of the different pelvic bones changed. Thus the pelvic architecture changed, from the shape of a simple cylinder from inlet to outlet still seen in great apes, to a complex structure where the planes of the inlet, midpelvis, & outlet all differ. Thus while in apes the baby can drop straight down & out, in man the baby must rotate as it winds its way down & out.  Figure 1 gives a simple overview, approaching the birth of the baby using our ‘flow” analogy described in an earlier blog.

The differences in the birth of a baby in humans & apes (click to enlarge)

This difficult route is of course made even more complicated by any variations in the size & shape of the maternal pelvis & fetal skull as well as position of the baby in utero.

These potential obstetric problems was aggravated by the growth of the brain that was required to cope with all the wonderful opportunities that walking upright offered e.g. using hands to work with tools. Thus because of the constraints on brain size by a small pelvis human babies are born relatively immature, with a lot of brain growth, development, & maturation occurring in infancy. See the excellent review by Anna Blackburn Wittman & L Lewis Wall for the fascinating details of this evolutionary process ((Obstet Gynecol Survey 2007; 62:739-748).

A further adaptation was a skull which could be gently compressed as it passed through the narrow pelvis and then expand in the months after birth to allow the brain to grow further. Human babies have openings in the skull; two fontanels & various sutures (see figure 2).

Sketch of babies head viewd from above to show the fontanels & sutures present. these allow the baby’s head to be gently compressed at birth & expand in infancy as the brain continues to grow postnataly. (click to enlarge)

These allow the baby’s head, containing the already large brain, to be gently compressed & its shape molded as the baby slowly passes down the relatively narrow & rigid female pelvis. Furthermore these two fontanels & the sutures only close gradually & fuse in months & years after birth allowing for rapid postnatal brain growth in infancy, which so essential for full human development. The fontanels are commonly known as the soft spot. The anterior is large at birth & only closes fully at 18 months. Parents are often scared to press on that area, although it is covered by tough membranes. I recall often examining spotlessly clean babies, who however had an accumulation of skin & debris at the soft spot, because mom & dad had been scared to clean too vigorously.


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