We have in the previous two parts of this 3 part series on the passage of babies through the birth canal briefly discussed the process as well as the differences in labor between humans & the giant apes. The title ‘passage over time” was referring to the time it takes in hours to pass through the birth canal. We discussed how much quicker & easier birth is in apes compared to humans, because of the changes bipedal locomotion produced in the pelvic skeleton & brain size. We now will turn to the evolution of the birth process in humans and discuss the passage of birth in terms of millions of years.
The newborn baby has a skull with open fontanels & unfused sutures. This offers the human baby 2 advantages. Their relatively large skulls can be compressed to pass through the pelvis and then expand over the infant years as the brain continues to grow.
Very recently Dean Falk & colleagues In the USA & Switzerland have compared these sutures in modern humans & great apes with the sutures shown in the fossil skull of Taung child in their paper “Metopic suture of Taung (Australopithecus africanus) & its implication for hominin brain evolution” in PNAS.
The skull of Taung is not complete (see Taung photograph in figure).
The right side of the skull is missing but the so called natural endocast i.e. fossilized debris gives an accurate representation of the overlying inner layer of the skull. It is fascinating that the scientists now report that the anterior fontanel as well as the metopic suture (see drawing of baby skull in the figure) can clearly be seen. Thus the “soft spot” & suture had remained open until Taung died at 3-4 years of age. This is evidence that selective evolutionary pressures were already at work to solve the so called “obstetric dilemma” of allowing development of a large brain despite having a relatively small pelvis.
In modern humans the anterior fontanel closes at around 18 months of age & the metopic suture fuses after that time. In contrast in the great African apes the metopic suture closes soon after birth. Thus the open suture appears to be a later development. They similarly found that the metopic suture was fused at birth in chimpanzees . Thus Taung was demonstrating this advanced “modern human” skull development 2.5 million years ago!!!!
This development would have allowed the birth of babies with larger brains as well as rapid expansion of the brain in infancy.