Evolution has nevertheless found a solution to overcome this first natural limit of brain growth.

Palace District, Machu Picchu, Peru
Palace District, Machu Picchu, Peru

Robert D. Martin continues:

It appears that because of the unique size of the adult human brain, part of the growth that would normally occur within the womb has been transferred to postnatal life.

It has been correctly said that human pregnancy actually lasts 21 months: nine months in the mother and another twelve months outside. This particular characteristic related to the development of the human brain explains why our newborns are so helpless, despite the fact that they correspond in many other respects to the standard pattern that nidifugous animals typically reveal. (p. 105)

Martin explains the consequences of the narrow human birth canal for the growth of the newborn’s brain:

A closer look reveals a special reason for the relatively helpless condition of human newborns. It has to do with the growth of the brain. The general rule for primates is that the brain has reached half its adult size at the time of birth.

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But there was never any doubt as to why the human brain could not continue to grow.

"Fountain of Ceremonies", Machu Picchu, Peru
"Fountain of Ceremonies", Machu Picchu, Peru

The answer lies in the biology of human reproduction. Robert D. Martin explains the problem in the essay The Evolution of the Human Body:

The dimension of the birth canal that passes through the pelvis represents a limit to the circumference of a newborn child’s head, and in humans the relevant dimensions are even smaller to enable the pelvis to be remodelled for bipedal walking.

At birth, the size of the human brain has already reached the limit set by the pelvis. The human birth process has therefore become exceptionally complex compared to that of other primates. It includes the continuous twisting and turning that a newborn has to do in order to be born head-first when he or she turns towards the mother’s back.
(Die Evolution des menschlichen Körpers, in: Fischer/ Wiegandt: Evo­lu­tion, pp. 74-109, p. 105)

This is where the development into a human being should have come to an end. The birth channel of the early human woman, who was still far from Homo sapiens and its culture, blocked brain growth and thereby preventing the development of the great ideas of a Descartes, Darwin, Kant and Mayr.