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.

Although the brain grows relatively rapidly during development in the womb, there is a significant slowdown at the time of birth. Humans are markedly different. The rapid growth that other primates show during foetal development continues in humans for about one year after birth.

The slowdown in growth that occurs around the time of birth in other primates begins at the end of the first postnatal year in humans. (p. 104)

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