September 18, 2019

The Marvel of the Human Dad

“To understand the role of the father, we must first understand why it evolved in our species of ape and no other. The answer inevitably lies in our unique anatomy and life history. As any parent knows, human babies are startlingly dependent when they are born. This is due to the combination of a narrowed birth canal – the consequence of our bipedality – and our unusually large brains, which are six times larger than they should be for a mammal of our body size.

This has meant that, to ensure the survival of mother and baby and the continued existence of our species, we have evolved to exhibit a shortened gestation period, enabling the head to pass safely through the birth canal. The consequence of this is that our babies are born long before their brains are fully developed. But this reduced investment in the womb has not led to an increased, compensatory period of maternal investment after birth. Rather, the minimum period of lactation necessary for a child to survive is likewise drastically reduced; the age at weaning of an infant child can be as young as three or four months. A stark contrast to the five years evident in the chimp. Why is this the case?

If we, as a species, were to follow the trajectory of the chimpanzee, then our interbirth interval (the time between the birth of one baby and the next) would have been so long; so complex and so energy-hungry is the human brain that it would have led to an inability to replace – let alone increase – our population. So, evolution selected for those members of our species who could wean their babies earlier and return to reproduction, ensuring the survival of their genes and our species. But because the brain had so much development ahead of it, these changes in gestation and lactation lengths led to a whole new life-history stage – childhood – and the evolution of a uniquely human character: the toddler.”

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