Human newborns arrive remarkably underdeveloped. The reason lies in a deep evolutionary trade-off between big brains, bipedalism and the limits of motherhood.

A newborn baby cannot walk, feed itself or even lift its head. Compared with most animals, human babies are remarkably helpless. Yet this dependence is not a weakness; it is an evolutionary advantage.

When babies are born, their brains contain billions of neurons. But how those neurons interact — and what they can do as babies grow through childhood into adulthood — is largely shaped by their ...