Lactose is the sugar that feeds our large brains. Primates nurse for a significant portion of their lives, partially because they have such long periods of infancy. Sharply curtailing the duration of breastfeeding below what the human child has evolved to expect, has significant deleterious health consequences for modern humans. Of course, humans do very little without regard to belief systems and customs. But this type of comparative study is helpful in countering the argument that children who nurse for an extended period of time are abnormal or that the mothers who continue to nurse them are encouraging dependency or are nursing for their own selfish reasons.
Cultural behavior can also be spectacularly non-adaptive.
Four of his six children died in the first three years of life primarily because as infants they were fed mainly sugar water. Human infants are so immature and need to feed so frequently that it should surprise no one that most human infants, in most places and at most times, have slept with their mothers and often, with both parents. This was still the pattern in the United States until about 75-100 years ago when parents began to be warned about the dangers of sleeping with their babies. Popular infant care books warned about the danger of sexually stimulating the infant by putting him in bed with the mother. Some experts warned that sleeping with infants predisposed them to homosexuality.