Humans beings are very slow to mature. We are the slowest of all primates, and probably the slowest of all animals. In other slow-maturing species, that means a long gap between babies. But not us. We can have new babies long before the earlier ones are mature because mothers get so much help from others.
In other apes, mothers rarely let go of their babies, not even to let their sisters or mothers hold or help, and certainly not the fathers of the babies. Human beings stand out for how much "alloparenting" - care by others - we do and accept. In foraging bands, fathers normally share in child care very extensively. This makes human mothers, especially in societies in which alloparenting is normal, confident that they can trust their babies to have extensive contact with other people.
Sarah Hrdy's main concern is to explain how intersubjectivity developed among humans. Alloparenting, she things, is the key difference. Mothers in many mammals, especially other primates, have reason to learn how to read the faces, noises, movements, and even minds of their babies, and babies need to develop similar skills in reading their mothers. But in humans, who are often in the care of "other mothers," it would be hugely valuable for babies to learn how to read other people, too, people with whom they did not share the whole range of smells and sounds that mothers and their babies do.
Hrdy reports that kids attached to their moms are better fed, but kids also attached to others were more empathetic, dominant, independent, and achievement oriented. This is an immediate fruit of a society, and species, trusting enough for alloparenting.