At the outset of Mothers and Others, Sarah Blaffer Hrdy poses a thought experiment. Each year 1.6 billion passengers fly around the world. We do so with remarkable ease. Just imagine, Hrdy asks, if our fellow human passengers suddenly morphed into another species of ape. We would be lucky to disembark with all ten fingers and toes still attached, or with any babies on board still alive. Bloody appendages would litter the aisles. It would be mayhem.