In the course of its evolution, a unique challenge was posed to Homo sapiens by two factors: we stood upright, which constricted the female pelvis, and we had bigger brains – a 300 per cent increase – which meant larger heads. So human babies had to be born more prematurely than any other species, and thus needed parental protection for much longer. This made parenting more demanding among humans than among other species, the work of two people rather than one.
