The placental steroid hypothesis of human brain evolution

The evolution of the human brain has long been framed in terms of sexual selection, with an emphasis on consistent but small on-average volumetric differences between males and females. In this review, we present new molecular, genetic and clinical findings regarding neurodevelopment, cortical expansion and the production of sex steroid hormones, such as testosterone and estradiol, by the placenta during pregnancy. We discuss converging evidence that on-average sex differences are relevant for human evolution but are characterised by significant overlap between the sexes and more adaptations in female, rather than male, physiology. We also consider recent accounts and modelling of evolutionary pressures in large social groups, regarding competition and fertility. Finally, we bring these findings together and present a novel hypothesis for understanding human brain development and evolution, which emphasises the role of sex steroid hormones, their prenatal production by the placenta and their roles in regulating physiology, fertility and cognition.