Oxidative stress and the evolutionary origins of preeclampsia

•Oxidative stress is a major component of preeclampsia.•Epitheliochorial placentation offers protection from placental oxidative stress.•Epitheliochorial placentation has evolved in species likely to be exposed to placental oxidative stress.•Risk of preeclampsia in humans results from a failure of apes to evolve an epitheliochorial placenta.

In this speculative paper, I consider the relationship between oxidative stress and the evolution of placentation in eutherian mammals. I argue that epitheliochorial placentation, in which fetal tissues remain separated from maternal blood throughout gestation, has evolved as a protective mechanism against oxidative stress arising from pregnancy, particularly in species with unusually long gestation periods and unusually large placentas. Human beings comprise an unusual species that has the life history characteristics of an epitheliochorial species, but exhibits hemochorial placentation, in which fetal tissues come into direct contact with maternal blood. I argue that the risk of preeclampsia has arisen as a consequence of the failure of human beings to evolve epitheliochorial placentation.