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Human Reproduction in an Evolutionary Context

Many years ago, I was browsing through Sydney Asdell's bibliographic masterpiece, Patterns ofMammalian Reproduction, idly wondering whether there was any rhyme or reason behind the bewildering array of reproductive patterns adopted by different mammalian species. For many wild animals, a pattern was not difficult to discern, especially for those living in temperate latitudes; seasonal changes in climate dictated seasonal changes in food availability, and the species in question had to harmonize its reproductive performance within these environmental constraints. In particular, it was important to give birth in due season, so that the ever-increasing metabolic demands of lactation to provide for the needs of a rapidly growth neonate could be met by an increased food supply for the mother. Since female mammals, with the added metabolic demands of pregnancy and lactation, have a far greater energy investment in reproduction than males, female reproduction must always be the limiting resource, and hence all reproductive strategies must ultimately be under her control. Many female mammals in temperate regions use changing daylight length to regulate the timing of the mating season, and many also make use of lactation to suppress further reproduction until the young are weaned. This much was clear. But when I turned the pages of Asdell's book to the section dealing with Horn sapiens, all was confusion! The massive number of entries dealt almost exclusively with the minutiae of human reproduction, usually based on data from Caucasian populations in Europe and North America. I was left wondering about natural patterns of human reproduction in our early ancestors, before modern Homo sapzens walked out of Africa to colonize the globe around 100,000 years ago. How had our reproduction evolved? That simple question germinated in my mind, and was to chart the course of my enquiries for the next two decades2 Let me begin by summarizing some of the factors that must have shaped the reproductive pattern of our hunter-gatherer ancestors, and then go on to speculate about the many ways in which modern civilizations have interfered with our reproduction, whether knowingly or unknowingly, and the consequences of having done so.