An Evolutionary, Biosocial Perspective on Variation in Human Milk Microbes and Oligosaccharides: An Example of Eco-Homeorhesis?

Abstract

Researchers have long known that milk composition varies among and within mammalian taxa. The origins of this variation among phylogenies are thought to be due to evolutionary shifts driven by Darwinian selection to maximize fitness across often substantially different physical, biological, and social environments. Conversely, milk composition variation within a taxon (e.g., Homo sapiens) is generally ascribed to more proximate factors such as dietary intake, time postpartum, acute and chronic maternal health, and (all-too-often, unfortunately) differences in sample collection, storage, and analysis. Here, we argue that at least a portion of this intraspecies variation in human milk composition-particularly as it relates to microbial and carbohydrate contents-may also be due to genomic variation driven over the millennia by historical environmental and biosocial selective pressures, particularly those related to pathogen exposure. We also outline a possible experimental approach, using combined population genetics and anthropological methodologies, to test the possibility that variation of "normal" milk composition is related to fitness within a specific context (an example of eco-homeorhesis).