May 20, 2019

The social worlds of infants, moms, and microbes

Courtney Meehan, Washington State University

Throughout our evolutionary history, and in much of the world today, human infancy has been characterized by a host of ancestral traits which include frequent maternal-infant contact, on-demand breastfeeding, co-sleeping, and cooperative breeding. These ancestral characteristics have, in part, supported the development of our altricial infants and enabled reproductive success in diverse environments, despite women’s narrow reproductive window and the costs associated with simultaneously rearing multiple dependent children. Here, I argue that these ancestral traits also serve as a critical pathway by which mothers and infants communicate, via microbiota, about their environment, priming the infant for the world in which he or she will be reared. Utilizing cross-cultural data on infant’s early environments and human milk composition, I characterize the diverse caregiving worlds of infants and explore how our early social environments and mothers’ life history characteristics are associated with the human milk microbiome (HMM). Human milk, once thought to be sterile, contains a diverse microbial community and as an early and consistent source of bacteria to infants, it is an important factor in the colonization of the infant gastrointestinal microbiome. Yet, the origins and role of the HMM are not yet fully understood. Our results identify multiple associations between maternal life history characteristics, our ancestral caregiving traits, and the HMM, providing initial evidence suggesting bi-directional maternal-infant communication during breastfeeding and that human milk composition may be socially-mediated.