I Contain Multitudes: The Microbes Within Us and a Grander View of Life

by Ed Yong

Human breast milk stands out among that of other mammals: it has five times as many types of HMO as cow’s milk, and several hundred times the quantity. Even chimp milk is impoverished compared to ours. No one knows why this difference exists, but Mills offers a couple of good guesses. One involves our brains, which are famously large for a primate of our size, and which grow incredibly quickly in our first year of life. This fast growth partly depends on a nutrient called sialic acid, which also happens to be one of the chemicals that B. infantis releases while it eats HMOs. It is possible that by keeping this bacterium well fed, mothers can raise brainier babies. This might explain why, among monkeys and apes, social species have more milk oligosaccharides than solitary ones, and a greater range of them to boot. Larger groups mean more social ties to remember, more friendships to manage, and more rivals to manipulate. Many scientists believe that these demands drove the evolution of primate intelligence; perhaps they also fuelled the diversity of HMOs. An alternative idea involves diseases. Pathogens can easily bounce from one host to another, so group-living animals need ways of protecting themselves against rampant infections. HMOs provide one such defence. When pathogens infect our guts, they almost always begin by latching onto glycans – sugar molecules – on the surface of our intestinal cells. But HMOs bear a striking resemblance to these intestinal glycans, so patho...