Our microbiome is like a hidden organ, helping us to break down foodstuffs that our own cells cannot cope with. And in turn, our food affects our microbiome. Our first set is laden with genes for digesting milk proteins, allowing us to make full use of our only source of nourishment as babies. Breast milk might even have evolved to nourish the most beneficial bacteria with special sugars.

Just before we move onto solid foods, our microbiome starts activating genes that break down the complex sugars and starches in plants, preparing us for the menu to come. As our diet diversifies, so do our bacteria. They activate genes that use carbohydrates effectively, produce vitamins, and break down unusual and diverse chemicals. As adults, our microbiome becomes relatively stable, but its membership roster depends on the food we eat. The guts of African villagers who eat high-fibre diets are dominated by plant-digesting specialists, which are much rarer in the guts of Europeans who eat high-fat diets.