We have evolved ways of selecting which species live with us, restricting where they sit in our bodies, and controlling their behaviour so they are more likely to be mutualistic than pathogenic. Like all the best relationships, these ones take work. Every major transition in the history of life – from single-celled to multi-celled, from individuals to symbiotic collectives – has had to solve the same problem: how can the selfish interests of individuals be overcome to form cooperative groups?
