Invisible to the naked eye, a frenetic marketplace buzzes all around you. The customers are bacteria and they are trading in genes, swapping them between individual cells as easily as humans swap presents or phone numbers. Some of the trades allow bacteria to cope with new sources of food. Others are more like arms dealing, with cells exchanging genes that allow them to beat antibiotics, or weapons that bestow the ability to cause disease.

These swaps are pervasive. At least an eighth of the genome of E.coli, a commonly studied species, has been borrowed from other bacteria. But this leads to an interesting puzzle. Genes don’t work in a vacuum; they interact with one another in a tangled web