By getting on with their usual activities, in a purely selfish way, they incidentally benefit their hosts. These “by-product mutualisms” are the perfect first handshake.6 Both partners get something out of the relationship, without either having to invest in it. The host can then evolve traits that solidify the partnership, from cells that house the tiny partners to molecular anchor-points for them to latch themselves to. And the most important of these traits – the one that does more than any other to seal a symbiosis – is inheritance.
