An embryo grows, and will continue to do so as long as it gets enough nutrients. The whole sequence seems self-contained, barrelling along like an immensely complicated computer program that runs itself. But the squid and other animals tell us that development is more than this. It progresses using instructions in an animal’s genes, but also in the genes of its microbes. It is the result of an ongoing negotiation – a conversation between several species, only one of which is doing the actual developing. It is the unfolding of an entire ecosystem.
