At one time, the ancient bacteria had combined with other microorganisms. They took up residence inside [our bodies]... The merged organisms went on to evolve into more complex oxygen-breathing forms of life. Here, then, was an evolutionary mechanism more sudden than mutation: a symbiotic alliance that becomes permanent. By creating organisms that are not simply the sum of their symbiotic parts – but something more like the sum of all the possible combinations of their parts – such alliances push developing beings into uncharted realms. Symbiosis, the merging of organisms into new collectives, proves to be a major power of change on earth.
– Microcosmos: Four billion years of microbial evolution by Lynn Margulis and Dorion Sagan