Pretty much all the life you can see without the help of a microscope–toadstools, poplars, shortstops–is multicellular. Life began as single-celled microbes over 3.5 billion years ago. But at least 25 times over the course of the history of this planet, microbes have come together to form multicellular collectives–otherwise known as bodies.

These transitions are particularly intriguing to evolutionary biologists, because the nature of evolution itself changed along the way. If you’re a microbe, natural selection favors mutations that affect your nature as a single cell. If you stumble across a way to feed on a new nutrient, your descendants may grow faster than the competition and come to dominate your population. Single-celled microbes can evolve to be altruistic–even to