The birth of cooperation

Mutually beneficial associations between individuals of different species, called mutualistic symbioses, have enabled major ecological innovations and underlie some of the major transitions in evolution (1). For example, the ancestor of plants domesticated endosymbiotic photosynthetic bacteria, today's chloroplasts, for carbon fixation. This association dramatically increased the habitat of these photosynthetic bacteria from the sea to terrestrial ecosystems. However, the colonization of land by plants required an additional symbiotic association, with fungal root symbionts that facilitate nutrient uptake (2). Yet, surprisingly little is known about how mutualistic symbioses evolved and persist. On page 94 of this issue, Hom and Murray show how mutualism may arise without prior coevolution (see the photo) (3)