According to pioneering biologist Lynn Margulis, cooperation is not a sideshow, but a primary driver of evolution:

Symbiogenesis is a theory of evolution. It argues that symbiosis is a primary force of evolution, because acquisition and accumulation of random mutations or genetic drift are not sufficient to explain how new inherited variations occur. According to this theory, new cell organelles, new bodies, new organs and new species arise from symbiosis, in which independent organisms merge to form composites. This challenges some standard textbook ideas of how evolutionary change occurs. To some degree, Darwin emphasized competition as the primary driving process of evolution, symbiogenesis emphasizes that co-operation can also be important to the process of evolution.