Why have only humans and social insects evolved a complex division of labor

Social species, those that have a complex division of labor, comprise about two thirds of the earth’s biomass. These social species – humans and social insects – are located at extreme points of the set of possible evolutionary paths. The queens of small social insects produce thousands of small larvae, whereas human females invest heavily in their children, who are born already with a very large brain. In spite of these and many other evident differences, social insects and humans have conquered the earth because they share two characteristics: a highly developed system of social cooperation, and a complex division of labor. These observations prompt two questions: If there are evident evolutionary advantages of cooperation and specialization, why have only few species been able to increase their fitness in this way? Why have these characteristics emerged as such extremely different forms of life? In order to answer these two questions, we will focus on possible “transition societies” in the evolutionary paths towards social species. We will argue that, in both the human and social insect cases, sexual selection had a crucial role in the development of the division of labor and entailed that the division of labor required either minimum or maximum unitary investments in the offspring. The species located in between these two extremes could not exploit the advantages of specialization.