Eusociality evolved repeatedly in different orders of animals, notably termites and the Hymenoptera (the wasps, bees, and ants). This 'true sociality' in animals, in which sterile individuals work to further the reproductive success of others, is found in termites, ambrosia beetles, gall-dwelling aphids, thrips, marine sponge-dwelling shrimp (Synalpheus regalis), naked mole-rats (Heterocephalus glaber), and many genera in the insect order Hymenoptera. The fact that eusociality has evolved so often in the Hymenoptera (between 8 and 11 times), but remains rare throughout the rest of the animal kingdom, has made its evolution a topic of debate among evolutionary biologists. Eusocial organisms at first appear to behave in stark contrast with simple interpretations of Darwinian evolution: passing on one's genes to the next generation, or fitness, is a central idea in evolutionary biology.

Current theories propose that the evolution of eusociality occurred either due to kin selection, proposed by W. D. Hamilton, or by the competing theory of multilevel selection as proposed by E.O. Wilson and colleagues. No single trait or model is sufficient to explain the evolution of eusociality, and most likely the pathway to eusociality involved a combination of pre-conditions, ecological factors, and genetic influences.

Eusociality can be characterized by four main criteria: overlapping generations, cooperative brood care, philopatry, and reproductive altruism. Overlapping generations means that multiple generations live together, and that older offspring may help the parents raise their siblings. Cooperative brood care is when individuals other than the parents assist in raising the offspring through means such as food gathering and protection. Philopatry is when individuals remain living in their birthplace.

The final category, reproductive altruism, is the most divergent from other social orders. Altruism occurs when an individual performs a behavior that benefits a recipient in some way, but at the individual's own expense. Reproductive altruism is one of the most extreme forms of altruism. This is when most members of the group give up their own breeding opportunities in order to participate in the reproductive success of other individuals. The individuals giving up their own reproductive success form a sterile caste of workers within the group. All species that practice reproductive altruism produce one or more queens, the only breeding females, who are larger than the rest. The remainder of the society is composed of a few breeding males, sterile male and female workers, and the young.

Charles Darwin considered the evolution of eusociality a major problem for his theory of natural selection. In The Origin of Species, he described the existence of sterile worker castes in the social insects as "the one special difficulty, which at first appeared to me insuperable and actually fatal to my whole theory". In the next paragraph of his book, Darwin describes a solution. If the trait of sterility can be carried by some individuals without expression, and those individuals that do express sterility help reproductive relatives, the sterile trait can persist and evolve.

Darwin was on the right track, except sterility is not a characteristic shared among all eusocial animals. Sterile workers of many eusocial species are not actually physiologically sterile. Male workers can still produce sperm, and female workers sometimes lay eggs, and in some species, become the new queen if the old one dies (observed in Hymenoptera, termites, and shrimp).

This insight led to inclusive fitness and kin selection becoming important theories during the 20th century to help explain eusociality. Inclusive fitness is described as a combination of one's own reproductive success and the reproductive success of others that share similar genes. Animals may increase their inclusive fitness through kin selection. Kin selection is when individuals help close relatives with their reproduction process, seemingly because relatives will propagate some of the individual's own genes. Kin selection follows Hamilton's Rule, which suggests that if the benefit of a behavior to a recipient, taking into account the genetic relatedness of the recipient to the altruist, outweighs the costs of the behavior to the altruist, then it is in the altruist's genetic advantage to perform the altruistic behavior.

William D. Hamilton proposed that eusociality arose in social Hymenoptera by kin selection because of their interesting genetic sex determination trait of haplodiploidy. Because males are produced by parthenogenesis (they come from unfertilized eggs and thus only have one set of chromosomes), and females are produced from fertilized eggs, sisters from a singly-mated mother share (on average) 75% of their genes, whereas mothers always share only 50% of their genes with their offspring. Thus, sisters will propagate their own genes more by helping their mothers to raise more sisters, than to leave the nest and raise their own daughters.