Eusocial insects are striking both for their enormous ecological success and for the unusual reproductive altruism of their workers. An explanation for both may lie in the advantage of providing extended care of the young. In insects, a single adult lifetime is often too short to allow extended care of the young. However, groups of adults can successfully provide overlapping care even if individual lifetimes are short, and the resulting high offspring survivorship may help account for the ecological success of social insects compared with solitary competitors. Models developed here show that the same advantage may have been a major selective force at the origin of eusociality.