Organizational citizenship behavior (OCB) and proactivity have garnered a great deal of research attention recently. Some scholars have subsumed proactivity under the concept of OCB because both concepts are based on discretionary behaviors. In this chapter, we want to summarize reasons to differentiate OCB from proactivity. We do this by showing that their functions are different even when they may be correlated, that there are different evolutionary paths for the development of OCB and proactivity, and that there are different biological components. At the same time, we argue that both OCB and proactivity have important evolutionary functions for humans, allowing humans to become a dominate animal in the world. We hope this chapter can spur more research on the intersection between the two constructs.