This talk examines the biological and cultural impact of menstruation in human evolution. It looks into which species menstruate and why this evolved. What effects would highly visible menstruation have on hominin social systems? Why did menstruation become a critical biological signal and how did this affect the emergence of symbolic culture? The widespread distribution of menstrual taboos and observances indicates their great antiquity in human belief systems. Can we offer any predictions about the various forms these take in different societies?