Homosexuality has been documented in Western societies as far back as Ancient Greece. Virtually every civilization since has had some record of homosexuality. According to one hypothesis, same-sex sexuality evolved through natural selection as it strengthened alliances and allowed low-status males to increase reproductive success. In ancient Rome, freeborn males could engage in homosexual sex freely as long as it met certain parameters, while many early scientists viewed it as a Darwinian paradox because it was non-reproductive. The modern LGBTQ pride movement and symbols like the rainbow flag originated in the 1970s to commemorate the Stonewall riots and allow open expression of sexuality.