That’s a great question! We didn’t really need sunscreen in prehistoric time because we adapted to the environments that we lived in and we didn’t migrate to new environments as quickly as we could in later times. Those adaptations are getting more tan more easily and growing thicker skin. We can still see this now in people who don’t use sunscreen and their skin looks tougher and more leathery. Also, there were some ancient sunscreens ranging from simple mud to pastes made from ground plants.

Human skin stood up better to the sun before there were sunscreens and parasols – an anthropologist explains why - The Conversation