Nudity is a state of being in which a human is not wearing clothing or specifically is not covering the genitals. Worldwide, social norms vary from banning nudity except in complete privacy to the acceptance of public nudity as a natural human state for some activities. The loss of body fur was one of the physical characteristics that marked the biological evolution of modern humans from their hominini ancestors. Adaptations related to hairlessness contributed to the increase in brain size, bipedalism, and the variation in human skin color. Although often used interchangeably, "naked" and "nude" are also used to distinguish between the various meanings of being unclothed. While estimates vary, for at least 90,000 years anatomically modern humans wore no clothing, the invention of which was part of the transition from being not only anatomically but behaviorally modern. As societies developed from being hunter-gatherers to being agrarian, clothing and other body adornments became part of cultural evolution as individuals and groups became differentiated by status and class. Through much of history until the modern era, people were unclothed in public by necessity or convenience when exercising for work or sport; or when bathing or swimming; often but not always in groups segregated by sex. Nudity is culturally complex due to meanings given various states of undress in differing social situations. In any particular society, these meanings are defined in relation to being properly dressed, not in relation to the specific body parts being exposed. Nakedness and clothing are connected to many cultural categories such as identity, privacy, social status and moral behavior.Generally, social norms regarding nudity are different for men than they are for women. It was not until the 17th century in Europe that the female breast became a part of the body that must be covered in public. It is only in the contemporary era that the nudity of children represents anything but innocence. Individuals may intentionally violate norms regarding nudity; those without power may use nudity as a form of protest, and those with power may impose nakedness on others as a form of punishment.