The title of Simone de Beauvoir's landmark feminist tract hints at the second-class status to which women have been consigned by men since time immemorial. That women are the "second sex" and not the first clearly implies their systematic subordination, a condition that has been near-universal throughout human history.
In the biblical story of Adam and Eve, it was Adam who was created first. Among other things, this makes him more godlike than Eve, who is created from Adam's rib. Here women can be seen as a kind of afterthought, created by, and to service the needs of, men.
The cultural values expressed in the story of Adam and Eve exist in virtually all cultures throughout the world. De Beauvoir, drawing on extensive research, cites numerous examples where women are treated as a second sex, where they are othered by men.
Far from being natural, this is an entirely environmental process. In virtually all societies, boys and girls are brought up to assume different gender roles, and in the process, boys are taught to be aggressive, assertive, and transcendent, whereas girls are taught to be passive, demure, and immanent.
Boys and girls are born equal, but due to a process of socialization, they come to be decidedly unequal when they get older. Once this happens, it can be observed that women are treated by society as the second sex, somewhat less than important to men.