Simone de Beauvoir's gender theory is considered to be one of the pioneers of feminist thought. Her book "The Second Sex" is seen as a milestone in explaining how and why women were and are subjected to men's rule. While some of Simone de Beauvoir's insights might be seen today as self evident (such as "a woman is not born a woman but becomes one"), other remain revolutionary till this day.
In the second chapter of "The Second Sex", titled "History", Simone de Beauvoir tries to explain how gender inequality established itself in the course of history and how is it that we came to take in for granted. de Beauvoir offers an existential approach to the examination of how is it that the woman was marked as society's "other" and subsequently "lesser".
Simone de Beauvoir argues that whenever there are two different human categories at the same time and place, there will always be one striving to subject the other to its rule. The burden of childbirth in ancient societies made women dependant on men's labor, and thus enabled the initial inequality. In this situation women were banished from activities such as hunting an fighting that were seen as man's purpose in his strive to elevate himself from nature (which was the woman's domain and there she stayed).
With the shift to agrarian societies and the introduction of the need to plan ahead, women gained importance by providing the continuation of the family/community/species, and her powers were recognized and feared (here the "witch" was born). The response was the objectification of women and they're treatment as property at the disposal of men. They were reduced to a mere function, and not an integral part of human existence. Women, Simone de Beauvoir argues, were the eternal and absolute "other" of men.