What is the topic and topic sentence of the text?

"All known cultures give meanings to sex categories; these meanings serve to create and maintain social distinctions between women and men. Looking around the world today, we find that such distinctions and hierarchies are usually built into institutions such as the family, law, and religion. For example, in all known societies, men have more formal political power than women, and men generally earn more money than women. However, the characteristics that are claimed to distinguish the sexes are not uniform from one culture to another. Nor are the distinctions necessarily stable across historical epochs within a society. Furthermore, societies vary in the extremity of the distinctions they draw and the rigidity with which these distinctions are enforced."

HI teachers. The above is from the book I study. I,m pretty much confused with the top of this text and the topic sentence. What is the point of this essay? What is the topic sentence?

Thanks in advance.