Is synonymy of "man" and "human" on decline? [closed]

Is the synonymy of man and human on the decline? By extension, I am interested in mankind and humankind.

I have no idea what online tools to use to investigate this.

If the synonymy of man and human were stagnant over time, one might expect their usage trends to be parallel. The fact that they're not and are, instead, undergoing separation may suggest that the synonymy is in decline. (Google Ngrams)

With mankind and humankind, however, the opposite seems to be true. (Google Ngrams)

Google's Ngrams Viewer is the tool I used for this. If you think my logic is sound, you can get there by following the links above.

When I was studying biology at school in the 1960s, I was once corrected (by a female teacher!) for referring to humans instead of man in an essay. In the decades since, it has become less and less acceptable to use man as a term for the human race. For instance, we say synthetic fibres rather than man-made fibres.

So the answer to your question is that the synonymy has declined to the point of being virtually non-existent.