Historians piece together what they can, and what they have found is that transgender people have existed for thousands of years, however they can. But if the word "transgender" and treatments for trans people won't exist for 1000 years, and there is no paper, how do you get a suicide note to confirm the reason someone committed suicide? You can't. 

There is an assumption that because we can recognize medical conditions / differences better, they are somehow more prevalent. They aren't though -- they are just better recognized. Many transgender people know they are different but needed the Internet to learn what they could do about it, as there is a huge stigma against them. 

And when I say brain bathed in testosterone, I am talking male levels -- voice dropping, hairy face, etc. If you're a woman and that was happening to you, it sounds like you would be more than a little uncomfortable. Otherwise, why would you transition? Or if you're comfortable looking like a man, why change?