The question “If it’s ‘everywhere’, why have I not heard of it before?” is indeed a question, but it is not itself an argument.
The obvious answer to why your reading material doesn’t contain certain information is that you have chosen not to read those books.
Relevantly: Recently, from a book, I learned something new. The most populous vertebrate on Earth is a bristlemouth. I reached the age of 43 without knowing this. But now I know what a bristlemouth is and where I could go see one.
Also relevantly: You have encountered transgender people before in your actual life. I am transgender, so, when I walk through the city, everyone who brushes elbows with me has brushed elbows with a transgender person. They just don’t know that they have. I worked at a 300-person company for a decade, and everyone at that company had a transgender colleague. They just didn’t know they did. I transitioned a decade before I started working there, and no one who interacts with me ever guesses that I am transgender, nor do they identify me as a woman. I am a man. People see and hear a man when they look at me and speak to me. So my 300 former colleagues continue not to know that they worked closely with someone who had already transitioned from one gender to another. That doesn’t mean I didn’t exist, and still less does it mean that my gender transition wasn’t valid or real. It suggests, rather, that my gender transition was quite successful according to the goals I set for myself (living as a man, not talking about my gender very much, not letting my transgender history define the kind of professional work I did, etc.). It suggests that gender transition is possible. It suggests that everyone already recognizes at least some trans people as the gender we are.
A transgender person is indeed a type of person “that you can physically touch and see and verify.” You and I may have brushed elbows at some point on the public transit. We trans people just don’t often wear pins that say “Hello, I’m Trans.” You look into the ocean and see “fish”; you don’t see “bristlemouth” if you haven’t learned to distinguish different kinds of sea creatures, or if the bristlemouths have started wearing pants so you don’t see the bioluminescence of their private parts. We are glowing, and if you can’t see it, then our glow-up isn’t for you.
Similarly, you have encountered references to historical societies that were aware of people of different genders and bodies—whether those societies included those individuals as “normal people” or persecuted them, or facilitated their transitions, or were complicit in doing any particular thing to them—and you’ve also encountered references to the individuals themselves. Whether the historian chose to highlight the person's gender/body was up to the historian. Whether you noticed subtle clues, or absorbed the information the historian provided, or questioned the historian’s narrative and sought more information from other sources to self-direct your continued learning, was up to you.
I would like you to sit with your statement that “We didn’t invent dogs, we just invented a word to describe them.” Humans in fact did domesticate wolves to invent dogs. This started in prehistory, a few thousand years before settlements and agriculture, and continues to this day, which is why we have teacup Yorkies. If you haven’t heard of this, this is also related to the type of history you choose to read and absorb. It doesn’t mean the history isn’t real.
Whether males are generally “better at aiming/throwing projectiles at moving targets” is not highly relevant to mammoth hunting. A spear is minimally helpful against a mammoth, and throwing the spear just a little bit harder than your girlfriend can throw it isn’t what’s going to kill the mammoth. Mammoths (depending on the species) weighed 2–5 times more than a car. A spear might not hit a lethal spot or might entirely bounce off their hide. The way early humans hunted mammoths was to outsmart them: frighten them with fire so they stampeded into a hidden pit. Of course it would be helpful to recognize who in your family group has been successful at hunting mammoths in the past, as well as whether they might be too injured, sick, pregnant, childrearing, etc. to go on a mammoth stampede right now. But knowing which of your group members is “a male” or “a female” seems much less relevant. Your uncle might have a broken leg, and there’s no reason to disqualify your aunt whose legs are fine. Anyway, as I was saying, mammoth hunting involves more strategy than strength. No matter how high someone’s testosterone is, he can’t win a strength contest with a mammoth, and he shouldn’t try. A person born with a vagina is explaining this to you. Many men do not understand this about the relevance of their testosterone levels, but I am a man who does. I have the testosterone levels of an average man because I inject testosterone. My testosterone vial wouldn’t help me catch a mammoth. Long-term observation of mammoth behavior is what would help me outsmart a mammoth.
Lifelong observation of cis (non-trans) people helps me understand a lot about how they think too.
