I never read this book but a few years ago I encountered it (I think in a vsauce video or another similar channel) and remember reading a wikipedia page about it. It was a fictional novel but written as if telling the history/scientific fact of the different species of the genus homo after homo sapiens dies out? I forget how many different kinds there were but it explored the history, anatomy, and competition between the species as well as how they adapted to global changes (ice ages, disease, warfare, etc.) I think the title was something like "# species of man" or "the # kinds of future men" but Googling different combinations of those words hasn't gotten me very far. I believe it was written in the early 20th century but I could be mistaken. Please let me know if this rings a bell!