While traveling across a barren landscape, caveman Yor arrives at a village in time to save the people from a dinosaur attack. During a feast held in his honor, Yor manages to rescue beautiful Ka-Laa and her guardian, Pag from a raid by a hostile neighboring tribe. After being told about another woman who wears the same mysterious medallion he has always had, Yor and his new companions set out to find her and learn about his destiny.
This sounds like a book I read in school where a guy got transported to the "past" became a member of a tribe and eventually adapted to his new life with a family. Then he gets taken back to the "future" and it turns out he was actually taken to the future where society collapsed because of some kind of disease.
I think about it regularly but can't remember the name