Forget Lord of the Flies!
This book with its Neanderthal and Homo Sapiens characters is one of my all time favorites.
I've read it three times.
Why? It grabs me from the first page. This is not science fiction as we know it. It's a work of the highest imagination. The way the Neanderthals move, the way they look, their difficulties, their problem solving, their emotions and their way of, well, there's no other phrase for it, their way of communicating. Add to this Golding's British feel for nature and you're well underway.
I won't reveal the story or the ending, will only say if you liked The Clan of the Cave Bear, this novel takes at least as gripping approach to our origin.