The Valley of Horses by Jean Auel

This book alternates between continuing Ayla's story after she leaves the Clan and has to fend for herself and telling the story of Jondalar, who leaves his family on a Journey with his brother. I enjoyed moving back and forth between these two characters, despite the fact that they don't meet till the last third of the book. There was a lot of thought about domestication in this book (Ayla, through chance and her good-hearted nature, ends up with both a cave lion and a horse), as well as communication and language. I think I liked the first one better, but I'm definitely going to want to see what happens to Ayla and Jondalar next.