"The Ancestor's Tale", also by Dawkins. Might be what you're looking for. It is basically a history of life on Earth, told backwards in time, with in-depth discussion of how and why evolution came up with new branches in the tree of life. I think of it as the applied/illustrative companion volume to the theoretical Selfish Gene. It is longer, but easier reading, and has Dawkins' usual lucid, very well informed, smart and occasionally witty prose.