This is classic Baxter. It spans millions of years, all the characters die, and by the end not only has everyone we know ceased to exist, the human race itself has de-evolved. No super-race of the future in Baxter's vision, humanity evolves eventually into little monkey like creatures that are symbiotic with trees. The first half of the book sticks pretty closely to what we know from the fossil record (apart from a few flights of speculation like intelligent dinosaurs using tools), and the real invention takes off in the second half, when we pass the present day and zoom off into the future, glimpsing some evolving machines on Mars as we pass through. It's appealing in spite of the sketchy characters because of the ideas fizzing off and the feeling that he takes his science seriously.