A good and interesting book. Maybe not SF, certainly not easy to pigeon-hole into any SF subgenre...
Most of the book is about the past - not an alternative history past, but our Earth's past (or at least educated guesses about it). Educated guesses are needed because we're mostly talking about a past before writing or even humans to know what happened. No, don't stop reading. It's well-written stuff by a good SF writer. It's sort of like a historical novel, only following the course of evolution from early primates to later primates to hominids to human - eventually beyond. It provides fascinating information and/or speculations on a variety of pivotal points in the branches of evolution most relevant to humans.
If you can do without ray guns, nanotech, hyperspace and interstellar lizards for a while (well, almost 600 pages of a while); it's a good read. No, if asked to give a list of themes that would make me choose a book, "glimpses into the lives of pre-intelligent animals" or "the road from mammal to human" would not be there. But if you're willing to try a book that doesn't fit your preconceived ideas, you'll probably enjoy this one.
The book begins just before the great impact 65 million years ago that lead to mass extinctions. We get to know the life of an early primate of the time, as well as various dinosaurs that shape its environment.
Step by step, we get a view of the lives, fellow animals, environment and continents of significant species over the ages. Not told like natural history text books, but as stories of individuals making their way through the world. It is written like stories of people in those senses that make it literature, while also describing them so we know how they are different than humans.
The book makes its way through hominids, early humans and early civilization. It makes a brief visit to more-or-less the world we are familiar with. At the end, it gives us some speculations on where evolution might continue after a new mass extinction at least partly caused by humans.
I can't say how much of Baxter's ideas are fanciful speculation or his educated guesses or leading theories in paleontology, primatology and anthropology. In any case, they are often thought-provoking and may lure you into reading the scientific literature on the subjects.