This is hard science fiction (ie, Baxter knows what he's talking about). It throws modern protagonists into an environment where they are confronted with Neanderthals, australopithecines and other human ancestors, including previously unknown species invented by Baxter. What's particularly impressive is Baxter's vision of the psychology of the almost-humans - speculative, but highly convincing. Themes of alternative history and a (silly but enjoyable) theory of human origins are also dealt with. The writing is good, and the characterisation is fair.
This is part of the Manifold sequence of books - I recommend you read 'Space' before you read this (and perhaps 'Time', although it's not so good) but it's not essential. They feature the same characters in alternate timelines. This works well, and is not a way to pad out a novel to saga length!
There aren't many authors writing 'proper' SF at the moment. That said, you don't need to be an expert in science to enjoy this. If you are interested in human evolution, you will find it an extremely enjoyable piece of speculation!