Have you heard of the clock of the long now? The clock of the long now will be built in Nevada, on land adjoining Great Basin National Park. When completed, the clock will tick once a year, with a century hand that advances once every hundred years. Every millennium, a cuckoo will come out and sing. The clock of the long now will be built to chart the depths of the future, to force us to think about timescales - to really think. Not just about the next year, or the next decade, or the next century, but about the next millennium, and all the years after that. The clock is being designed to last ten thousand years. This one artefact is intended to last as long as humanity's entire technological history to date. In short, the clock of the long now is a Big Idea. An idea that forces us to think about our place in the universe. It is the sort of idea Stephen Baxter seems to come up with every other week. The Big Idea at the heart of Evolution is a doozy. "Evolution follows the ebb and flow of one stream in the great river of DNA," states the blurb. "It turns the story of Darwinian evolution into a constant drama, a daily life and death struggle. It is a story that transcends species, mankind and, in the end, the Earth itself." In practice this means that although nominally a novel, at times the book reads more like a collection of short stories. It is made up of many segments, each closer to the present than the previous instalment, and each focusing on a primate one step closer to humanity. Indeed, from a purely narrative point of view it would be entirely possible to skip any given segment. This structure is perhaps the only practical way to dramatise evolution, but it gives rise to the book's two significant flaws. Firstly, the quality is less even than you might otherwise expect from a novel, since the quality of each segment is independent of the surrounding segments. Secondly, there is inevitably a degree of repetition, as successive generations grapple with similar challenges. After all, the current era of frenzied progress has lasted only 100,000 years or so, and Evolution covers a great deal more history than that. It is tempting to see Evolution as the logical culmination of Baxter's work to date. His fascination with the destiny of species, and of mankind in particular, runs through the Xeelee sequence. His experimentation with writing about different forms of consciousness has moved from altered humans (Flux) through animals (the Mammoth books) to a variety of hominid species (Origin). The implications of Darwin's great theory for humanity are present in the Manifold series. We are just another species, says Baxter. Look at those who have fallen before us; why should we fare any better? Are we really any different? What sets this novel apart is simply that to date it is far and away Baxter's most successful examination of these themes. This is partly due simply to better writing; where previously he tried with limited success to convey a sense of how other hominids would think, here the focus is external - evolution viewed through the lens of humanity. At times, he falters; but if some of the parts are weak, the whole is hugely rewarding. The narrative glides between tight third-person and omniscient perspectives, at times feeling like the natural history program you wish the BBC could make. 'Walking with Hominids,' if you will. The themes arise from anthropology, evolutionary biology and sociology. This is a story about humanity: How we are, and how we came to be this way. At times, the story is bleak; at others, it is strangely uplifting. The very best segments of the book - such as the bittersweet 'Mother's People', perhaps the crux around which the story turns, perhaps the moment when hominids truly become humans - are when it is both. Evolution opens with the biggest bang in our history: The KT boundary event, the great dinosaur-killing rock. It ends, as it must, with a far distant futurity to rival the dreams of Stapledon or Wells. In between, there are wonders to ruin the imagination and moments of painful, visceral humanity. Evolution is our story. Understand it.