The Plausibility of Life, by Marc Kirschner & John Gerhart

The basis of evolution is that there is variation among the individual members of a species in each generation, and selection of the individuals best fitted to their environment. So those individuals do better and, crucially, leave behind more offspring to pass on desirable characteristics to future generations.

Selection is the easy part to understand. But where does the variation come from? This was a great puzzle to Darwin, and remains inadequately understood today. Small variations can comfortably explain how a species becomes more and more exquisitely adapted to its environment: ancestors of giraffes acquired longer necks because in each generation those with longer necks found more food in the treetops, so they thrived and produced more descendants.

But it is not immediately obvious how this gradual process can give rise to new species. Is it just a question of a few individuals getting cut off and evolving in a different direction? Or are there sudden jumps - dramatic mutations - from one generation to the next? If so, what causes them? This is the question that gives Marc Kirschner and John Gerhart their subtitle: "resolving Darwin's dilemma".

They bring an impressive weight of authority to the investigation and offer an intriguing and compelling possible solution. After a clear introduction to the basics of evolution, Kirschner and Gerhart soon get to the nitty-gritty of how things operate at the level of cells and molecules.

Discussion of living systems that can flip between two different states, like the alligators whose sex depends on the temperature at which eggs develop, takes us into the realm of feedback. Recent discoveries in molecular biology explain how a single cell develops into a complex organism such as a giraffe or, indeed, a human being.

If this seems daunting, don't despair. The Plausibility of Life is a serious book, in the same sense that, say, histories by Anthony Beevor are serious, but no harder to read. It deals with topics such as chaos and complexity that are also relevant to an understanding of human history, as well as of human origins.

The bottom line of the argument is that life is organised in such a way that potentially useful variations are thrown up in every generation, and that this facility for variation has itself evolved. That helps, perhaps, to explain why life on Earth existed only in single-celled forms for billions of years before exploding into the variety around us today.

This idea, which the authors dub "facilitated evolution", carries a bonus. Instead of making it hard to understand how complex structures like the eye or wing could have evolved, facilitated evolution works precisely because the organisms on which evolution operates have great potential for variation. As the authors point out, this pulls the rug from under the argument for "intelligent design". The secret lies "in understanding the organism on its own terms", which are "nothing like a brass watch or a divine creation". Tactfully, they do not rule out the possibility of the divine, merely suggesting that those with religious views should "draw the line between faith and science" elsewhere.

The Plausibility of Life can be highly recommended not only as an insightful look at an important area of scientific work but as powerful ammunition on the side of good - at a time when some states in the US are again trying to make creationism part of the school science curriculum. Most of all, though, it can be recommended as a good read.

John Gribbin's 'Science: a History' is published by Penguin