The huge complexities of life and sex

<i>Mendel's Demon: gene justice and the complexity of life </i>by Mark Ridley (Weidenfeld &amp; Nicolson, &pound;20)

This really is a long-awaited book. Mark Ridley, who works in the department of zoology at Oxford University (and is not to be confused with the science writer Matt Ridley), has long been known to be working on a popular book about life, sex and evolution. Not another one, I hear you cry. But this is a work with a difference, well ahead of the pack of "me too" evolution books around today.

This really is a long-awaited book. Mark Ridley, who works in the department of zoology at Oxford University (and is not to be confused with the science writer Matt Ridley), has long been known to be working on a popular book about life, sex and evolution. Not another one, I hear you cry. But this is a work with a difference, well ahead of the pack of "me too" evolution books around today.

The book's success is partly because the story is so intrinsically interesting; partly because Ridley knows it inside out; and partly because he tells this story so well. The unusual slant, for a popularisation, is that he tells the story in terms of complexity. As Ridley puts it, the existence of life no longer seems to be a puzzle for biology; but the existence of complex life-forms like ourselves certainly is.

There are two ways of putting this statement in perspective. One is simply to look at the enormous numbers of bacteria and to compare them with the numbers (or rather, the total mass) of complex life-forms. In a telling example, Ridley mentions that the average human large intestine has a lining of bacteria about two centimetres thick. Single-celled life forms such as bacteria together contain about the same amount of carbon tied up in living form as all of the plants on the earth today.

The other way of looking at the importance of so-called simple life-forms is to consider how long they have been around. The Earth itself formed about 4,500 million years ago, and for the first few hundred million years of its existence it was simply too hot for life to exist. Yet the fossil evidence suggests that simple life-forms already inhabited the earth by 4 billion years ago. It took until between 500 million years and one billion years ago for complex life-forms to emerge.

In other words, for 10 per cent of Earth history, it was uninhabitable; for 75 per cent of the Earth's history, nothing more complex than a single cell was around; and complex life-forms have been present for only 15 per cent of the time. Ridley argues a plausible case that this implies that it is somehow "easy" for simple life to arise, but difficult for complexity to arise.

Why, then, should complexity have arisen at all? The argument that most appeals to me originates with Charles Darwin himself. Any evolutionary change must make a simple form more complex. Once you have more complex forms, some changes will lead back to simplicity, but other changes will produce greater complexity.

Although Ridley goes on at perhaps unnecessary length in giving a detailed explanation of how evolution works, it's worth wading through this part to get to the meat of the book. Here he discusses the role of sex in keeping complex life-forms going - and evolving to greater complexity.

He borrows a neat analogy from John Maynard Smith involving two broken-down cars, one of them with dodgy brakes and the other with faulty ignition. By combining the best bits of each car, you end up with one runner and one wreck. Sex does the same thing. If each parent has a mixture of good and bad genes, some of their offspring will inherit all the bad genes and die young, while others will inherit only the good genes and prosper. Ridley's own variation on this theme borrows from the New Testament: sex is equivalent to taking as many sins as possible, combining them into one scapegoat and crucifying the creature.

But sex, of course, is not the same as gender. Another mystery of evolution is why there should be both males and females instead of every individual being able to mate with every other individual. Here, too, Ridley is both informative and readable.

As a tailpiece, he considers whether, even with the aid of sex, life-forms like ourselves are as complex as is possible, or whether evolution could produce even greater complexity. It may be that nature will need a little help from human ingenuity. But I don't want to give away all his secrets; read the book yourself; you certainly won't regret it.

The reviewer's latest book is 'Stardust' (Allen Lane)