Thursday Book: In favour of Deep Earthers

PAUL DAVIES has been writing excellent books about science for so long that it is hard to believe that he is still getting better. But on this evidence, he is. I approached The Fifth Miracle with a little trepidation. It is subtitled "the search for the origin of life", and there is something of a tradition among eminent astronomers and physicists that, once they reach a certain age, their thoughts turn to semi-philosophical mysteries in the mistaken hope that such a soft subject must be easier than the physics which is now too taxing for their brains.

The results can be embarrassing. But Davies succeeds not only in being provocative and controversial, but in maintaining the rigorous scientific approach of the physicist. He also uses his writing skills to bring a fascinating but largely unsung idea into the limelight.

The theme of The Fifth Miracle is not so much how life came into being, or even what life is (although Davies offers as neat a summary of its meaning as you could hope to see), but rather where life can exist - in particular, where life can originate.

The key to life, at this level, is a flow of energy from a hotter system to a cooler one. Life on Earth today exists because the Sun is hot and the surface of the Earth is cool. The shift of energy from the Sun to the Earth implies that the thermodynamic quantity known as entropy, which is inversely related to the existence of complexity and information, can decrease on the surface of the Earth even though it always increases in the universe at large. Interesting things can happen on the surface of the Earth because elsewhere things are getting more and more boring as the stars die. Eventually, the Sun will be a burnt-out cinder at the same temperature as its surroundings.

Using this kind of physical insight, Davies pounces on the recent discovery of bacteria that live in the depths of the ocean. They feed off the entropy flow associated with underwater volcanic vents, and off other superbugs that seem to exist deep within the "solid" Earth (actually, honeycombed with microscopic pores). Their existence is revealed by, among other things, the deep-drilling programmes of the oil explorers.

But perhaps "pounces" is the wrong word. Rather, after introducing the idea of the relationship between life and thermodynamics, Davies sidles round the back of his main theme, introducing us to the genetic code and to traditional ideas about the origin of life. He prepares the ground so that his dramatic suggestion that life began "hot and deep" - far below the surface of the young Earth, where it was protected from the battering that took place at the planet's surface as debris from the formation of the solar system rained down - seems both natural and compelling.

The key step in this preparation involves a thorough discussion of the difficulties of the conventional view: that life on Earth originated in what Charles Darwin called a "warm little pond" at the surface of the planet. J B S Haldane elaborated this idea to encompass the entire primordial ocean, in a condition he memorably described as having "the consistency of hot dilute soup". Davies clearly spells out the problems with the primordial soup hypothesis, before offering his alternative.

The argument is beautifully constructed, and quite persuasive. It happens that I am not persuaded, even though I share Davies's doubts about the primordial soup. This is because I find the evidence for a cosmic origin of life, in the clouds of material between the stars, even more compelling (largely because of the immense amount of time offered by this scenario for life to emerge).

The cosmic connection has been promoted by another eminent astronomer, Sir Fred Hoyle, who has unfortunately tended to antagonise his critics by seeming to stray across the fine line separating respectable speculation from cranky theorising. To his credit, Davies also discusses these ideas, explaining why he is dissatisfied with them on scientific grounds. But science usually makes rapid progress when two (or more) rival hypotheses purport to explain the same phenomenon, and we now have the delicious prospect of a rivalry between the Deep Earthers and the Cosmic Connectors which will stimulate the debate about how and where life first appeared.

I don't by any means agree with all conclusions Davies reaches in this delightful book. But I do emphatically agree with the way he sets out his stall. Right or wrong, it is a classic example of how to present a scientific case, and an insight into the way good scientists work. A theory doesn't have to be right for it to be good science, and, like the book, this is one of the best.

The reviewer's latest book is `Almost Everyone's Guide to Science' (Weidenfeld & Nicolson)

John Gribbin