Perhaps the three greatest outstanding challenges to science at the turn of the Millennium are the origin of the universe, the origin of life and the origin of consciousness. The first and last of these subjects have had much attention, and although they are still deeply puzzling, the basis of a scientific theory is there.

The problem of the origin of life, by contrast, is highly problematic. Darwin's theory of evolution gives a good account of how life has developed over the aeons from simple single-celled organisms to the exuberant biosphere of today.

But Darwinism only provides a mechanism for one species to transform into another. It does not explain how life started.

As even the simplest organism is immensely complex, it is hard to understand how such a thing could have arisen spontaneously from lifeless chemicals. This problem has led some distinguished scientists either to declare that life is an outlandish accident, or that it must simply be accepted as a special state of matter of impenetrable origin. Francis Crick, who discovered the structure of DNA, wrote: "The origin of life appears to be almost a miracle, so many are the conditions which would have had to be satisfied to get it going." The celebrated quantum physicist Niels Bohr argued that: "The existence of life must be considered as an elementary fact that cannot be explained, but must be taken as a starting point in biology."

ÖйúAƬ

Part of the problem of under- standing biogenesis, as it is technically known, is that it all happened a long time ago. The oldest fossil microbes have been dated at 3.6 billion years, although there are traces of life at work in rocks as old as 3.85 billion years. Nobody knows where life began, although my best guess is that it was deep beneath the sea bed near a volcanic vent.

It is not even certain that life began on Earth. It could have started on Mars, for example, and come here in a meteorite. Since all traces of terrestrial biogenesis are certain to have been obliterated, the best course is to recreate the likely steps in the laboratory, and to elucidate the basic principles that govern the evolution of organised complexity in chemical mixtures.

ÖйúAƬ

Although it is possible to fabricate some of the basic building blocks of important bio-molecules in the lab, it is unclear whether the same processes would occur in nature. Scientists are as far as ever from demonstrating how a primitive living organism might arise from scratch.

On the theoretical side, there is disagreement at even the most basic level. Biogenesis is the point where physics, chemistry and biology meet, and the very different modes of thought in these disciplines is brought into stark conflict. Biologists recognise the key role that contingency plays in evolution. Nature has no foresight. Evolution proceeds by the accumulation of small adaptations generated by random mutations, subjected to the sieve of selection by the environment. If the biosphere were to be destroyed by a cataclysm, leaving only a few microbes to evolve anew, the pathway of evolution on the replay would probably bear scant resemblance to our own evolutionary history. This is because so many of the features of familiar organisms are due to chance.

By contrast, physicists tend to focus on universal laws of nature that enable accurate predictions to be made. For example, if a ball is thrown in the air at a certain speed and angle, its point of fall can be computed in a dependable way. Steeped in this tradition, physicists, and to a certain extent chemists, tend to assume that there is a predictable chemical trajectory leading towards life.

The assumption that biogenesis is a road down which a chemical mixture is inexorably conveyed by the passage of time, with "life" as its destination, is called biological determinism. The hope of biological determinists is that there are deep laws of nature, perhaps involving the organisation of complexity, that will explain how life can spring into existence from non-life.

ÖйúAƬ

The problem is, we have no idea how many of the crucial steps in biogenesis are due to chance, and how many to the operation of universal laws. If life were a product of chance alone, it would be an accident of stupendous improbability. The likelihood of creating a short protein by the random shuffling of amino acids - the molecular building blocks of all proteins - is 1 in 10130! This seems so absurd that many scientists flatly reject chance as an explanation.

Unfortunately, because we have a sample of only one biosphere, it is impossible to draw any statistical conclusions. Biogenesis could have been the result of hitting an astronomically unlikely jackpot in a gigantic molecular lottery, but because we human beings are the direct product of it, we cannot regard ourselves as independent observers. Thus the search for life elsewhere in the universe is crucial. If we were to find even a single microbe on Mars or Europa, and if we could rule out cross-contamination from Earth, then life would be written into the laws of the cosmos - two cosmic near-miracles in one solar system would be too much of a coincidence. But until such time as we can be sure that we are not alone, it remains an open question whether we are merely insignificant chemical quirks or the expected product of an ingeniously bio-friendly universe.

Paul Davies

The Fifth Miracle: The Search for the Origin of Life is published next month by Penguin, Pounds 18.99.