At one level, of course, this is literally the greatest story ever told and one that most of us only know in the form of dim, heavily edited highlights - volcanoes to coal forests to dinosaurs to humans. Fortey fills in the gaps as he takes us on a fascinating and revealing gallop through the whole story. In the beginning were a few replicating molecules sitting in something close to a medieval vision of hell - "a torrid cauldron of acid, emitting sulphurous fumes". He sings a paean for the thousands of millions of years when algae and single-celled creatures ruled the world, an Eden of co-operation before sex and aggression fueled competition. A Carboniferous forest from 330 million years ago is vividly imagined - dragonflies the size of seagulls flit through the trees and millipedes as big as cobras march through the undergrowth. We meet all sorts of other wonders such as the six-foot water scorpion, Slimonia, which lurked in estuaries when the land was first colonised; encounter the tricky mechanics of growing giant trees and discover the answer to the chicken-and-egg problem - the egg came first.
But how do we know all this is true? If we were to think about it at all we might imagine that piecing together the jigsaw of evolution was something like the painstaking business of sequencing human DNA. At the end you have the true picture - this is how it happened. But Fortey believes that what he is doing is perhaps closer to literature.
Why did he call the book a biography? "It was deliberate", he said. "Any biography is an interpretation. What we think of as the truth about someone changes over time. For instance, Richard III was traditionally cast as a villain after Shakespeare's hatchet job but later historians have done a convincing job of rehabilitating him by drawing on different evidence. The complexity of life's story is such that it allows for an infinite variety of tellings."
Fortey is no revisionist historian but he does believe that blind chance was much more of a factor in evolution than the traditional picture of species surviving because they were more perfectly adapted. "Very often evolution just had to work with whatever was left over from a catastrophe," he says. In fact, what makes the book particularly intriguing is that it gives a hint of the bitter battles for survival that went on between the palaeontologists themselves for the survival of their theories as new fossils came to light.
The role of comets is a good example of the way the biography of evolution has to be regularly re-written. For generations of palaeontologists, the merest suggestion that comets had played any part in the story was heresy. Evolution, the text books declared, was a slow and gradual process, proceeding according to principles still at work today. Two comet heretics - the astronomer Fred Hoyle and the polymath Immanuel Velikovsky - both built elaborate theories in which comets either seeded life or sparked catastrophes. In the 1960s and 1970s they were, scientifically speaking, beyond the pale.
Then at the end of the Seventies came the dramatic news that iridium had been found in the thin layer of clay known as the KT boundary, found at sites all round the world, that marks the end of the Cretaceous period 65 million years ago. Below it the fossil record of the dinosaurs is plentiful, above it there are no dinosaur bones anywhere. Extinction, but how? The importance of iridium is that it is rare in terrestrial rocks but common in meteorites. Suddenly comets were back in the scientific cannon, at least as a possibility.
"There's still a debate going on but the evidence for some sort of extra- terrestrial arrival does seem strong," says Fortey. "But if you look at the scientific references you won't find anyone quoting Velikovsky or Hoyle. As far as the comet saga is concerned, like so many species, these characters have left no trace of their existence."
Putting in this sort of human dimension helps to make the dizzying sweep of geological time more comprehensible, but the narrative becomes even more cosy when we discover that several of life's chapters have a distinctly British flavour. Did you know, for instance, that a key site for investigating "one of the most profound events in the history of life" - the colonisation of the land - is near Ludlow in Shropshire?
Because the Empire was in full swing when palaeontology got going, many of its key periods - such as Devonian and Silurian - are named after regions in Britain. "The geology of these islands is particularly interesting," says Fortey, "but having an empire did mean that gentlemen could follow such leisurely pursuits as geology. The outlines of the Cambrian, Ordovician and Silurian periods were all traced by clergymen and gentlemen scholars in Wales." The way their debates were settled affects the story Fortey tells today.
One of the basic ideas that most modern palaeontologists share with their Victorian forerunners is that scientific research must be based on reductionism - the way to understand anything is to understand its parts. But it's a doctrine that runs into terrible difficulties when unravelling the childhood biography of life.
In the early days there were a number of crucial moments, which we still don't understand, when some organism didn't make just a quantitative change - a stronger this, a longer that - but a qualitative one. The first and greatest was going from a soup of inorganic chemicals to an organic entity that could replicate itself. Almost as miraculous in terms of the unbelievably complex chemistry involved, were the next two - photosynthesis and the creation of the cell.
As a palaeontologist Fortey doesn't have much to say about events at a molecular level but he does make two intriguing points. The first is that they all seem to have happened just once - our DNA and that of the simplest bacteria have features in common which you wouldn't find if all sorts of different life lines had emerged. Yet, Fortey asks, if they are the result of a combination of natural processes, as the reductionists insist, then why didn't they happen more often?
"It's a question that relates to debate about life on Mars and extra- terrestrial life in general," he explains. "If life is something that is very hard to create, and if it only happened once, that suggests it is, then it's much less likely to have happened anywhere else." But how we think about problems has an effect on the sort of solutions we come up with, and Fortey believes there is another way to look at the origins of life problem.
He sees a strong parallel between the origin question and one of the other big questions of our time - how does subjective consciousness emerge from the objective networks of neurons in our brains? "It is the same sort of problem, involving a qualitative jump," he says, all too aware that his reductionist colleagues refer to consciousness as the `C' word, which shouldn't be mentioned in scientific society. But it was only because someone thought comets a likely cause of dinosaur extinction that they looked for iridium in the KT boundary.
The inexhaustible nature of the fossil evidence means that the complete story will never be told. The heartening side of this for Fortey is that the palaeontologists' hammer will never be pushed aside by more hi-tech equipment. "Recent advances in computers and optics do help enormously," he says, "but even in the year 2050 we are still going to depend on the people out in the field just banging on a remote rock face for months on. There are still surprises to be had on the Welsh borders and in Surrey."
And doubtless the struggle for the survival of the new theories at the university high tables will be just as fierce toon
`Life: an unauthorised biography' is published by Harper Collins at pounds 20.
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