In almost every children's book on dinosaurs, there is an illustration of a giant long-necked herbivore bellowing in agony as flesh is shredded from its flanks by a much smaller but fleeter-footed predator. It is the kind of image that must haunt a large, sluggishly performing organisation. Yet the BBC, which has lately been enduring a savaging such as any pack of velociraptors might have been proud to inflict, is turning to dinosaurs to help patch up its wounds. They are the subject of its flagship science series this autumn, and the fascination that they inspire should ensure that Walking with Dinosaurs is an enormous worldwide hit.
It is certainly doubtful whether the colossal £6 million budget required to bring the Mesozoic convincingly to life would ever have been committed to any other nature series. This reflects the rather depressing fact that dinosaurs are simply more intriguing to people, and more familiar as well, than the vast majority of creatures still alive upon the planet. In fact, so prevalent have images of them become, in books, films, toys and cartoons, that they must surely rank among the most familiar icons of a century that they predate by millions of years. Can there be anyone easier to buy a present for than a friend who likes dinosaurs? When it comes to dead celebrities, Elvis has nothing on tyrannosaurus rex.
Of course, ubiquity can deaden us to wonder. It might seem strange that creatures so awesome to contemplate should also lend themselves so wholeheartedly to kitsch. But this is an ambiguity that has always lain at the heart of the dinosaur's image. Stephen Jay Gould has explained dinomania by referring to "the epitome proposed by a psychologist colleague: big, fierce, extinct - in other words, alluringly scary, but sufficiently safe". Dinosaurs are seen as "safe", patronisingly regarded as bywords for evolutionary failure, and it is that which makes them seem so ripe for commercial exploitation, even as they are also regarded as the ultimate in ferocity and size. Such a tension, between T Rex as a child's stuffed toy and as the lawyer-crunching, jeep-mauling monster of Jurassic Park, helps to illustrate why dinosaurs have no simple metaphorical value. They are defined by evolving scientific research, and yet science is unable to explain them altogether: into the gaps can be projected all our manifold obsessions and concerns, as mutable and contradictory as society itself. It turns out that the B-movies were right all along: just when you think you've got dinosaurs pinned down, they always break free.
Nor has there ever been a time when they weren't roaming beyond the boundaries of scholarship. The first full-scale replicas of prehistoric monsters were built not for a museum but for the grounds of the Crystal Palace exhibition centre, where they remain to this day. Constructed of concrete, the most sophisticated building material then available, they were consciously presented as both proof and symbol of British scientific supremacy. The first dinosaurs had been discovered and categorised in England some 30 years before; however bizarre the Crystal Palace models may appear to us today, they were constructed according to the most up-to-date specifications of Richard Owen, the world's leading anatomist and the man who had first coined the word "dinosaur". Queen Victoria herself attended their opening, to admire the monsters which, in the words of their sculptor, Benjamin Hawkins, "the Almighty Creator designed to inhabit and precede us in possession of this part of the earth called Great Britain".
By contrast, if dinosaurs are not a feature of the Millennium Dome then it is presumably because they have long since ceased to be seen as exclusive national property. As with industrialisation and cricket, so with dinosaur studies - a British lead soon melted away. Finds in England had mostly consisted of isolated teeth or bones, but from the 1870s complete skeletons began to be dug up in the Rockies. The lead in palaeontological research passed across the Atlantic, never to return, but it was the revelation of the dinosaurs' sheer scale that really impressed the American public, for many of the skeletons were the size of apartment blocks. Even in prehistoric times, it seemed, things in America had been the biggest and the best.
In fact, the great rush to excavate dinosaurs was a more fitting metaphor for the way their country was developing than most Americans would have cared to admit. Palae-ontologists were borne west on the same flood of immigrants that wiped out both the buffalo and the Indian way of life. The greed for spectacular specimens led America's two leading palaeontologists, Professors Cope and Marsh, to pursue the most vicious personal rivalry in the history of science, as their rival gangs stole or smashed up each other's collections of bones. Eventually their struggle to establish a monopoly led to the ruin of both their fortunes; in a parody of his customary ruthless business practice, it was left to that arch-monopolist Andrew Carnegie to step in and take over the funding of the hunt for dinosaur bones. It was size he was interested in, colossal specimens that would match the monstrous scale of his own business interests. The skeleton that dominates the main hall of the London Natural History Museum was a gift from him, and still bears witness to Mammon's appropriation of the dinosaur: its scientific name remains Diplodocus Carnegii.
As an example of advertising, this was almost as provocative a tempting of fate as it had been for Victorian Britain to identify itself with a vanished empire of monsters. At least as much as for their size, dinosaurs were already celebrated for being extinct, condemned by the same processes of Darwinism that both capitalism and imperialism invoked to justify themselves. As excitement at the first great wave of discoveries faded, so public wonder at dinosaurs grew ever more tempered by contempt. Diplodocus, the very specimen that bore Carnegie's name, became the epitome of ponderous failure, a dun-coloured, tail- dragging behemoth. It was an image that Gary Larson drew on for his witty cartoon of an assembly hall full of despondent stegosaurs. "The picture's pretty bleak, gentlemen," a lecturer admits. "The world's climates are changing, the mammals are taking over and we all have a brain about the size of a walnut."
For much of the century, scientists would have largely agreed with this assessment. In fact, it was precisely because dinosaurs were seen as such an evolutionary dead end that palaeontologists were content to leave them to cartoonists and children: what professional would willingly waste his career in a cul-de-sac? All the same, a few scientists did start to take the dinosaurs seriously. The beginning of what has since been called the dinosaur renaissance is customarily dated to the discovery in Montana of a single sickle-like claw in the early 1960s. It was shown to have belonged to an agile, leaping predator, which had used it rather as a kick-boxer might have used a flick-knife, to rip apart its prey. Such a creature seemed far removed from the sluggish reptiles of convention, and it began a process of re-evaluation that has become one of the most significant scientific stories of the past 30 years. By the end of it, the skeleton of Diplodocus Carnegii would no longer be dragging its tail but swinging it like a whip over the heads of spectators, and, for the first time in over a century, the public's fascination with dinosaurs would not be despite what the academics thought but directly inspired by and swelling in its wake.
At the heart of what their chief propagandist, Bob Bakker, has called "the dinosaur heresies" is the issue of whether dinosaurs were reptiles at all. If, as Bakker has argued, they were warm-blooded, then it would require their reclassif-ication as a quite separate class of vertebrate, a development loaded with taxonomic implications. For the public, however, the most immediate effect of the debate has been to make dinosaurs seem even scarier than before, not only fierce but intelligent and pacy. Popular illustrations now reflect this consensus. Dinosaurs gallop or bound, and even their skins are all the colours of the rainbow. More than anything, though, it was the film of Jurassic Park that cemented the dinosaurs' new-found reputation, and in particular the portrayal of the pack-hunting, problem-solving velociraptors. This was only fitting, for on each of their feet was a familiar-looking claw - retractable and sickle-like, the weapon of a kick-boxer. One moment of brilliant cinema summed up the whole revolution in dinosaur studies: a velociraptor, peering through the window of a locked door with fearsomely intelligent eyes, flicks out its claw, then snorts on the glass and mists it with its breath.
Steven Spielberg had a film-maker's licence: the case that velociraptor - or any other dinosaur - was warm-blooded remains unproven. So does a whole host of other physiological and behavioural mysteries: it can sometimes seem as though each new find simply stirs up fresh passions and controversy. The classic example from the past 20 years has been the discovery in Montana and China of vast colonies of eggs. The implication that the dinosaurs that laid them must have nested communally is a wonderfully evocative one, and yet the chief effect of it has been to fuel a much more far-reaching debate, the question of whether birds, too, might really be dinosaurs. One authority on the cultural mutations of the dinosaur has gone so far as to argue that this issue threatens "to kill the romance and mystery of its object, and to disperse it into a dead metaphor, a framework for seeing almost anything 'as' a dinosaur". It seems likelier, though, that the very opposite is the case, so intriguing are the issues of speciation and extinction that the controversy raises. Dinosaur fascination, in short, has come of age. For the first time ever, perhaps, dinosaurs are less interesting as cultural fetishes than as objects of science.
So the BBC has timed its series well. Its recreation of the Mesozoic cannot be considered definitive - nothing ever will be - but to watch it is an exhilarating, almost overwhelming experience. It is also a profoundly moving one, similar to the effect of watching epic tragedy, for over all the beasts portrayed hangs the shadow of the greatest dinosaur mystery of them all. Whether they were wiped out by an asteroid or not, we know their doom is coming. Nor can we any longer scorn them for this. In the midst of environmental catastrophe as we are, we know that the dinosaurs' world is not the only one that may soon have passed away. More so than ever, it seems, when we walk with the dinosaurs, we walk with ourselves.
"Walking with Dinosaurs" begins on BBC1 on 4 October