Life on Earth has suffered five mass extinctions, each leaving an indelible mark, since the emergence of complex life just over half a billion years ago. They were all what geologists call "sudden". Something must have caused them – but what? Did each have a single cause? Did they have different causes? Did each have many causes? Could any single factor be enough to extinguish 90 per cent of all living things? These questions are important, because asking the wrong one – or failing to examine one's implicit preconceptions – is a frequent trap for the unwary scientist, especially when that scientist is operating beyond his or her original discipline, and even the unwary bystander.
When meteorites fall from the sky, witnesses are drawn, like readers scanning the news, to search for meaning. What does this event mean for me? The answer depends, for the most part, upon context – life experiences, intellectual baggage, expectations. Alsatian peasants, seeing a meteorite fall in 1492 near the town of Ensisheim, saw disaster. The Emperor Maximilian, passing nearby, foresaw victory. Oddly, both were right – Maximilian defeated the French, but the peasants of the Rhineland had to endure wave upon wave of war for decades. And academics? They didn't believe any of it. Until the last years of the 18th Century, learned men deferred to Aristotle and Newton – neither of whom allowed any grit into the celestial clockwork.
This was an easy line to hold, because until 1794 (when a meteorite finally exploded in full view of Siena's professors and educated Grand Tourists) the only witnesses had been peasants. For example, in 1768, a meteorite shower not far from Le Mans, France, was investigated by a group of aristocratic savants (including Antoine Lavoisier, father of modern chemistry). They dismissed eyewitness accounts and ascribed the fallen stones to lightning.
Thirty-five years later, in 1803, a similar fall only a hundred kilometres away, in a radically changed political context, had a very different outcome. Post-revolutionary France had formed an Institut National out of its royal predecessor, and sent in one Jean-Baptiste Biot. Paysans were now citoyens, and their observations treated with respect – enabling Biot finally to break free of centuries of prejudice and bring the idea of "stones from the sky" closer to the scientific mainstream. Same event, different context, different outcome. Similarly, the effect of a major meteorite strike upon life on Earth is, also likely to depend heavily upon circumstances – upon everything else that was going on at the time.
It took a long time for geologists to embrace meteorite strikes. In its quest to understand the Earth, geology draws in experts from many disciplines. Classically trained geologists and palaeontologists have a deep-seated feeling for their subject's historical nature; and like all historians, tend to mistrust pat explanations. But scientists from non-historical disciplines, physics in particular, tend to apply subtly different criteria for judging whether a scientific story sounds more or less likely to be true. In the mid-1970s, Earth science was galvanised by the plate-tectonic revolution but was otherwise still in thrall to a Victorian assumption that nothing "sudden" could possibly achieve anything lasting in Earth history. Mass extinctions were, therefore, a little embarrassing – and tended to be rationalised away. The leading British opponent of such "gradualism" Professor Derek Victor Ager (1926-95) realised a 24-hour hurricane could leave more trace of itself in sediments than intervening ages without name. The rock record was, he held, a scandal-sheet, recording the Earth's rare, exciting moments, and largely ignoring its more frequent longueurs. This was the "neocatastrophist" revolution.
By the time the Seventies were out, suddenness had been rehabilitated – with a vengeance. While looking for something else entirely, geologist Walter Alvarez, his Nobel-prize-winning physicist father Luis Alvarez, discovered (and in 1980, with Frank Asaro and Helen Michel, published) a paper about a thin, layer separating the Cretaceous and Tertiary periods rich in the rare element iridium; a horizon that also marked one of the "big five" mass extinctions, when dinosaurs and much else of the Mesozoic world order vanished for ever. Because iridium could only have come from space (Earth's crust being heavily depleted in it), there must have been a massive impact. The extinction, it seemed, had not been merely "geologically" sudden (a million years or so). It had happened in a day – a day that put an end to the world of dinosaurs and ammonites, and gave birth to the world of mammals, with (eventually) us in it.
Those who have been brought up to think like physicists (as most scientists who study impacts are) found it easy to accept that they had at last cracked the problem of "what killed off the dinosaurs". But many geologists were, and remain, wary. Partly, they felt aggrieved that physicists had apparently shot their fox; but that wasn't all, and only explains their initial reaction. Physicists like the idea because it is simple and parsimonious, whereas many geologists mistrust it for precisely the same reason. As historians, they feel in their bones that there is no imperative for the simplest explanation to also be right one, and the nagging conviction remains – despite the rehabilitation of the rare event – that major changes in Earth history simply cannot have single causes.
So what do we know about the impact at the end of the Cretaceous? We know that it happened. There is even a candidate crater called Chicxulub, which lies offshore the Yucatan Peninsula of Mexico. Doubt persists about its precise age, but despite decades of research, when we have found hundreds more craters on Earth, only one has been linked to a mass extinction event, and only one of the "big five" mass extinctions can be linked to any extraterrestrial causes. None of the others coincides with impact evidence, including the biggest one of all, the end-Permian extinction 250 million years ago.
However, we do know that the end-Cretaceous Earth was choking to death from the effects of unimaginably intense volcanic eruptions in India – effusions that were orders of magnitude bigger than anything humanity has experienced (or will ever experience, if we're lucky). Unlike impacts, convincingly linked to only one mass extinction, every mass extinction event can be correlated with a Large Igneous Province (a region of the Earth's surface where a series of large eruptions over a geologically short time period) somewhere on the globe. So could it be that the meteorite that hit the Earth at the end of the Cretaceous only produced its marked effect because of timing – because the impact or impacts were reinforcing other inimical factors? As new evidence comes in, this is beginning to look increasingly likely.
New discoveries are now even linking meteorites to one of life's greatest-ever diversifications. Some 470 million years ago, when the world was thinly colonised by simple marine organisms, Earth suddenly found itself bombarded by countless meteorites, over a period perhaps exceeding 10 million years, following a major collision in the Asteroid Belt. Geologists are finding fossil meteorite material in sediments of this age everywhere. Most intriguingly, these amazing discoveries (first made in Sweden, now being extrapolated worldwide by Professor Birger Schmitz of Lund University) may help explain a baffling burst of evolutionary diversification – the biggest to affect life after the so-called "Cambrian explosion", when complex animals first appeared. This is known as the "Great Ordovician Biodiversity Event (GOBE), and it has puzzled palaeontologists since it was uncovered by computer analysis of species data in the early 1980s.
The theory goes that by sterilising large areas, bombardments helped break the stranglehold of endemic species, allowing new opportunistic organisms to invade, increasing biodiversity by an ecological phenomenon known as the Intermediate Disturbance Effect. Such biodiversity increases would feed through, in time, to faster evolutionary diversification.
So when we read of a meteorite fall today, we should reflect that what may have been bad for T Rex 65 million years ago was good for birds and aardvarks and us; and had it not been for a collision between asteroids that showered the mid-Ordovician Earth, T Rex himself might never have had his big chance.
Indeed, as with all incoming news, the meaning you derive from it rather depends on where you're standing.
Incoming! Or, why we should stop worrying and learn to love the meteorite, by Ted Nield, is published by Granta (£20). To order a copy for the special price of £18 (free P&P) call Independent Books Direct on 08430 600 030, or visit www.independentbooksdirect.co.uk