LONG before Scotland was Scotland, when the population consisted only of green algae and the Highlands were as dry as Death Valley, a large natural object fell out of space and struck the Earth near where the village of Stoer now stands, in Sutherland. This incident occurred 1.2 billion years ago, but it has only been confirmed in the last few months. "If the same thing happened today," says planetary geologist Scott Thackrey, "all the trees in Aberdeen would be felled. The trees in Inverness would actually ignite. Most man-made structures would collapse. Everything made of paper would burn. You wouldn't be safe in Glasgow. But sitting here, we would be vapourised."

As it happens, we are sitting out a rainstorm in a rented car beside the Stoer church graveyard. And if tombstones are monuments to life's impermanence, then geologists serve to remind us that even the mountains and seas are not nearly as eternal as we tell ourselves.

Like most in his profession, and possibly more so, given his self-confessed addiction to energy drinks, Thackrey perceives time in such a way that eons seem to whizz back and forth before his eyes. Even as we speak, he is forming a mental picture of the impact made by the meteorite in question. ("Meteorite" is the generic name given to any space-rock that hits the ground at least partly intact. The same object is referred to as a "meteoroid" before it enters Earth's atmosphere, and a "meteor" when it appears in our sky, bursting into flames and becoming what we also call a "shooting star".) "I can see the crater now," says Thackrey. "Smoking, somewhere to the east of us."

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Several of his colleagues see it differently. "They think the crater would be to the west," he says. "We're all still arguing about it at this point. But they're wrong." This argument is literally academic, not least because the crater may never be found. After more than a billion years, it is either buried deep beneath the Minch Basin or the Moine Thrust, both defining features of the local topography, or it has been eroded out of existence.

So, this particular discovery was made not in the field, but under the microscope. The "eureka moment", according to Thackrey, came when two collaborating teams at Aberdeen University and Oxford's Department of Earth Sciences found "shocked minerals" inside "melt fragments" within rock samples from Stoer. "That's when we twigged," he says. "F***ing hell, this is impact-related. We were dancing around in the lab."

Even if a layman could get equally excited about the composition of Scottish stones, the ones that he shows me today are far too slippy to dance on safely. Thackrey leads the way out of the car park, along a coastal path, and down to the sea, where the rocks are still warped from the ground-surge that must have followed seconds after the initial impact, when extreme heat and pressure briefly turned the land into a rolling semi-liquid.

It has previously, and wrongly, been assumed that this misshapen corner of the country was once part of a volcanic horizon. "Oops," says Thackrey, having helped to prove otherwise. At 25, and still a PhD student at Aberdeen University, he is the youngest contributor to the research paper, published earlier this year, which effectively rewrote geological history, although the title didn't exactly set the world on fire: A Precambrian Proximal Ejecta Blanket From Scotland.

Even so, I wouldn't know what I was looking at or standing on - irrefutable physical evidence of the first known meteorite strike on what we now call British soil - unless Thackrey told me. He seems concerned that I won't find the site sufficiently dramatic, beyond the obvious Earthly beauty. To illustrate, he produces space probe photographs of a "directly analogous" site on Mars, where meteorite craters are vastly better and longer preserved.

"There is so much interest in impacts at the moment," he says. "From exobiology the search for life, or the remains of it, on Mars and elsewhere, to cratering as a process." His own interest is both cosmic and domestic. Thackrey is a local boy, born in Inverness and raised in Aberdeen. Having spent most of his recent summers in the Canadian High Arctic, conducting research at the Haughton Crater (which NASA uses as a base to test equipment for its ongoing Mars project), he says he is "amazed" to be working so close to home.

He is also a recent applicant to the European Space Agency's astronaut programme, although he has no plans to travel any further than the moon, after hearing too much gruesome medical conjecture about the possible skin and bone damage that might result from more prolonged journeys. In the meantime, Thackrey has become a hot-shot within a school of scientific thought that allows him to contemplate "the whole wonder of massive explosions".

"What would a big impact look like?" he asks. "The only thing we can compare it to is a nuclear detonation, but even that doesn't really come close. I'd love to see one ..." The biggest impact of the recent past is not, in fact, very far removed from living memory.

This year marks the centenary of the so-called Tunguska Event, when something like an atom bomb went off deep in the Siberian forest before atom bombs had even been invented, knocking millions of trees down for hundreds of miles. The only humans around to witness it were a few Russian settlers and Shanyagir tribespeople, and when scientists belatedly reached them for comment more than 20 years later, they testified that the sky had "split in two" and "it was as if there was a second sun". Peasants from the isolated village of Korelina had assumed that the world was ending, and sent a delegation to ask the nearest archpriest how they should prepare. "As the minutes passed," wrote the wry astronomer Kevin Zahnle, with the benefit of another few decades' hindsight, "this theory was dropped in favour of other, less final theories, until today one is hard-pressed to find anyone who truly believes that the world ended on the morning of June 30, 1908."

In the conspicuous absence of a crater, however, it took the best part of a century, and many wild guesses (some wilder than others, with certain physicists proposing that a black hole had passed through the Earth), before the only adequate explanation was confirmed on another planet.

When the comet Shoemaker-Levy 9 (SL9) collided with Jupiter in 1994, it was seen to disintegrate under "impact pressure" in mid-air, bursting high in the atmosphere to cause shockwaves and firestorms on the ground, without so much as touching the surface. Something similar must have happened at Tunguska.

And according to the renowned Scottish research astronomer Bill Napier, "the fact that this happened within the last 100 years has to be a matter of concern". "Even a relatively small air-burst like that could flatten London," Napier tells me. "From the city centre all the way out past the M25. And we would expect to see a couple of hundred of these impacts in quite a short timeframe."

Napier and his frequent collaborator Victor Clube are the leading proponents of a theory known as "coherent catastrophism", which suggests that the Earth is pelted at regular intervals by asteroid and comet fragments dislodged from distant, ancient rubble fields strewn across the known universe.

The mechanics of this are barely fathomable and currently unquantifiable, involving galactic rotation, orbital deviation, and fantastical outer space place-names such as The Oort Cloud, The Kuiper Belt and The Taurid Complex. "Astronomy," admits Napier, "is too uncertain a subject to make any definite statements." But the core of his idea is grounded, if you like, in our geological record, and supported with first and second-hand accounts from our ancestors.

"From peak to loftiest peak, the Earth takes fire," wrote the Roman poet Ovid in his masterwork Metamorphoses, which was published circa 8AD but quoted much older Greek legends - in this case the story of Phaethon, son of the Sun, who tried to ride his father's chariot across the sky but lost control and crash-landed it. "Walled cities melt away," continued Ovid, "whole tribes and peoples turn to ashes gray ... The mountain masses with their forests burn."

"There is a long tradition of describing strange meteoristic events," says Napier. "The Book of Revelations seems to borrow from Hellenic, Persian and even early Sanskrit myths for its details of celestial conflicts which have catastrophic effects on Earth."

This sounds like the stuff of popular fiction, and Napier has published several novels - Nemesis, The Lure, Shattered Icon - that take his research to speculative extremes. The Cosmic Serpent, however, which he co-wrote with Clube in the early 1980s, was an entirely non-fictional and highly influential attempt to revive the study of meteorite impacts as a deciding factor in the past and future existence of life on this planet.

That kind of thinking had not been taken seriously since Charles Darwin and Charles Lyell published their respective theories of evolution and "uniformitarianism" over 100 years earlier. At that time, the only known impact site in the world was the famous Barrington Crater in Arizona, which allowed Darwin, the legendary naturalist, and Lyell, the elder geologist, to assert that such strikes were so rare as to have no real bearing on the long, slow and steady processes of terrestrial change. Science and technology have since advanced to prove them both wrong in this respect, tracing craters all over, and under, the Earth's surface.

It may, indeed, have been minerals from iron-rich asteroids and ice from vaporous comets that "seeded" this planet with the ingredients for life in the first place. And in the late 1970s, while Napier was formulating his own theory, an oil survey worker found a vast ring of destruction deep beneath the Mexican village of Chicxulub - 65 million years old, 180 kilometres wide, and subsequently linked by Dr Luis Alvarez to the extinction of the dinosaurs.

Others in the field have recently gone further, to associate possible strikes and airbursts with the sudden death of almost all known marine animals 200 million years earlier, and the killing off of North American mammoths and mastodons around 11,000 BC. The original Flood myth, disseminated through ancient literature from the Epic Of Gilgamesh to the Book Of Genesis, has been reconsidered in terms of a meteorite landing in the Indian Ocean, and the resulting tsunami.

Several potential impacts have been dated to the mid-sixth century AD, when the Byzantine historian Procopius wrote that "a most dread portent took place ... the sun gave forth its light without brightness". Northern Europe appears to have suffered something like a nuclear winter in this period, with forest fires, airborne debris and corrosive rains leading to climate change and crop failures that may in turn have weakened the population to their first encounter with the Black Death.

All of which seems to bear out Napier and Clube's follow-up book, The Cosmic Winter. And if they're right about meteorites behaving like bad weather, then we can reasonably expect another turn for the worst. "I wouldn't necessarily be looking out for a single, one-off Hollywood spectacular," says Napier. "More of a widespread battering of the environment over a sustained period."

The experts are not remotely agreed on how worried we should be. Telescopes and satellites are now in a position to detect potential "planet-killers" - asteroids or comets over one kilometre in diameter - many years or even decades before they might collide with the Earth.

The problem, according to Jay Tate, director of the public information resource Spaceguard UK, is that no measures are in place to avert them, and smaller objects could still "whistle through" to strike us at any time. "Do you want to know how much warning we would have of a country or city-killing asteroid?" asks Tate. "About four seconds. A major impact is just as likely to happen before the end of this conversation as 10,000 years from now."

A retired British Army officer and guided-weapons specialist, Tate became curious about government plans to defend against meteorites after the SL9 comet bombarded Jupiter in 1994, and was furious to find that they didn't have a single one. He and several eminent scientists formed a task force on Near Earth Objects (NEOs), presenting parliament with a series of recommendations for action in 2000.

"Very little has been done since," says Tate, from the Spaceguard Centre at the former Powys Observatory in Wales. "The official British line seems to be that we don't need to bother, because the Americans are taking care of it. But if that's the rationale, then what the bloody hell are we doing in Iraq?"

He goes on to explain that even the US is not particularly invested. NASA is now spending the vast bulk of its federal budget on the long-term goal of a manned mission to Mars, as opposed to the supposedly simple probes and rockets, known as "gravitational tractors", which would nudge a killer comet off-course. "NEOs represent a proven science and a definite problem," says Tate. "Unlike the dinosaurs, we could do something about it. I think the biggest obstruction might actually be the human psyche, which only stretches one or two generations in each direction. If something happened to mum or dad, or directly threatened our kids, then it might seem real to us. Beyond that, who gives a damn?"

Standing on the northwest shore of Scotland, it does not seem likely that the world will end in the next four seconds. But the world has already ended here at least once, if only for the simplest, earliest organisms who happened to be living near Stoer 1.2 billion years ago, on the day of the impact.

"A lot of algae would have been killed, yeah," says Scott Thackrey, pointing out traces of long-dead lifeforms preserved in the rock.

"Blasted. Obviously, they weren't made extinct, because eventually they evolved into us. But it would have been one hell of a show."