There was no warning. One minute, the Earth was quiet, just as it had been for countless millennia. On the land, the great, shimmering deserts supported a population of insects and primitive dog-sized reptiles called therapsids.
   Around the coasts, forests of strange flowerless trees and miasmic swamps were home to giant crocodiles and trillions of dragonflies - in an era before birds, the kings of the skies. In the seas, the waters teemed with mini sea monsters, and sharks hunted bizarre bony armoured  fish.
   Then it came. Out of the sky, with a roar that must have sounded like the slamming of the doors of hell itself, an object that would change the course of life forever on our planet.
   When it hit, the cataclysm it unleashed was beyond imagining. The ground shook with the force of a million earthquakes, and a crater hundreds of miles across was punched in the crust.
   Yesterday, scientists announced that they have solved one of the greatest mysteries in geology - the cause of The Great Dying that came at the end of the Permian era, 251 million years ago.

A giant asteroid, seven-and-a-half miles across, hit our planet at some 30,000mph, with so much force that 90-95% of all animal and plant species vanished.
   It was a catastrophe unparalleled in Earth's history - even the famous asteroid that hit the Yucatan peninsula in Mexico and felled the dinosaurs millions of years later was on a far smaller scale.
   And the news that a relatively small asteroid is capable of killing off virtually everything alive has given new impetus to plans to map and study these lethal cosmic bombs.
   This is why the landing of the NASA probe, called NEAR-Shoemaker, on a 21-mile-wide asteroid called Eros last week may prove to be one of the most significant space missions in history, as I shall explain. After the impact of the asteroid that triggered The Greta Dying, every living thing for a thousand miles around was fried by the titanic fireball.
   The coasts were lashed by giant waves three miles high, obliterating everything in their path.
   The impact triggered a series of giant volcanic eruptions in what is now Siberia, releasing huge amounts of deadly gases into the atmosphere. Within a week or so, half the animals on the land were dead.
   Months of darkened skies, freezing temperatures and acid rain followed, so that the remaining plants and the sea creatures started to die too.
   The roll-call of the dead stretched to tens of thousands, maybe millions of species. The sea creatures called Trilobites vanished entirely. Only a few corals survived - the ancestors of all of today's reefs.
   Most of the primitive reptiles were killed, leaving the continents virtually devoid of large animals. Amphibians, fish and molluscs were reduced to a few hardy surviving species.
   Then, as the aeons passed, life slowly recovered - and a hitherto insignificant group of little reptiles recovered their numbers and came to dominate the planet. In the absence of any competition, they grew in size, and in number.
   The great asteroid impact that we now know caused the biggest mass extinction of life in Earth's history also ushered in the age of the dinosaurs.
   If this event were a one-off, it would be merely a geological curiosity, an ancient catastrophe buried in the mists of time.

However, it did happen again. For 180 million years, the dinosaurs prospered. Reptiles dominated the land, the seas and the air. Nothing could stop them, or so it seemed.
   By sixty-five million years ago, some dinosaurs had evolved into fast-moving, warm blooded and efficient killing-machines.
   Then the party ended. Another asteroid, or maybe a comet, struck the coast of Mexico. Again, billions of animals perished, in fires and earthquakes, giant waves and acid rain.
   The dinosaurs went entirely, so did the coiled ammonites (marine molluscs), the flying reptiles and the great fish lizards.
   One group of survivors included an obscure group of little furry animals - the mammals - which in the millennia after the impact evolved in both size and in number.
   Sixty-five million years later, mammals dominate the planet; and at least one species, Homo sapiens, has evolved beyond the first flickering signs of intelligence.
   This is just as well because we or our descendants will need all that brainpower if we are to avert another disaster from outer space.
   The discovery that an asteroid was responsible for The Great Dying brings home just how dangerous our solar system can be. Certainly the threat of a space collision is now being taken seriously, by the U.S and British governments.
   Asteroid strikes are rare, but when they happen they are devastating, and one could happen tomorrow. It seems that for the past few thousand years we have just been lucky; the era of recorded time has not coincided with any major impacts.
   But there have been smaller ones, Ninety-three years ago, a small cousin of the asteroid that destroyed the dinosaurs struck the Tunguska region of Siberia, flattening millions of acres of forest and incinerating everything for a hundred miles.
   If it had struck just eight hours later in the earth's orbit around the sun, it would have obliterated Edwardian London.
   In 1933, a fragment of comet or asteroid struck the Brazilian jungle. Again, if this had hit a city it would have killed millions. Clearly, it is only a matter of time before we have major disaster from space.
   What the NEAR-Shoemaker space probe, which landed earlier this year on Eros, tells us about asteroids may one day prove to be vital.
   For example, if we do detect an asteroid hurtling our way - and we have the time to do something about it - it would probably involved sending a nuclear missile out in to space to deflect the asteroid or to blow it up.
   We know now from NEAR-Shoemaker that large asteroids are solid lumps of rock, not loose piles of rubble as had been suspected - important to know when selecting the warhead. Scientists believe that a solid lump will be easier to knock off course than a pile of gravel.
   Our telescopes have detected hundreds of these 'Near Earth Objects' whose orbits take them worryingly close to Earth. Most of these are harmless, and will never hit us. But every now and then one streaks past, too close for comfort.

But it would take only a slight nudge from the gravity of another planet such as Mars to alter the course of one by a few thousand miles and send it careering into our world.
   And if a seven-mile-wide asteroid hit us tomorrow, we would be as helpless as the amphibians and trilobites of long ago.
   Experts calculate that the chance of this happening in an individual's lifetime is about one in 20,000; remote perhaps but a thousand times more likely than scooping the National Lottery jackpot.
   What is certain is that one day, in the near or not-too-distant future, the sky will again blaze with fire, the ground rumble and huge waves pulverise the coasts.
   If we haven't taken steps to protect ourselves - and that means lots more asteroid missions and increased efforts to map the skies for space debris - we will go the way of the therapsids and the tyrannosaurs… paving the way for who knows what insignificant group of creatures to take their turn in the limelight.