Some say the world will end in fire, some say in ice, wrote Robert Frost. But whatever is to be our fate, it is now overdue.
After analysing the eradication of millions of ancient species, scientists have found that a mass extinction is due any moment now.
Their research has shown that every 62 million years - plus or minus 3m years - creatures are wiped from the planet's surface in massive numbers.
And given that the last great extinction occurred 65m years ago, when dinosaurs and thousands of other creatures abruptly disappeared, the study suggests humanity faces a fairly pressing danger. Even worse, scientists have no idea about its source.
'There is no doubting the existence of this cycle of mass extinctions every 62m years. It is very, very clear from analysis of fossil records,' said Professor James Kirchner, of the University of California, Berkeley. 'Unfortunately, we are all completely baffled about the cause.'
The report, published in the current issue of Nature, was carried out by Professor Richard Muller and Robert Rohde also from the Berkeley campus. They studied the disappearances of thousands of different marine species (whose fossils are better preserved than terrestrial species) over the past 500m years.
Their results were completely unexpected. It was known that mass extinctions have occurred in the past. During the Permian extinction, 250m years ago, more than 70 per cent of all species were wiped out, for example. But most research suggested that these were linked to asteroid collisions and other random events.
But Muller and Rohde found that, far from being unpredictable, mass extinctions occur every 62m years, a pattern that is 'striking and compelling', according to Kirchner.
But what is responsible? Here, researchers ran into problems. They considered the passage of the solar system through gas clouds that permeate the galaxy. These clouds could trigger climatic mayhem. However, there is no known mechanism to explain why the passage might occur only every 62m years.
Alternatively, the Sun may possess an undiscovered companion star. It could approach the Sun every 62m years, dislodging comets from the outer solar system and propelling them towards Earth. Such a companion star has never been observed, however, and in any case such a lengthy orbit would be unstable, Muller says.
Or perhaps some internal geophysical cycle triggers massive volcanic activity every 62m years, Muller and Rohde wondered. Plumes from these would surround the planet and lead to a devastating drop in temperature that would freeze most creatures to death.
Unfortunately, scientists know of no such geological cycle.
'We have tried everything we can think of to find an explanation for these weird cycles of biodiversity and extinction,' Muller said. 'So far we have failed. And, yes, we are due one soon, but I would not panic yet.'
