They were the most successful animals on the planet – and the most ferocious. They ruled the world for 100 million years. Some grew to a gigantic size: stegosaurus, diplodocus, Tyrannosaurus rex. Others became fearsome underwater predators, like icthyosaurus and plesiosaurus, while pteradons, with their vast wing-spans, dominated the skies. And then they died and left the way clear for shrew-like mammals to evolve into lions, lemurs and lemmings.
The debate about what killed the dinosaurs has been equally fearsome. Depending on who you believe, it was an asteroid impact, a supervolcano, or a gamma ray. They were starved, poisoned, frozen, boiled, drowned, dried, asphyxiated, irradiated or all of the above. "A colleague of mine said, 'Paleontologists are responsible for the third law of mass extinctions: for every extinction, there's an equal and opposite mechanism,'" says Shanan Peters, a professor of geology at the University of Wisconsin-Madison.
Peters has come up with a new theory to explain the demise of the dinosaurs, and all the other extinctions that have written their fragile, fossil messages within the bones of the earth. "One of the remarkable things about this work is that it is a statistical smoking gun. It's in the background for all extinctions, but it's predictive about which species are more likely to survive and which will go extinct," Peters says. His study was published in the journal Nature.
Since life began 3.5 billion years ago, there have been five mass extinctions. The dinosaurs died in the last one, 65 million years ago, but the worst was at the end of the Permian period, 250 million years ago. Known as "The Great Dying", it wiped out 95 per cent of all species. Some scientists suggest we are now on the brink of a sixth mass extinction.
Peters's theory is that it was changing sea levels that did for the dinos as well as other species throughout evolutionary history. A few years ago, in geological time, the world looked rather different. Europe was a shallow sea, 100 metres deep, and a band of ocean ran through the middle of America. This stretch of sea teemed with giant sharks and mosasaurs – massive marine predators. As the sea drained away, the sharks and mosasaurs became extinct.
Clearly, a rise or fall in sea level can have a dramatic impact on marine animals, but it also affects terrestrial fauna and flora. Peters points out that we in the UK are warmed and moistened by the Gulf Stream, whereas parts of Canada on a similar latitude are much colder. Withdrawing an ocean from Colorado would suddenly have turned the region hot and dry. "Life on land would have known about the loss of that shallow sea," Peters says.
The last great extinction was triggered by a fall in sea levels, but others have been caused by a rise. Peters arrived at this conclusion by collecting rocks from 540 sites across America. He looked at two main types: carbonates, which he likens to the white sand you see in beautiful marine environments like the Bahamas, and siliciclastics, which are like the muddy sand beaches we have in the UK, dark with sediment that's been washed off the land.
"At each spot, I asked what the record of sea-level rise and fall was and what the environmental consequence of that was," Peters says. This enabled him to plot sea level against the numbers and types of species found in these two different types of marine sediment. "It doesn't mean that other physical perturbations aren't important," says Peters, referring to supervolcanoes and asteroid strikes. "The Yucatan crater is the biggest crater caused by an asteroid in the last 540 million years. If it happened now, I'd feel the shock wave and see the debris and dust at my desk in Wisconsin. But the problem with this theory is that we have lots of large craters – 100 kilometres in diameter – caused by asteroids that aren't linked to any extinctions."
So what led to the rise and fall in sea levels? One explanation is the shift in the Earth's tectonic plates. The other is climate change. Over the past 500 million years, ice sheets have been forming, advancing, melting and retreating. These vast perturbations in climate change were produced by shifts in Earth's orbit around the Sun. Geologists say this is natural and normal: we are in a cold phase right now.
However, as we know, after the death of the dinosaurs, one species came to dominate the planet – and it is warming the earth up rather rapidly. The 20th century saw the greatest increase in temperature of any century in the last thousand years. The last decade has been the warmest since records began. "The rate at which we're changing the climate is unusual," Peters says, with scientific reticence. "The only similar rate and magnitude of global warming seen in the fossil record was 55 million years ago. Something caused massive amounts of carbon dioxide to be injected into the atmosphere and dramatically altered the climate."
If global warming continues at its current pace, Peters says, "sea levels will rise for sure". The biggest and most immediately noticeable impact on the natural world will be the death of coral reefs. But vast numbers of people could lose their homes, livelihoods and lives. For every centimetre the sea rises, about one metre of land is lost. By 2100, it's predicted, the sea will have risen by 50 centimetres. It may not sound a lot, but most of the human population lives by the sea. In Bangladesh, such a rise would result in 17 million people losing their land, yet a rise of 10 metres is nothing in geological terms, Peters says.
Global warming has already led to changes on every continent. Scientists examined reports dating back to 1970 and found that in at least 90 per cent of cases, shifts in wildlife behaviour and populations could only be explained by global warming. The team also found that 95 per cent of environmental changes, from melting permafrost to retreating glaciers, were caused by an increase in global temperature.
But even without global warming, we are going to have a mass extinction, claims Dr Peter Mayhew, a biologist from the University of York, who's studied the impact of climate change on mass extinctions. "It's due to habitat loss, and there's no realistic way we're going to avoid that without curtailing human population growth. Extinction will be insidious; we may almost not recognise that it is happening. To you and me in the UK, mass extinction is not going to be something that impinges much on our lives. It'll mean we won't see some species we saw in the past." It's already rare to see large blue butterflies, badgers and bee orchids.
As for extinction on a global scale, according to Dr Richard Leakey, the famous paleoanthropologist and author of the book The Sixth Extinction, every year between 17,000 and 100,000 species vanish from our planet.
By the end of this century, the human population is predicted to have reached 10 billion and the consequence is thought to be the loss of half the world's total number of species. Leakey says: "Homo sapiens is poised to become the greatest catastrophic agent since a giant asteroid collided with the Earth 65 million years ago, wiping out half the world's species in a geological instant."
Extinctions that shaped our planet
Cretaceous-Tertiary, 65 million years ago
Thought to have been aggravated by the impact of large asteroid on the Yucatan peninsula, it led to the death of 16 per cent of marine families and 47 per cent of marine genera, and 18 per cent of land vertebrate families, including the dinosaurs.
End of the Triassic, 200-214 million years ago
This extinction was aggravated by a supervolcano that triggered a rise in sea levels and global temperatures. The death toll included 22 per cent of marine families and 52 per cent of marine genera.
End of the Permian, 250 million years ago
Earth's worst mass extinction, annihilating 95 per cent of all species, including 53 per cent of marine families, 84 per cent of marine genera and 70 per cent of land species such as plants, insects and vertebrate animals.
Late Devonian, 364 million years ago
About 22 per cent of marine families and 57 per cent of marine genera were lost.
439 million years ago
Sea levels fell as glaciers formed, and then rose as glaciers melted, resulting in the extinction of 25 per cent of marine families and 60 per cent of marine genera.