A few weeks ago, I read David Marinelli’s article in this newspaper, ‘Sleepwalking Towards Extinction’, in which he drew stark attention to the danger humanity is facing “as we sleepwalk towards the end of civilisation”. He highlighted that it is unprecedented, in the millions of years of life on Earth, for one species to be responsible for “annihilating all other species, be they plants or animals”.
He blames humans for single-handedly causing a mass extinction of life (in which people will also perish in unprecedented numbers) as a result of our plunder of the planet, the destruction of forests, the clearing of land for agriculture and the global warming caused by rising carbon emissions. He points out that the last time this happened was 66 million years ago when “a meteor hit the planet”.
While the word “extinction” may be slightly over-blown, the fear of severe, life-changing climate change is deserved. The existential threat is imminent unless nations combine to mitigate it.
Marinelli’s article got me thinking about previous proven mass extinctions. About 466 million years ago, long before the dinosaurs, Earth plunged into an ice age. As the poles began to freeze, lower temperatures around the world led to an extraordinary proliferation of new species. Scientists now believe that they have identified what caused this explosion of life.
According to Philip Heck at the University of Chicago, co-author of a paper published in Science Advances, an asteroid collision between Mars and Jupiter showered our world with millions of tonnes of dust. This extra-terrestrial dust shaded the Earth from the sun for over two million years. The cooling led to more complex ecosystems as new categories of animals emerged.
I was also fascinated to read a recent report – published in the Proceedings of the National Academy of Sciences in the United States – which shed dramatic light on the last mass extinction, for it is worth recalling that of all the species that have lived on Earth, more than 99 per cent are extinct.
This most recent extinction happened when an asteroid several miles wide, travelling at 12 miles a second – twenty times faster than a bullet – crashed into the Earth’s ocean. A huge crater off the Yucatan Peninsula in the Gulf of Mexico is the lasting evidence of this catastrophe. Scientists have found compelling evidence of the blast that has shaped the destiny of mankind.
A minute-by-minute account of the aftermath of the strike which hit the Earth, with the power of 10 billion atom bombs on one single violent day, has been pieced together using a rock sample from that 90-mile wide crater. The study confirmed the theory that it was not fire or flood that caused the dinosaur’s extinction. It was a sudden denial of sunlight.
Rather like the ice age 466 million years ago, the meteorite instantly vaporised millions of tonnes of rock as it pierced the Earth, forming a sulphurous cloud. Researchers estimate that at least 325 billion tonnes of sulphur were released. This blocked out the sun and plunged the Earth into a freezing, decade-long global winter.
The dinosaurs died, but our ancestors were able to thrive
Moments later, a vast tsunami was storming across North America. The impact ended the reign of the dinosaurs, giving way to the ‘Age of the Mammals’.
Excavation of the site has allowed scientists to recount in detail the impact of the collision and to understand better its likely longer-term effects. The crash unleashed about ten billion times as much energy as the atomic bomb that destroyed Hiroshima in 1945. Researchers conclude that much of life on the planet was first scorched and then – because of the weight of debris ejected into the atmosphere, which blocked out the sun – frozen and starved.
The research from the impact site focuses on a column of rock drilled from the central zone of the 90-mile wide Chicxulub crater.
In the 24 hours after the asteroid struck, about 130 metres of material was deposited, a rate of accumulation seldom matched in the geological record. According to Professor Sean Gulick of the University of Texas Institute for Geography, the layers tell a clear story: much of the life on earth was first fried then frozen.
The analysis has revealed the cataclysmic forces unleashed inside the crater in the minutes after impact, as well as holding clues about the longer-lasting effects of an event that would snuff out three quarters of life on the planet. The bottom 40 or 50 metres of the extracted column are melted rock and fragments of stones cemented by finer material deposited within minutes of the asteroid strike, amid clouds of super-heated dust, ash and steam.
Another 90 metres of material were deposited by ocean water flowing back into the vast crater caused by the tsunami backwash. The patterns show how the charred landscape was sucked into the crater as the ocean settled.
This was a momentous day in the history of life on Earth and this study is a very clear documentation of what happened at ground zero. The most prominent victims were dinosaurs. Fossil evidence shows these had dominated the Earth for about 230 million years.
Yet, there is no fossil trace of non-avian dinosaurs in rocks younger than 66 million years. They all perished. Without the mass extinction, humans would never have evolved. The dinosaurs died, but our ancestors were able to thrive.
The beneficiaries were mammals, which evolved from reptiles about 200 million years ago. Starting with early hominids some four million years ago in East and southern Africa, mammalian species multiplied after the dinosaurs’ extinction. Having been predominantly nocturnal creatures, our distant ancestors began to emerge in daylight.
Biologists stress that the evolution of life is not an inexorable process of greater complexity. Rather, the theory of evolution explains how organisms adapt to existing conditions. The evolution of sophisticated, self-aware intelligence has happened only once and in one place, among humanoid primates in Africa. Ultimately – according to Vanessa Hayes of the Garvan Institute in Sydney – some 130,000 years ago the first migration of modern human beings started in East Africa (arriving in Europe 45,000 years ago).
All the research underlines how chancy is our existence. A catastrophe of scarcely imaginable scale was our species’ greatest stroke of good fortune. Will climate change lead to another mass extinction?