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Asteroids may have had a deeper impact on Earth than previously thought. Scientists have found evidence that the planet could owe its biological diversity to a 10m-year bombardment of giant rocks about 470m years ago.
They suggest that the break-up of a huge asteroid at that time meant Earth was repeatedly struck by its fragments at a key period in the evolution of life. Before the impacts, Earth’s animal life was dominated by a small number of species. After they began, however, the fossil record shows that the number of life forms soared.
The scientists involved in the research suggest that the ecological disruption caused by the impacts spurred existing life forms to evolve much faster — eventually giving rise to the ancestors of dinosaurs, mammals and fish.
The theory makes a stark contrast to the common view that meteorites are destructive. The best-known impact, which caused the giant Chicxulub crater on the coast of Mexico, happened 65m years ago and is thought to have wiped out the dinosaurs.
The theory that other meteorites helped evolution is set out in a new book, Incoming!, by Ted Nield, editor of Geoscientist, the journal of the Geological Society of London.
He said: “The picture emerging from new research is that around 470m years ago a stupendous collision in the asteroid belt bombarded the Earth with meteorites and this may have been responsible for the single greatest increase in biological diversity since the origin of complex life.”
Nield’s book is based partly on the work of Birger Schmitz, professor of bedrock geology at Lund University in Sweden. He analysed rock samples taken from five Swedish quarries, finding that the layers of rock from 470m years ago contained an amazingly high number of fossilised meteorites.
The number of meteorites that hit the earth at present is about one for every 7,700 square miles per year. In the Swedish quarries Schmitz found a 100-fold increase in meteorite strikes, starting 470m years ago and lasting around 10m years.
He said: “The original asteroid that broke up was perhaps a few hundred kilometres in diameter so there were a lot of fragments. The size would have been up to 10km — the same size as the one that wiped out the dinosaurs.
“We think that there were quite a few really big impactors and these would have had the power to change life on earth.”
Schmitz has travelled to China and other parts of the world where rocks of a similar age and type can be sampled — and found similar increases in the number of meteorites.
The evidence that Earth underwent an intense bombardment is increasingly accepted — but did this really cause the sudden increase in life forms also seen in the fossil record over the same period?
Here, Schmitz is more cautious. He said: “The theory is controversial because most scientists believe that meteorite impacts cause extinction.
“This is true, but 470m years ago diversity was pretty low — so there were far fewer species to wipe out. Our theory is that the impacts changed the climate and geology of the earth and species had to adapt rapidly to big changes or go extinct.”
The fossil record suggests that at the time the bombardment started Earth was going through an evolutionary lull. The first great expansion of animal life, the Cambrian explosion, had taken place about 80m years earlier. During the Cambrian period animals with hard parts to their bodies, such as shells, appeared, leaving a fossil record. After that, however, evolution appears to have paused. Most life was marine-based and there were relatively few species.
Nield points out that the Earth was a different place then, with most of the land concentrated in the southern hemisphere around a single giant supercontinent that researchers call Gondwanaland.
With relatively few complex species on land, the most advanced forms of life were primitive jawless fish and squid-like cephalopods, mostly living in shallow seas. Something influenced evolution — and meteorites could have been the cause.
