Life after disaster

Richard Gregory

The End of Evolution: Dinosaurs, Mass Extinction and Biodiversity By Peter Ward Weidenfeld and Nicholson Pounds 18.99 0 297 81475 3 STORY: This book has the most dramatic theme imaginable, extending from the start of life on Earth to our present concern for the human role in the life of our planet and whether something like the present is sustainable. As a geologist and paleontologist, the author is well qualified as a time traveller, reading the past in layers of rock. His account makes science fiction seem feeble by contrast.

His theme is that the Earth has encountered at least three devastating events, the first at the boundary between Palaeozoic and Mesozoic rock formations of almost 250 million years ago showing sudden death of protomammals - not over a period of ten million years, as often assumed, but very rapidly from some global disaster. The event itself is tantalizingly absent in the record, but the strata of the youngest Palaeozoic age, preserved in the United States and Europe, contain brachiopods and trilobites - then a sudden break, followed by clams, ammonites and new kinds of fish. The notion is that sudden death of many species opens the way for new evolutionary experiments. But what caused the first event remains unknown.

The second event is the mass extinction of 65 million years ago which included the death of the dinosaurs. A layer of clay, originally found in Italy, is associated with this event which lies on the Cretaceous-Tertiary boundary. In 1980, the distinguished scientists Luis and Walter Alverez, as Ward says "startled the scientific world with their theory that the great extinction closing out the Mesozoic Era had been brought about by lethal effects following the impact of a giant meteor with the earth". They predicted similar clay layers in other parts of the world, which have been found, and the evidence is growing that the hypothesis of bombardment from space is correct, the meteorite being some six miles in diameter, with far greater kinetic energy than all of our atom bombs. In this layer, around the world, are concentrations of iridium and platinum which are rare on earth, but common in asteroids.

An alternative disaster theory favoured volcanism, as iridium has been found associated with volcanoes. But when Bruce Bohor disaggregated the clay with solvents, he found mineral grains with microscopic lines on their surfaces identified as due to shock. Such lines are seen only at meteor impact craters and sites of thermo-nuclear bombs. Shocked quartz grains have now been discovered in many Cretaceous-Tertiary boundary sites world-wide.

The originally reported extra-terrestrial materials have not been confirmed for the first event - from which life returned much more slowly: "It took 10 million years before coral reefs once again reappeared . . . Not only was diversity greatly affected, but the composition of creatures as well, for so great was the marine extinction at the end of the Permian Period that they completely reset the composition of subsequent marine life. The Mesozoic oceans contained a suite of creatures almost completely different from those of the Palaeozoic." The time-evidence for corresponding extinction of land animals is less clear, but Peter Ward concludes, partly from his own work in the thick strata of the Karroo in South Africa, that they also died out very rapidly.

The third event is the entrance of mankind. Some two million years ago, our intelligence and tools changed the flora and fauna of the world even more drastically than a giant asteroid from space. Unlike the first two events, losses of species were not world wide and were mainly of large animals. Extinction of entire species could occur within 300 years.

Ward says, "Who wants to believe that the first Americans were hunters of such skill that they could destroy thirty-five genera (and many more species) of large mammals in a single millennium after arrival in North America? Yet this is an issue that must be clarified. If mankind could so quickly destroy the majority of the world's big game with primitive stone age technology, what hope have the world's creatures in the face of our far more advanced technology? If this hypothesis is false and the extinctions can be shown to have been caused by natural forces, such as the extensive climate change coming with the end of the Ice Ages, we face an even more disturbing set of implications. No-one disputes that the extinctions took place, or that they occurred very quickly. But if such massive extinctions can take place because of climate perturbations, the world's remaining biota is in very grave danger in light of what our species is currently doing to the global atmosphere . . .(This) alternative paints a horrifying picture for the next millennium in the earth's history, a time when the Third Event will be in full swing. Perhaps we can teach ourselves to stop killing animals and thus stave off the worst potential ravages of a mass extinction. But can we change the weather? Can we stop global warming?" There are paradoxes here, for it seems that without ancient disasters evolution would have got stuck, or more likely moved into different channels, so that nothing like mankind would have appeared. We owe our existence to disasters, yet we may be creating a disaster that could even be the end of evolution.

This is not a technical work, but rather a personal account from a professional enthusiast who is an expert on fossil ammonites, who shares with us his adventures in wild places, and even his political comments when he was digging in Georgia at the collapse of the Russian Empire. He sees analogies between mass extinctions and painful recovery through evolution and the traumas of politics. He concludes that although we are destroying the flora and fauna of our world, our intelligence is such that we, at least, may survive.

Richard Gregory is emeritus professor of neuropsychology at Bristol University.

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