Durin hundreds of millions of years, the shape of the world changed out of recognition many times. Great rifts opened and closed in its surface, coasts rose and fell; at times huge areas were covered with a long-since vanished vegetation. Many species of plants and animals emerged and proliferated. Most died out. Yet these ‘dramatic’ events happened with almost unimaginable slowness. Some lasted millions of years; even the most rapid took centuries. The creatures who lived while they were going on could no more have perceived them than a twenty-first century butterfly, in its three weeks or so of life, could sense the rhythm of the seasons. Yet slowly the earth was taking shape as a collection of habitats permitting different strains to survive. Meanwhile, biological evolution inched forwards with almost inconceivable slowness. Climate was the fi rst great pacemaker of change. About 65 million years ago – an early enough point at which to begin to grapple with our story – a long warm climatic phase began to draw to a close. It had favoured the great reptiles and during it Antarctica had separated from Australia. There were no ice-fi elds then in any part of the globe. As the world grew colder and the new climatic conditions restricted their habitat, the great reptiles did not manage to adapt, though it is likely that it was a sudden event – the impact of a giant asteroid – that killed them off completely. But the new conditions suited other animal strains which were already about, among them some mammals whose tiny ancestors had appeared 200 million years or so earlier. They now inherited the earth, or a considerable part of it. With many breaks in sequence and accidents of selection on the way, these strains were themselves to evolve into the mammals which occupy our own world – ourselves included. Crudely summarized, the main lines of this evolution were probably determined for millions of years mainly by astronomical cycles and a few sudden events, such as eruption of massive volcanoes or the impact of asteroids. Climate was the all-important factor, changed by the earth’s position in relation to the sun or by short-term circumstances. A huge pattern emerges, of recurrent swings of temperature. The extremes which resulted, of climatic cooling on the one hand and aridity on the other, choked off some possible lines of development. Conversely, in other times, and in certain places, the onset of appropriately benign conditions allowed certain species to fl ourish and encouraged their spread into new habitats. The only major sub-division of this immensely long process which concerns us comes very recently (in prehistoric terms), slightly less than 4million years ago. There then began a period of climatic changes which we believe to have been more rapid and violent than most observed in earlier times. ‘Rapid’, we must again remind ourselves, is a comparative term; these changes took tens of thousands of years. Such a pace of change, though, looks very different from the millions of years of much steadier conditions which lay in the past.
How to Understand the Roots of History Best?
