Impact by comets and asteroids

Although Norman Newell’s pioneering research was published in 1967, general interest in mass extinctions provoked by catastrophic changes in the environment was not aroused until 1980, when a paper appeared in the journal Science proposing that the end-Cretaceous extinction was caused by the impact of a huge asteroid. Before this time several people had suggested an extra-terrestrial cause for particular mass extinctions. Thus, in the middle of the twentieth century, the German palaeontologist Otto Schindewolf, who had long been preoccupied with the marine mass extinction at the end of the Palaeozoic era, concluded on the evidence of fieldwork in the Salt Range of Pakistan that the event must have been a catastrophic one for which he could literally conceive no earthly explanation. He was consequently led to speculate that the causal factor was a nearby supernova explosion. The increased cosmic radiation impinging on the Earth could, he thought, have destroyed the ozone shield and have led to lethal exposure of numerous organisms. A few other such speculations invoking some kind of extraterrestrial factor were put forward at about the same time, and in 1970 Digby McLaren, an expatriate British palaeontologist who had risen to become Director of the Canadian Geological Survey, made a startling proposal. He was an expert on the late Devonian marine mass extinction at the end of the penultimate, Frasnian, stage. Like Schindewolf, he agreed that the event was much too wide spread, dramatic, and ‘geologically instantaneous’ to have been caused by a merely terrestrial process, and he speculated that the world’s ocean of the time had been severely disturbed by the impact of a giant meteorite. Three years later, the American chemist Harold Urey, a Nobel Prize-winner, published a paper in the journal Nature in which he argued that several extinction events within the past 50 million years had been caused by the impact of comets. These various suggestions, together with a few others invoking increases in radiation from outer space, either in the form of cosmic radiation or solar protons, were virtually ignored. This is unsurprising in view of the almost total absence at that time of any supporting evidence, with the possible exception of a few tektite layers in Tertiary deposits.