In science, as in most fields of human endeavour, fashion plays a role. Two decades ago, evidence was discovered that the dinosaurs (and a great many other, less well-known, creatures) were exterminated by a collision between the Earth and an extra-terrestrial rock. The evidence came in the form of a layer of clay rich in iridium that has been identified in sites all around the world, and appears to be the result of such a collision. One decade ago, a crater that seemed to be the same age as this layer was identified in southern Mexico. Since then, it has become fashionable to look for evidence of impacts at the time of the other four so-called mass extinctions that the record suggests have happened since fossils became abundant 545m years ago. Conversely, alternative explanations for mass extinctions, such as the huge volcanic eruptions that often seem to coincide with them, have fallen out of fashion.
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