The Alvarez impact hypothesis is published

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Luis Alvarez, Walter Alvarez, Frank Asaro and Helen Michel published in Science the measurement of anomalously high iridium in the boundary clay at Gubbio and Stevns Klint, and the inference that a body about ten kilometres across had struck the Earth and injected enough dust into the stratosphere to suppress photosynthesis. The paper is unusual in that its central evidence is a single element abundance rather than a fossil or a structure, and the argument depends on a fact about planetary differentiation rather than about biology. It was resisted for most of the following decade, partly because no crater was known, partly because it looked like catastrophism, and partly because sampling of the fossil record near the boundary appeared at the time to show a gradual decline — an appearance later attributed in large measure to the difficulty of sampling the last occurrence of a rare species.

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