Teroid Impacts

The Extinction Hypothesis

In 1980, geologist Walter Alvarez, with his father, the Nobel-Prize-winning physicist Luis Alvarez, [Page 65]and nuclear chemists Frank Asaro and Helen Michel, proposed that a layer of iridium-enriched clay found in a gorge in northern Italy was evidence of an impact by a 10-km-wide asteroid. They predicted that a similar layer would be found at other sites around the world, a prediction later confirmed. They also presented geochemical evidence that showed that the layer was located at the boundary between the Cretaceous and Tertiary periods (the K-T boundary) and thus was contemporary with the extinction of the dinosaurs. Their paper, published in the journal Science, reached a multidisciplinary audience of scientists. Physicists, geologists, astronomers, paleontologists, and evolutionary biologists joined in debate about the validity of the impact hypothesis. Broadly, while physical scientists were quick to accept the hypothesis on the basis of the geological evidence and astronomical knowledge about asteroids, life scientists rejected the hypothesis as unable to explain the full complexities of the fossil record.

The ensuing controversy, with its newsworthy combination of dinosaurs, global catastrophe, and celebrity scientists—for example, television astronomer Carl Sagan and paleontologist and popular science writer Stephen Jay Gould offered their views—was readily converted into newspaper and magazine articles and television documentaries. Time magazine, for example, published a brief account of the Alvarez team's findings a year before their paper appeared in Science, and the magazine returned to the story a further five times in the following years. It helped that dinosaurs already had a significant presence in public culture. Although earlier researchers had proposed impacts as an extinction mechanism and others had recorded the concentration of particular metals in the K-T boundary clay, the identification of a specific impact event at the time of the dinosaur extinction ensured that the Alvarez hypothesis reached a wide audience.

Both media reports and commentaries in scientific journals helped draw the geochemical findings to the attention of scientists in disciplines other than geology and geochemistry. A survey in the mid-1980s found that 30% of British and German paleontologists reported having first heard of the impact extinction hypothesis in scientific commentaries rather than in research papers, and a further 10% reported first hearing of it in the mass media.

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