One of the boldest assertions ever published in the scientific literature started with a single modest observation. In the late 1970s, geologist Walter Alvarez of the University of California at Berkeley and his father Luis, a Nobel Prize–winning physicist, found an unusual chemical signal in an ancient layer of Italian clay. The clay was enriched in iridium, a rare metal that comes mostly from meteorites, interplanetary dust, and other cosmic debris. The iridium spike appeared in sediments 65 million years old, at the so-called K-T boundary between the Cretaceous and the Tertiary periods. It coincided with the demise of the dinosaurs.
Contamination from local sources or a glitch in the iridium-counting machine could have explained the finding. But the Alvarezes found an even bigger spike in another Cretaceous-Tertiary deposit in Denmark. Their interpretation, published in 1980, was heretical.
The clay at the K-T boundary was high in iridium, they said, because it was made of the ash and dust from a six-mile-wide asteroid that had crashed into Earth with the energy of
100 million megatons of TNT. The impact instantly killed every living thing within hundreds of miles. The animals that weren’t incinerated or gassed by fumes froze or starved to death soon after, when dust kicked up by the impact blotted out the sun for more than a year, killing plant life around the globe. Dinosaurs were only the most conspicuous casualty of an epic disaster that eradicated half of all the species on Earth.
“Their idea was met by instant ridicule and derision by most geologists and paleontologists,” recalls paleontologist Michael Benton of the University of Bristol in a recent book. It took another decade of evidence gathering, including the documentation of an impact crater off the Yucatán Peninsula, for the impact theory to win acceptance, he notes. Now “ ‘Extraterrestrial Cause for the Cretaceous-Tertiary Extinction’ is considered . . . one of the most influential publications in earth sciences in the twentieth century,” Benton writes in When Life Nearly Died: The Greatest Mass Extinction of All Time.
