Many scientists argue that a single disastrous event caused the extinction not only of the dinosaurs, but also of a large number of other species that coexisted with them. In 1980, the American physicist Luis Alvarez (1911–1988) and his geologist son Walter Alvarez (1940–) proposed that a large comet or meteoroid struck Earth sixty-five million years ago. They pointed out that the sediments at the boundary between the Cretaceous and Tertiary periods contain a high concentration of the element iridium—an element that is rare on Earth, but not in space. This iridium anomaly has since been discovered at more than fifty sites around the world.

In 1990, tiny glass fragments, which could have been caused by the extreme heat of an impact, were identified in Haiti. A 110-mile (177-kilometer) wide crater in the Yucatan Peninsula, long covered by sediments, has been dated to 64.98 million years ago, making it a leading candidate for the site of this impact. A hit by a large extraterrestrial object, perhaps as much as 6 miles (9.3 kilometers) wide, would have had a catastrophic effect upon the world’s climate. Huge amounts of dust and debris would have been thrown into the atmosphere, reducing the amount of sunlight reaching the surface. Heat from the blast may also have caused large forest fires, which would have added smoke and ash to the air. Lack of sunlight would kill off plants and have a dominolike effect on other organisms in the food chain—including the dinosaurs.