By Monastersy, Richard
Science News , Vol. 151, No. 9
In the fall of 1973, a young geologist named Walter Alvarez explored a limestone gorge just outside the medieval walls of Gubbio, Italy. Chipping away at the layers of rock with his hammer, Alvarez stumbled onto something that would revolutionize how scientists view our planet and the history of life.
Alvarez, together with his father, Nobel prizewinning physicist Luis W. Alvarez, and two other researchers at the University of California, Berkeley, discovered a pencil-thin layer of clay containing unearthly amounts of the element iridium. In 1980, the team interpreted the iridium layer as evidence that a huge comet or meteorite slammed into Earth 65 million years ago with a strength 10,000 times the explosive power of the global nuclear arsenal.
This planetary concussion, posited the scientists, offered the long-sought explanation for the disappearance of the dinosaurs and many other animals at the end of Earth's Cretaceous period.
The Berkeley team's work focused unprecedented attention on events at the close of the Cretaceous, making this distant time one of the best-studied moments in Earth's history. William Glen, a geophysicist and historian who has tracked the impact debate, counted more than 2,500 papers and books published on the topic by 1993. In terms of its influence on future science, Glen ranks the impact hypothesis even above that of the plate tectonics revolution of the 1960s.
"The Alvarez hypohesis has provided a stimulus to the earth sciences that I think, eventually, will turn out to be virtually unprecedented in this century," says Glen. "The Alvarezes started a new industry. The implications of the idea are enormous."
Initially, the Alvarez hypothesis walloped scientists almost as hard as the Cretaceous cataclysm itself would have. The idea had such a drastic effect because it challenged one of the most revered tenets of geology and evolutionary biology-a concept of gradual change known as uniformitarianism.
Championed by Charles Lyell in 1830, this doctrine supplanted the once-popular theories that cast ancient catastrophes such as floods and volcanoes as the major architects of change on Earth.
Uniformitarianism teaches that the history of the world has been shaped by the same slow processes that can be seen today wearing away mountains or replacing one species with another. Because no one has ever witnessed a planet-wrenching impact, uniformitarians regarded such events as an outlandish explanation for past extinctions.
Evidence of the Cretaceous impact mounted quickly, culminating in the discovery in the early 1990s of a giant crater hidden beneath the Yucatan Peninsula. The success of the Alvarez hypothesis has prompted researchers to begin focusing on other crises in Earth's history, some even more devastating than the Cretaceous catastrophe. …