Born in 1940, Walter Alvarez is a geologist and author from Berkeley, California. Alvarez studied geology at Carleton College and Princeton University, where he completed his PhD in 1967. Alvarez worked as a research geologist for American Overseas Petroleum and then for the Lamont Geological Observatory at Columbia University. In 1981, he became a professor at the University of California, Berkeley. Alvarez continues to work at the Earth and Planetary Science graduate school at Berkeley, researching global tectonics, Earth history, and asteroid impacts.

Alvarez is best known in his field for his “impact hypothesis,” the theory that an asteroid impact triggered the mass extinction of the dinosaurs and other species at the end of the Cretaceous period. While researching plate tectonics in Gubbio, Italy, Alvarez became interested in the KT boundary: the rock layer marking the end of the Cretaceous period and the beginning of the Tertiary period. The sudden shift in materials and fossils in the rock convinced Alvarez that an abrupt mass extinction event occurred at that time, and he hypothesized that a catastrophic event, such as an asteroid impact, was the cause. When he formally articulated this hypothesis in the early 1980s, it was highly controversial among geologists: Many doubted that a catastrophic event could trigger mass extinction.