Geologist Walter Alvarez was on an expedition in Italy during the early 1970s when he noticed something fascinating in the limestone mountains outside Gubbio: two dark layers of rock sandwiching a lighter, half-inch-thick seam. The darker sections were filled with fossils of microorganisms known as forams, while the center swath was virtually devoid of fossil life.
Alvarez and colleagues from Columbia University’s Lamont-Doherty Earth Observatory later determined that the middle layer was laid down at the exact time of the extinction of the dinosaurs. More exciting, he and his father, Nobel Prize–winning physicist Luis W. Alvarez, found that the fossil-free layer was rich in iridium, an element that is rare on the earth but relatively abundant in rocks from space.
Piecing those clues together, the two Alvarezes proposed a radical idea: The mass extinction that wiped out the dinosaurs 65 million years ago was caused by the impact of a giant asteroid, which unleashed a globe-spanning cloud of debris and plunged the planet into darkness for months. In 1990, geologists found the crater from this disaster off the north coast of the Yucatan peninsula, validating the impact theory for most of the scientific world.
After that triumph, Walter Alvarez, now at the University of California at Berkeley, began seeking other ways of using science to illuminate history—not just remote geological epochs, but also the events of the human era. “Naturally, a geologist thinks historically, and human history entangled itself with the work I was doing,” he says.
That thinking began to take concrete form one day when he received an e-mail from Fred Spier, a sociologist at the University of Amsterdam. Spier was working in the field of “big history,” a unified, multidisciplinary narrative of everything from the Big Bang to the present. He asked Alvarez if he would be interested in teaching a course on this topic, and Alvarez agreed. “It was a seed that fell on fertile ground,” he says.
