Sixty-six million years ago, three-quarters of species — the dinosaurs among them — suddenly vanished (the Cretaceous–Paleogene extinction). The cause of this mass death was debated for a century: slow climate change, volcanoes, disease? In 1980 the Nobel-winning physicist Luis Alvarez and his geologist son Walter Alvarez proposed a striking answer with an unexpected piece of evidence.

While examining a thin clay layer in Italy that coincided with the moment of extinction, Walter noticed an oddity: in this layer the element iridium, extremely rare in Earth's crust, was tens of times more concentrated than it should be. Iridium is scarce on Earth but abundant in asteroids. The Alvarezes showed that the same iridium enrichment was found in layers of the same age all over the world. The only reasonable explanation was a vast source from the sky.

The hypothesis was this: an asteroid about 10 kilometres across struck the Earth; the dust and ash the impact threw up cloaked the atmosphere for months, cut off sunlight, halted photosynthesis, and collapsed the food chain. The iridium was the ash of this asteroid scattered across the world. The idea was at first met with great scepticism — geologists were wary of catastrophist explanations.

But when in the 1990s a giant impact crater 180 kilometres across (Chicxulub), of exactly that age, was found on the Yucatán peninsula in Mexico, the chain of evidence was complete. The Alvarez hypothesis is now widely accepted. This discovery not only explained the end of the dinosaurs; it showed that the history of life is shaped not only by slow evolution but also by sudden, random catastrophes from the sky. Even the rise of our own kind owes itself to the impact that cleared the way for mammals.