has yielded more than 100 fossil meteorites, found in rock layers that date to soon after the asteroid breakup. "The flux had to be enormous at this time," says Bill Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who has studied the breakup.

The meteorites fell about the time early animal life boomed. Major animal groups had already evolved, but during this Great Ordovician Biodiversification Event (GOBE), ocean animals doubled or even tripled in biodiversity. Reefs built by microbes gave way to some of the first coral reefs, trilobites grew larger, and tentacled predators such as nautiloids diversified and swarmed the mostly fishless seas. In 2008, Schmitz argued that the asteroid breakup might be responsible. Perhaps, he and colleagues proposed, large fragments striking Earth spurredthe GOBE by shaking up ecosystems, clearing out ecological niches for new species to evolve into.

But the idea didn't catch on, in part because there was no sign of large impact craters from just after the collision. Instead, some paleontologists offered a different trigger for the GOBE: a succession of ice ages that took place at the same time. Colder water can hold more dissolved oxygen, fueling life. And as water froze into glaciers, sea levels dropped, isolating shallow seas and creating niches for speciation.

Schmitz's team now thinks the asteroid breakup brought on those ice ages, by creating a cloud of dust that hung in space and in the atmosphere, reflecting sunlight away from Earth and allowing the ice to build up. In layers of limestone spanning just