The researchers found that there was a 60% chance we were hit in the past 1 billion years or a 90% chance if one looks back 5 billion years—consistent with previous estimates based on preliminary models of GRBs. It’s possible, then, that a nearby GRB could have caused one of the mass extinctions recorded in the fossil record. In fact, the Ordovician extinction, which occurred around 440 million years ago, had certain signatures of a GRB-induced event, such as a greater extinction rate among surface-dwelling marine life as compared with bottom-dwellers