We live on a planet shaped by violent extraterrestrial visitors: close encounters of the asteroid and comet kind. That idea, once considered controversial, now lives thoroughly in the mainstream. Even school kids know that a 6-mile-wide asteroid strike at the end of the Cretaceous Period, 65 million years ago, helped usher the dinosaurs offstage. Evidence suggests similar impacts have contributed to other regional and global extinctions as well, possibly including the Permian-Triassic event 251 million years ago, the greatest recorded wipeout of them all.
Yet in many ways, scientists have barely begun to comprehend the full, complex relationship between life on Earth and death from above. All of the well-documented mass extinctions have occurred within the past 550 million years. Most of the known impact craters are no older than that, either. That may sound like a long time, but life on this planet began at least 3.5 billion years ago, meaning that more than 80 percent of the story is missing. It is a vast, highly obscured and oft-overlooked stretch of history that a few dogged researchers are only now piecing together.
Bruce Simonson of Ohio’s Oberlin College is one of the leading sleuths. He and the few others in this small field have excavated evidence of 15 enormous asteroid strikes that occurred between 1.7 billion and 3.5 billion years ago. “We’ve documented a minimum of four big impacts right around the end of the Archean [2.5 billion years ago], all of which I would bet money are at least as big as the one at the end of the Cretaceous,” he says. Those impacts marked, in essence, the final stages of Earth’s development into the planet it is today.
