Sometime between the demise of the dinosaurs and the rise of primates. Earth suffered a one-two punch from two unrelated asteroids. The double whammy could signify chaos in the inner solar system, triggering a cascade of events that perhaps plunged Earth into the recent ice age. The impacts that made the Popigai crater in Siberia and the Chesapeake Bay crater on the US East Coast were two of the largest asteroid strikes since the one that killed the dinosaurs, so big that their dust left thin layers of debris all over the globe. The depth of debris layers shows that the asteroids struck in quick succession - within 20,000 years of each other, around 36 million years ago. A Popagai-sized impact happens just once every 20 million years. So the thinking was that the two rocks were fragments of one larger asteroid.
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