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17 December 2014
The asteroid that smashed into the Yucatan Peninsula a little more than 66 million years ago left behind the Chicxulub crater, but it also left behind something else: iridium, a rare element, which settled in a fine layer all over the world. When scientists discovered this layer between rock strata in the 1980s, it eventually led them to the crater as well, and an explanation for the disappearance of the dinosaurs. But on either side of that layer, which serves as a geological boundary between the Cretaceous and the Paleogene, determining the age of rock is more difficult. This fuzziness makes it harder for paleontologists to piece together the timeline of life’s evolution after the mass extinction, which included the emergence of humans and all other mammals.