Jan Zalasiewicz, of the University of Leicester who has spent much of his professional life studying the beautiful shapes left behind by these long-gone animals, now thinks he's found the perfect marker for the Anthropocene, or the new epoch of humans, so dubbed for Homo sapiens’s world-changing impacts. It's a rather precise start date, thanks to some unusual isotopes: July 16, 1945, at 5:29 in the predawn New Mexico desert. That's when U.S. scientists exploded the world's first atomic bomb and when the human-induced radioactive isotope clock started ticking.

The three isotopes in question are cesium 137 and plutonium 239 and 240, which will take millennia or more to decay. There are no known natural sources of cesium 137. As a result of the subsequent detonations of hundreds of such weapons around the globe, there will be plenty of these isotopes still around far into the future. Like the meteorite that helped end the Cretaceous period about 65 million years ago, and possibly the reign of the dinosaurs as well, the nuclear detonation may mark for future geologists a turning point in Earth's history.

Nuclear blasts may prove best marker of humanity's geologic record