National Geographic : 2011 Oct
• Not the same planetary fever exactly; it was a di erent world the last time, around 56 million years ago. The Atlantic Ocean had not fully opened, and animals, including perhaps our pri- mate ancestors, could walk from Asia through Europe and across Greenland to North America. ey wouldn't have encountered a speck of ice; even before the events we're talking about, Earth was already much warmer than it is today. But as the Paleocene epoch gave way to the Eocene, it was about to get much warmer still---rapidly, radically warmer. The cause was a massive and geologically sudden release of carbon. Just how much car- bon was injected into the atmosphere during the Paleocene-Eocene ermal Maximum, or PETM, as scientists now call the fever period, is uncertain. But they estimate it was roughly the amount that would be injected today if human beings burned through all the Earth's reserves of coal, oil, and natural gas. e PETM lasted more than 150,000 years, until the excess carbon was reabsorbed. It brought on drought, oods, insect plagues, and a few extinctions. Life on Earth survived---indeed, it prospered---but it was drastically di erent. Today the evolution- ary consequences of that distant carbon spike are all around us; in fact they include us. Now we ourselves are repeating the experiment. e PETM "is a model for what we're star- ing at---a model for what we're doing by playing with the atmosphere," says Philip Gingerich, a vertebrate paleontologist at the University of Michigan. "It's the idea of triggering something that runs away from you and takes a hundred thousand years to reequilibrate." Gingerich and other paleontologists discov- ered the profound evolutionary change at the end of the Paleocene long before its cause was traced to carbon. For 40 years now Gingerich has been hunting fossils from the period in the Bighorn Basin, a hundred-mile-long arid plateau just east of Yellowstone National Park in north- ern Wyoming. Mostly he digs into the anks of a long, narrow mesa called Polecat Bench, which juts into the northern edge of the basin. Polecat has become his second home: He owns a small farmhouse within sight of it. One summer a ernoon Gingerich and I drove in his sky blue '78 Suburban up a dirt track to the top of the bench and on out to its south- ern tip, which a ords a ne view of the irrigated elds and scattered oil wells that surround it. During the recent ice ages, he explained, Pole- cat Bench was the bed of the Shoshone River, which paved it with cobbles. At some point the river shi ed east and began cutting its way down through the so er and more ancient sediments that ll the Bighorn Basin. Meanwhile the Clark's Fork of the Yellowstone River was doing the same to the west. Polecat Bench now stands between the two rivers, rising 500 feet above their valleys. Over the millennia its anks have been sculpted by winter wind and summer gully washers into rugged badlands, exposing a layer cake of sedi- ments. Sediments from the PETM are exposed right at the very southern tip of the bench. It is here that Gingerich has documented a great mammalian explosion. Halfway down the slope a band of red sediment, about a hundred feet thick, wraps around the folds and gullies, vivid as the stripe on a candy cane. In that band Gingerich discovered fossils of the oldest odd- toed hoofed mammals, even-toed hoofed mam- mals, and true primates: in other words, the rst members of the orders that now include, respec- tively, horses, cows, and humans. Similar fossils have since been found in Asia and Europe. ey EARTH HAS BEEN THROUGH THIS BEFORE. Environment editor Robert Kunzig wrote about population for the January issue. is is Ira Block's 37th story for the magazine.