The road to the Irish Gulch gold mine is rugged and winding. To get there, you head south out of Dawson, in Canada’s Yukon Territory, then bear right at the sign for GuggieVille, an RV park named after the Guggenheim family of New York. (The Guggenheims once managed a gold-dredging operation here.) From there, a washboard gravel road leads past a slew of small mines—the Lucky Lady, Last Chance, and Gold Run—then dead-ends at Irish Gulch.
The North American megafaunal extinction, as the event is known, was not an isolated incident. At various times, sudden, sweeping die-offs have been recorded in Madagascar, New Zealand, Australia, and South America. Unlike the meteor that hastened the demise of dinosaurs 65 million years ago, the North American megafaunal extinction has not been linked to a single cataclysm. Disease is one suspect, as are rapid climate changes and global warming. But the most popular scenario is simpler and bloodier: The megafauna were hunted to death by bands of humans that crossed the land bridge behind them.
Many paleontologists have argued that the people who crossed the land bridge into North America were too few and poorly armed to wipe out mammoths and scimitar-toothed cats. But Cooper, until recently, disagreed. He grew up in New Zealand, where he spent most of his youth spelunking through caves. On occasion, he would run across the remains of moas, extinct birds that once populated the country. Moas were tall, flightless, and evidently tasty: In the space of 300 years, the native Maori had wiped them out. “You find barbecue pits full of their bones,” Cooper says. “So I wasn’t stopped by the idea that these guys had primitive weapons.”
As it happens, moas were also the subject of Cooper’s first foray into ancient DNA: He spent a year in the late 1980s working on them at the University of California at Berkeley, when paleogenetics was still an embryonic field. Cooper first compared moa and kiwi DNA to see if the two flightless birds were closely related. Much to everyone’s surprise, they were not. Then he took a closer look. Taxonomists had long divided moas into two types, one of which, Dinornis, included three species: a 5-foot-tall bird, a 3-foot dwarf, and a 10-foot-tall, 550-pound “supermoa.” The bones of all three groups covered New Zealand, but after cloning and comparing their DNA, Cooper made a startling discovery: All three species were genetically identical. The so-called dwarf moa was actually the male, and the two taller birds were female. “Here we had thousands of bones of birds that went extinct only 500 years ago, and no one worked out that we were looking at males and females until we compared the DNA,” Cooper says. “Think of early human ancestors, where we have only scattered fossil fragments, and the problem of relying on morphology is obvious.”
