or millions of years before the first humans came, big animals migrated to and from North America, crossing to Asia on the Bering land bridge and to Europe on a long-drowned land passage that once connected Canada to Greenland. Camels first evolved in North America; the Nebraska camel stood about twice as tall as its modern counterparts in Asia. Horses also originated here. But the last native American horses died in the late Pleistocene; their kind only returned millennia later, carried over the ocean by Spanish conquistadors who unwittingly performed an act of Pleistocene rewilding.
For some scientists, steeped in this broader view of creatures that globe-trotted long before humans could influence their travels, the idea of reshuffling megafauna among the continents makes sense. Robinson, for one, is an enthusiastic supporter of Pleistocene rewilding. "One important reason to study Pleistocene paleoecology is to help inform current conservation and restoration goals," he says. "Why should Columbus and the conquistadors be our reference point?"
Just outside the Fordham University campus, on a hectic street in the Bronx, grows a honey locust tree, its trunk and branches studded with forbidding six-inch thorns. The fruits are designed to lure the giant herbivores that, in times long gone, spread the locust seeds; the thorns deterred mastodons and giant sloths from munching on the rest of the tree.
After spending a couple of days with Robinson, I've begun to see Pleistocene ghosts everywhere. We can never raise the mastodon, yet the beast persists, its absence brooding over uneaten pods of honey locust in New York City and new housing developments sprouting on the ancient peat of Orange County. I have my own deep reservations about Pleistocene rewilding: The assumption that we know what we're doing when we tinker with nature has so often been proved wrong. But I've begun to understand the people who are passionate advocates for the idea.
For Robinson, analyzing the catastrophe that hit North America's giant mammals is more than an exercise in armchair time travel. It's about understanding where we've come from as a species, and deciding where we're going. A penchant for driving other creatures to extinction may be among the defining human traits, along with a big brain and opposable thumbs. Or maybe it's just a very old bad habit, one that looking backward can help us change.