The World Without Us

In 1956, a year before arriving here, Paul Martin had spent the winter in a Quebec farmhouse, during a postdoctoral fellowship at the University of Montreal. A case of polio contracted while collecting bird specimens in Mexico as a zoology undergraduate had rerouted his research from the field to the laboratory. Holed up in Canada with a microscope, he studied sediment cores from New England lakes that dated back to the end of the last ice age. The samples revealed how, as the climate softened, surrounding vegetation changed from treeless tundra to conifers to temperate deciduous—a progression some suspected led to mastodon extinction.

One snowbound weekend, weary of counting tiny grains of pollen, he opened a taxonomy text and started tallying the number of mammals that had disappeared in North America over the past 65 million years. When he reached the final three millennia of the Pleistocene epoch, which lasted from 1.8 million until 10,000 years ago, he started to notice something odd.

During the time frame that coincided with his sediment samples, starting about 13,000 years ago, an explosion of extinctions had occurred. By the beginning of the next epoch—the Holocene, which continues today—nearly 40 species had disappeared, all of them large terrestrial mammals. Mice, rats, shrews, and other small fur-bearing creatures had emerged unscathed, as had marine mammals. Terrestrial megafauna, however, had taken an enormous, lethal wallop.

Among the missing were a legion of animal kingdom Goliaths: giant armadillos and the even-bigger glyptodonts, resembling armor-plated Volkswagens, with tails that ended in spiked maces. There were giant short-faced bears, nearly double the size of grizzlies and, with extra long limbs, much faster—one theory suggests that giant short-faced bears in Alaska were why Siberian humans hadn’t crossed the Bering Strait much earlier. Giant beavers, as big as today’s black bears. Giant peccaries, which may have been prey to Panthern leo atrox, the American lion that was considerably bigger and swifter than today’s surviving African species. Likewise, the dire wolf, the largest of canines, with a massive set of fangs.

The best-known extinct colossus, the northern woolly mammoth, was only one of many kinds of Proboscidea, including the imperial mammoth, largest of all at 10 tons; the hairless Columbian mammoth, which lived in warmer regions; and, in California’s Channel Islands, a dwarf mammoth no taller than a human—only the collie-sized elephants on Mediterranean islands were smaller. Mammoths were grazing animals, evolved to steppes, grassland, and tundra, unlike their much older relatives, the mastodons, which browsed in woods and forests. Mastodons had been around for 30 million years, and ranged from Mexico to Alaska to Florida—but suddenly they, too, were gone. Three genera of American horses: gone. Multiple varieties of North American camels, tapirs, numerous antlered creatures ranging from dainty pronghorns to the stag moose, which resembled a cross between a moose and an elk but was larger than either, all gone, along with the saber-toothed tiger and the American cheetah (the reason why the sole remaining pronghorn species of antelope is so fleet). All gone. And all pretty much at once. What, Paul Martin wondered, could possibly have caused that?

The following year, he was on Tumamoc Hill, his big frame again perched over a microscope. This time, rather than pollen grains saved from decay by an airtight covering of lake-bottom silt, he was viewing magnified fragments preserved in a moisture-free Grand Canyon cave. Soon after he arrived in Tucson, his new boss at the Desert Lab had handed him an earthen gray lump the approximate size and shape of a softball. It was at least 10,000 years old, but unmistakably a turd. Mummified but not mineralized, it yielded identifiable fibers of grasses and flowering globe mallow. The plentiful juniper pollen Martin found confirmed his subject’s great age: temperatures near the floor of the Grand Canyon have not been cool enough to sustain juniper for eight millennia.

The beast that excreted it was a Shasta ground sloth. Today, the only surviving sloths are two tree-dwelling species found in the Central and South American tropics, small and light enough to quietly inhabit rain forest canopies far from the ground, out of harm’s way. This one, however, was the size of a cow. It walked on its knuckles like another of its surviving relatives, the giant South American anteater, to protect the claws it used to forage and to defend itself. It weighed half a ton, yet it was the smallest of the five sloth species that lumbered around North America, from the Yukon to Florida. The Florida variety, the size of a modern elephant, topped three tons. That was only half the size of a ground sloth in Argentina and Uruguay, which at 13,000 pounds stood taller than the largest mammoth.