turning up didn’t seem all that old. These weren’t heavily mineralized fossils embedded in solid layers of rock. Tusks, teeth, and jawbones at places like Kentucky’s Big Bone Lick were still strewn on the ground, or protruding from shallow silt, or on the floor of caves. The big mammals they belonged to couldn’t have been gone that long. What had happened to them?
The Desert Laboratory—originally the Carnegie Desert Botanical Laboratory—was built more than a century ago on Tumamoc Hill, a butte in southern Arizona overlooking what was then one of North America’s finest stands of cactus forest and, beyond that, Tucson. For nearly half the lab’s existence, a tall, broad-shouldered, affable paleoecologist named Paul Martin has been here. During that time the desert below Tumamoc’s saguaro-covered slopes disappeared under a snarl of dwellings and commerce. Today, the Lab’s fine old stone structures occupy what developers now covet as prime view property, which they continually scheme to wrest from its present owner, the University of Arizona. Yet when Paul Martin leans on his cane to gaze out his lab’s screened doorway, his frame of reference for human impact is not merely the past century, but the last 13,000 years—since people came to stay.
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
The best-known extinct colossus, the northern woolly mammoth, was only one of many kinds of
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.
A decade would pass before Paul Martin got to visit the opening in the red Grand Canyon sandstone wall above the Colorado River where his first sloth dung ball had been collected. By then, extinct American ground sloths had come to mean much more to him than simply more oversized mammals that had mysteriously toppled into oblivion. The fate of sloths would provide what Martin believed was conclusive proof of a theory forming in his mind as data accumulated like layers of stratified sediment. Inside Rampart Cave was a mound of dung deposited, he and his colleagues concluded, by untold generations of female sloths who took shelter there to give birth. The manure pile was five feet high, 10 feet across, and more than 100 feet long. Martin felt like he’d entered a sacred place.
When vandals set it on fire 10 years later, the fossil dung heap was so enormous that it burned for months. Martin mourned, but by then he had been setting blazes of his own in the paleontology world with his theory of what had wiped out millions of ground sloths, wild pigs, camels,
“It’s pretty simple. When people got out of Africa and Asia and reached other parts of the world, all hell broke loose.”
Martin’s theory, soon dubbed the Blitzkrieg by its supporters and detractors alike, contended that, starting with Australia about 48,000 years ago, as humans arrived on each new continent they encountered animals that had no reason to suspect that this runty biped was particularly threatening. Too late, they learned otherwise. Even when hominids were still
The first Americans, Martin believes, were the ones who expertly produced the leaf-shaped flint projectile points found widely throughout North America. Both the people and their lithic points are known as Clovis, named for the New Mexico site where they were first discovered. Radiocarbon dates of organic matter found in Clovis sites have sharpened past estimates, and archaeologists now agree that Clovis people were in America 13,325 years ago. What exactly their presence signifies is, however, still a matter for hot dispute, beginning with Paul Martin’s premise that humans perpetrated the extinctions that killed off three-fourths of America’s late Pleistocene megafauna, a menagerie far richer than Africa’s today.
