Imagine a world with only half the variety of large animals that we know today.

Imagine an Africa with hyenas but no lions, an Australia with wombats but no koalas, a North America with elk but no bison.

Imagine zoos and televised nature programs featuring rhinos without hippos, giraffes without gorillas, zebras without camels, leopards without cheetahs. The missing animals simply do not exist; we know them only from fossils.

Without realizing it, we are in exactly this situation today. In what paleontologists have begun to call "near time," the last 50,000 years, datable by radiocarbon, the world lost half of its 200 genera of large mammals (those weighing more than 45 kilograms or 100 pounds). Beyond the living bears, bison, deer, moose, and other large mammals familiar to us now, an additional 30 genera and over 40 species lived in North America, and even more in South America. Most of the Western Hemisphere's charismatic large mammals no longer exist. As a result, without knowing it, Americans live in a land of ghosts.

Some of these great creatures—the extinct megafauna—appear in popular museum displays in our large cities. Even the names of others are utterly unfamiliar to most of us. North America lost mastodons, gom-photheres, and four species of mammoths; ground sloths, a glyptodont, and giant armadillos; giant beavers and giant peccaries; stag moose and dwarf antelopes; brush oxen and woodland musk oxen; native camels and horses; short-faced bears, dire wolves, saber-toothed and dirk-toothed cats, and an American subspecies of the king of beasts, the lion. After the extinctions, the mean body mass of North American mammals was the lowest it had been in 30 million years (Alroy 1999).

The survivors of the big wipeout are those large animals familiar to us now, such as bison, brown (grizzly) bears, cougars (mountain lions), deer, elk (wapiti), moose, musk oxen, and pronghorns. Most people regard these as defining "wild America." They do not. To give so little attention to the dozens of big animals we have lost so recently simply sells North America short. Before extinction of our native big mammals, the New World had much more in common with an African game park than most of us realize.

South America also lost heavily. Extinction struck many species of ground sloths, one monster weighing over 4,500 kilograms (10,000 pounds) (Fariña, Vizcaíno, and Bargo 1997). The biggest native herbivore in the New World tropics today is Baird's tapir, which may reach 225-300 kilograms (about 500-650 pounds).

Australia lost giant animals of its own. Though not as massive as the largest in the Americas, they included giant wombat-like creatures the size of rhinos, giant kangaroos larger than any of the living kangaroos, many other large marsupials, and even some oversized koalas and echidnas. (Echidnas, or spiny anteaters, differ from all other mammals in that their shell-covered eggs are incubated and hatched outside the mother's body.)

If we could travel back just those 50,000 years—a third of the age of our species in Africa, but a mere i/80,000th of the roughly 4.5-billion-year age of the Earth—we would find ourselves in a "Quaternary zoo" far more spectacular and much richer in species of large mammals than any zoo that exists today.

Any fan of modern wombats—muscular 4-foot-long marsupials resembling badgers with elongated koala faces—would delight in seeing diprotodons, which looked like one-ton wombats. The diprotodons and their entire family suffered extinction over 40,000 years ago.

Those who enjoy America's modern armadillos would take particular pleasure in the glyptodonts, another extinct family. The size and overall shape of a giant tortoise or of a Volkswagen "beetle" (Hulbert 2001), glyptodonts were completely armored in bone and had long, muscular tails ending in a club or a mace-like cudgel, presumably used to beat off attackers.

We would gaze in awe at mastodons, mammoths, and gomphotheres, all relatives of modern elephants. The American display would also feature several species of ground sloth, some of them as large as the mammoths. The Shasta ground sloth was about the size of a large black bear. The public would flock to the viewing platform at feeding time, when buckets of this animal's favorite vegetable, mallows in the hollyhock family, would attract patient mothers carrying their young on their backs. It would be harder to feed the biggest ground sloths, elephant-sized Megatherium, able to reach high into trees, pulling down branches with their long arms and heavy claws. Some paleontologists suspect they may also have been scavengers, eating carcasses of large dead animals.

The carnivore displays would be equally impressive. The short-faced bear, Arctodus, exceeded all living bears in size and probably in speed. The famous saber-toothed cat (Smilodon) was about the size of today's African lion, with curved 7-inch-long upper canines, while the canines of the scimitar cat (Homotherium) were "only" 4 inches long. Among the other carnivores were a subspecies of lion, Panthera leo atrox, as well as the dire wolf, Canis dirus, along with the dhole, Cuon, a wild dog that survives in Asia.

These are some of the more spectacular mammals we would see in a Quaternary zoo, side by side with our familiar bears, bison, and hippos. If the zoo included birds and reptiles, we would also see New Zealand's moa—10 extinct species of flightless, hairy-looking birds, the largest of them bigger than ostriches. Among the wondrous Australian giants were fearsome monitor lizards weighing up to 150 kilograms (330 pounds); two terrestrial crocodiles; and a giant extinct python.

All of these animals were present on the planet until well into the lifetime of our own species. Why are they gone from the Earth today? In this book I argue that virtually all extinctions of wild animals in the last 50,000 years are anthropogenic, that is, caused by humans. To get our history right we need to know more about the extinctions of near time. And we need to give thought to reversing prehistoric extinctions when we have the chance. That leads us to the most controversial vista of them all, the contemplation of bringing back the elephants and representatives of other lineages that evolved over tens of millions of years in the Americas.

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