Tim Flannery’s book will forever change your perspective on the North American continent. The Eternal Frontier is history, but on a scale unimagined by most historians, for it begins with the scorching of our continent 65 million years ago and ends in the future. Flannery guides us on a sweeping odyssey through time, in which he synthesizes vast amounts of information from geology, paleontology, and human history. Crisis and recovery are the leitmotifs, as continents separate and collide with profound consequences for the earth’s climate and its living things.

Flannery is a distinguished paleontologist, accomplished writer, and director of the South Australian Museum. Why, one might ask, is an Australian writing about the history of North America? As a student of the evolution of mammals and the environments in which they lived, Flannery unavoidably took an interest in North America. Thanks to abundant fossil deposits well distributed over time, the history of mammals is writ more clearly on our continent than anywhere else. North America gave rise to some globally successful lineages—horses, camels, dogs—and served as a proving ground for numerous others as they migrated in and out over intermittent land bridges to other landmasses. But The Eternal Frontier is far more than a bestiary of extinct mammals. Half the book is a penetrating, introspective account of the role we humans are playing on the evolutionary stage. As an outsider, he sees us North Americans without blinders, and takes us to task for our foibles with a candor that will surely make some readers squirm.

The history of life on earth is largely driven by events of a kind scientists call nonlinearities, radical breaks from the status quo. Among them are continental mergers and breakups, drastic changes in climate, the opening and closing of corridors of intercontinental (or interoceanic) migration, and, of course, extinctions of species. Over one lifetime, such events are so rare as to escape the power of natural selection. No creature adapts to something that has never happened before. Thus the all-too-human response to the prospect of nonlinearities is denial. We do it as individuals when we smoke cigarettes or build expensive houses on Atlantic barrier islands, and we do it as a matter of public policy in connection with global climate change and the use of natural resources. One cannot come away from this book without being deeply unsettled by the repeated failures of our species to heed the warnings of impending non-linearities.

The story opens with one of the most extraordinary nonlinearities in the earth’s history, the moment when an asteroid the size of Manhattan sliced into the underbelly of North America at a speed of 15 miles a second, gouging a trough in the earth’s crust so broad that from one rim it would have been impossible for someone to see the other. The heat generated by the impact is estimated to have been a thousand times that received from the sun, enough to fry the continent. The existing forest of magnolias, araucarias, and palms was literally incinerated, and the Age of Dinosaurs came to an abrupt and untimely end.

The tale of what happened afterward can be read in the rocks. The impact itself is marked by a thin line containing high levels of iridium and an isotope of chromium found in a rare class of meteorites known as carbonaceous chrondrites. Lying above the iridium line is a lifeless deposit of gray clay representing fallout of the debris injected into the atmosphere by the blast. Above that is a much greater thickness of another clay eroded from the naked land. Much of America had effectively been sterilized.

Plants did not recolonize until long afterward. The first to arrive were ferns, which for thousands of years carpeted the landscape in a continent-sized thicket of excruciating monotony. When trees finally did return, they were entirely different from those that had been prevalent before the blast. More than 885 feet above the impact line, implying an immense passage of time, relatives of today’s magnolias, basswood, and dawn redwood reconstituted a credible forest. What was so bizarre about this forest was that its species had been previously restricted to the polar region. How odd that polar trees with a deciduous habit of annually shedding leaves had invaded the latitude of New Mexico, which then basked in an equable tropical climate. Northern trees remained important for ten million years because North America’s only connections to the rest of the world were polar. No botanist would ever conceive of such a strange sequence of events.

Meanwhile, in the Southern Hemisphere, the effects of the impact were moderated by the independence of the climate systems north and south of the equator, although dinosaurs and many other animals died there as well. Plants, however, fared much better, as indicated by the persistence of “vegetable dinosaurs” in such places as Tasmania, New Zealand, and New Caledonia.


Among the few survivors in central North America were species associated with fresh water, perhaps because the tiny algae that support the aquatic food chain found refuge on the bottoms of ponds and rivers. In the absence of either dinosaurs or large mammals, a strange collection of turtles, amphibians, and crocodiles appeared in what is now Montana and flourished for millions of years in a weird evolutionary reminiscence of the Paleozoic, a time some 200 million years earlier when archaic amphibious vertebrates first ventured onto land.

The revival of a terrestrial fauna waited half a million years until the first immigrants arrived from Asia, giving rise to an explosive growth of placental mammals in which fifty new genera appeared every million years. Eight million years after the impact we see a continent whose trees lose their leaves in a tropical climate, its forests altered in the absence of large herbivores, and a few survivors and immigrants rapidly filling the ecological vacuum. “It is an utterly foreign place,” Flannery writes, “as different from modern North America as is the island of Borneo today.”

Fifty-one million years ago the climate of North America was warm enough to make today’s Borneo seem positively fresh. The surface of the polar sea was a mild 14 degrees centigrade (58 degrees Fahrenheit), and the Dakotas were cloaked in tropical forest, even though they were 10 degrees of latitude north of where they are today. This greenhouse earth came to a decisive end 33 million years ago when Australia and Tasmania pulled away from Antarctica, making way for today’s Antarctic Circumpolar Current. Described as the “switch that turned on the modern world,” the circumpolar current served to isolate Antarctica from warm tropical water, an event that put Antarctica in an icehouse and brought about the birth of the South Polar Ice Cap.

The icecap in turn reflected sunlight away from the earth, which served to accelerate cooling. Global mean temperature dropped by 5 degrees centigrade and North America cooled a prodigious 8–12 degrees centigrade. The shock was so severe that 97 percent of marine snails and 89 percent of clams went extinct along the Gulf Coast. Deciduous trees spread, turtles, crocodiles, and alligators vanished, and mammal diversity plunged to a nadir.

The climate subsequently warmed again, but was drier than before. Vast tracts of North America became wooded savanna—ideal habitat for browsing and grazing mammals. Flannery writes:

North American fossil deposits of this age regularly yield the remains of fifteen or more species of mammals weighing more than five kilograms, a diversity never seen before or since in North America and in our time existing only in the richest grasslands of east and south Africa.

By 10 to 15 million years ago, some seventy to eighty genera of large mammals inhabited the continent, compared to today’s twenty-two.

Once again the inexorable drifting of the continents brought an era to a close. This time it was the formation, 2.8 million years ago, of the Panamanian isthmus. The warm, westbound North Equatorial Current, which had traveled unimpeded from Atlantic to Pacific, was now deflected to the north, creating the Gulf Stream. The proximity of warm water to the North American coast increased rainfall, which encouraged the spread of forests and contributed to the coming Ice Age.

The emergence of the Panamanian isthmus initiated an unprecedented biogeographic experiment. It simultaneously connected two previously isolated continent-sized landmasses and imposed a barrier in the midst of what had been a continuous seaway. What ensued on land was one of the most remarkable interchanges in the history of life, one that brought together an archaic fauna of marsupials and primitive placental mammals with a much more modern set of animal lineages, many of which remain prominent today.

North America had two advantages in the interchange: it possessed a larger area and it had been repeatedly connected to Asia, so that many of its species were not truly autochthonous but derived from an even larger landmass. The populations of many of these northern groups exploded when they reached South America, giving rise to large numbers of descendant species, most spectacularly rats and mice, which in four million years proliferated into 200 genera and 424 species. Other northern groups that became more numerous in South America were peccaries, horses, dogs, bears, weasels, cats, squirrels, rabbits, tapirs, camels, deer, and elephants.

Flannery tells us that southern creatures made their way north in almost equal variety, including opossums, porcupines, capybaras, primates, anteaters, armored glyptodonts and armadillos, ponderous mixotoxodonts, lumbering ground sloths, and a fearsome carnivorous bipedal bird weighing 880 pounds. As the southbound and northbound immigrants intermingled, they gave rise to one of the most unusual assemblages of animals ever to walk the earth. Surprisingly, the merger of disparate faunas did not lead to the prompt elimination of the archaic by the modern, since both coexisted for nearly three million years. However, immigrants from the south generally failed to diversify in the north, perhaps in part because the Ice Age intervened shortly afterward, greatly restricting the livable area of the North American continent.


Continental glaciation altered the face of North America almost as drastically as did the meteorite impact that undid the dinosaurs. The sheer magnitude of the ice sheet beggars the imagination. Stretching from British Columbia to Newfoundland, and from the high arctic to Wisconsin, ice accumulated to a depth of two miles, weighing down the land to create Hudson Bay and the contemporary shoreline of the Arctic Ocean. A mere 18,000 years ago, North America was buried under more ice than there is today in Antarctica. Iowa was tundra, and spruce trees grew in Florida. Yet, incredibly, most of North America’s fauna survived—not just one advance of the ice, but at least seventeen major episodes of advance and retreat over a period of more than two million years.

When at its fullest extent, the great ice sheet cut North America in two. South of the ice, encompassing most of what is the US today, was a cold, dry land occupied by a strange array of beasts derived from Eurasian, South American, and indigenous North American sources. Northwest of the ice sheet lay Beringia, a thousand-mile-wide land bridge between Asia and America that had been left exposed by a sea that had receded as much as 400 feet, while glaciers claimed so much of the world’s water. Beringia remained ice-free because the frozen Arctic Ocean offered no source of free water to generate precipitation.

One’s wildest daydream wouldn’t conjure up Beringia as it was during the Ice Age: a vast arid plain, colder (if that is conceivable) than today’s North Slope, and, of course, dark for nearly half the year. Surely, one would think, not a fit place for large animals. But large animals there were. Woolly mammoths, 11.5 feet tall at the shoulder and weighing up to thirteen tons—much larger than the largest African elephant—roamed the Arctic night grazing the “mammoth steppe” of tussock grass. Sharing this bleak world with the mammoths were bison, caribou, musk ox, and others, as well as their predators, wolves and lions—yes, the self-same lion that survives today only in dwindling portions of Africa and in one tiny reserve in India. In the day of megafauna, lions thrived over much of the Old World, including even the high Arctic.

The Ice Age was a time of gigantism. Seemingly everything grew bigger, including birds, perhaps partly as an adaptation to cold. South of the ice sheet, what is now the US supported a remarkable menagerie of outsized beasts. In addition to mammoths and mastodons, there were giant ground sloths, giant peccaries, giant armadillos, giant capybaras, giant beavers, and giant bison. With such prey, there were predators to match: the American lion was a heavyweight cousin of the Old World lion; the dire wolf was larger and more heavily built than the gray wolf; and perhaps the most fearsome beast to stalk the earth since Tyrannosaurus rex was the short-faced bear. Standing tall on long legs and capable of running down swift prey, the short-faced bear, at more than 1,500 pounds, was the largest mammalian carnivore ever to have trod the earth. Even stranger were the birds, for example Teratornis incredibilis, a predator borne aloft on sixteen-foot wings. But compared to its South American relative, Argentavis magnificens, Teratornis was small-fry. Argentavis had a wingspan equal to that of a small Cessna and weighed around 175 pounds, as much as an adult human. It was one of those creatures, like the bumblebee and pterodactyl, which in principle couldn’t fly but did.

The climate changes that drove the advances and retreats of the ice were typically abrupt, for there seemed to be no in-between condition. Either the northern latitudes were buried under a mile or more of ice or they were, as today, largely ice-free. The transitions were brusque and required animals to move great distances to find suitable habitats. Species of lemmings which today inhabit the high Arctic lived in Kansas as neighbors of the eastern chipmunk. My imagination fails me when I try to think of the habitat that could support them both. Mammoths, mastodons, giant long-horned bison, camels, horses, flat-headed peccaries (the commonest large mammal on the continent), pronghorns, saber-toothed cats, and a host of others adjusted to the changing times, for their remains are sprinkled liberally through the entire period.

From the reference point of today, some animals seemed strikingly out of place. Jaguars stalked the boreal forest, and cheetahs sprinted after pronghorns on the Great Plains. Notwithstanding the colder clime, peccaries, capybaras, and armadillos lived in mid-continent US, far to the north of their contemporary descendants. These apparent anomalies are not anomalies at all, but artifacts of the human invasion of the continent. We are greatly misled by taking the present status quo as a point of reference.

As the great North American ice sheet began to melt for the seventeenth time, about 14,000 years ago, a narrow, ice-free corridor opened to the east of the mountains, connecting Beringia to the lower half of the continent and opening an immigration route to the species that would forever change the face of America—our own. It remains controversial when the first humans arrived in the Americas, but there is wide agreement among specialists that a culture employing large and expertly crafted spear points appeared about 13,200 years ago. The first of these points was discovered near Clovis, New Mexico, and the culture that made them is now identified with that name. In the brief span of three hundred years or so after the first Clovis hunters appeared, the megafauna was gone—mammoths, mastodons, ground sloths, horses, camels, some thirty genera in all, including every animal on the continent weighing over 220 pounds, save two—the black bear and mule deer. Clovis hunters brought about an extinction crisis almost as drastic as the one that finished the dinosaurs.

Ever since University of Arizona paleontologist Paul Martin proposed the “megafaunal overkill hypothesis” twenty-eight years ago, his idea has been disparaged by skeptics who have passionately championed alternative versions of history. The mystery of the disappeared megafauna is by no means a new issue, for none other than Alfred Russel Wallace, the coauthor with Darwin of the theory of evolution by natural selection, recognized that “we live in a zoologically impoverished world, from which all the hugest, and fiercest, and strangest forms have recently disappeared.” Doubters have clung to all sorts of explanations—from “‘racial senility’ to disease, cosmic rays and even God’s desire to clear the way for His chosen people and their herds.” I myself suspect that the fierce intellectual resistance to Martin’s thesis has been inspired less by a weighing of the scientific evidence than by a subjective reluctance to accept that we humans single-mindedly and systematically destroyed the Garden of Eden. The most credible opponents of the overkill hypothesis have argued that climate change was to blame. But their argument completely fails to account for the fact that the megafauna quite handily survived all previous glacial advances and retreats and the climatic upheavals that accompanied them.

Subsequent research has left no doubt of the correctness of Martin’s inference. Throughout the world, large mammals and birds disappeared coincident with or shortly after the first evidence of human settlement. If climate change had been the cause, then one would expect to see some synchrony in the disappearances, but even in the glacial Northern Hemisphere, the megafauna disappeared at different times from different regions: before 14,400 years ago in northern Eurasia, and between 13,200 and 12,900 years ago in North America. Clinching the argument is the recent discovery of a frozen mammoth on Wrangell Island in the Arctic Ocean 125 miles north of Siberia. The mammoth died only 4,500 years ago, which is just when the first humans reached Wrangell, and about 10,000 years after the last Siberian mammoth perished.

Elsewhere in the world, except in Africa and southern Asia, where man or his evolutionary forebears have been present for at least the last 1.5 million years, the megafauna swiftly disappeared as soon as humans appeared on the scene: 50,000 years ago in Australia, 13,000 years ago in South America, 6,000 years ago in the Caribbean islands, 1,200 years ago or thereabouts in Madagascar, New Zealand, and other Pacific islands, including Hawaii. It is a disturbing fact to face, but the evidence overwhelmingly implies that we humans ate our way through most of the earth’s largest birds and mammals long before the invention of gunpowder. European sailors were able to exterminate the dodo and great auk only because of the failure of Paleolithic man to reach the remote island hideaways of these last survivors of a bygone age.

We now inhabit a drastically diminished continent, for what little was left undone by Clovis hunters was largely completed by Europeans in their westward march across North America. Fifty million bison were reduced to a pathetic remnant of six hundred. Wolves and grizzly bears were systematically exterminated and relegated to the sparsely inhabited north. Market hunters extinguished the passenger pigeon, the most abundant bird on earth, and then went on to decimate waders and waterfowl, eliminating the eskimo curlew and reducing other species to rarity. Even the common herring gull was fair game. So reduced was the eastern population that it survived in only one breeding colony.

The few large mammals that now embellish our western parks are not even true Americans and might not be here at all if it were not for the ecological vacuum left behind by the Clovis rampage. Moose, bison, elk, brown (grizzly) bear—all invaded via Beringia after the Clovis culture had been replaced by others that now had to depend on harvesting fruits and nuts more than on big game.

Growing recognition of these excesses of the past has engendered feelings of loss and nostalgia. Could America once again be home to a respectable megafauna? Flannery thinks yes, by introducing the closest living relatives of our lost animals. “Perhaps a century from now elephants will once again roam North America, together with large numbers of bison, llama, tapir, jaguar, camel and Chacoan peccary.” Of course, with so many herbivores, we would need predators, including lions, cheetahs, and grizzly bears. Other lost members of our fauna have no contemporary surrogates—glyptodonts, ground sloths, mixotoxodons—for their South American relatives were also liquidated by Clovis man. But still, even a half-reconstituted megafauna would be immensely exciting, with elephants, lions, horses, camels, buffalos, and more—all wild—right here in America!

Is such a dream pure fantasy, or might some such scenario one day become a reality? To me, Flannery’s vision seems improbable, but even now, megafaunal restoration of a kind is taking place, both officially and unofficially. Officially, wolves are being reintroduced to Yellowstone, Arizona, and North Carolina, and lynx to Colorado, through programs of the US Fish and Wildlife Service. Other releases are in the planning stage, all of them restorations of native species to native habitat.

As these carefully conceived efforts go forward, unofficial introductions threaten to take us in another direction. Feral hogs, many descended from introduced European boars, are already widespread in the Southeast, where they do great damage to native vegetation. Sika deer from Asia are established in parts of Maryland and Virginia and elsewhere. Donkeys roam the deserts of the Southwest. But these examples are peripheral. The main event is in Texas, where the habitat has the potential of supporting a megafauna as rich as that of South Africa’s Kruger National Park.

Texans have discovered that game ranching offers greater economic rewards than traditional cattle ranching, for big-city hunters are quite willing to pay thousands of dollars for a trophy specimen. At latest count, the state boasts some two hundred thousand “exotics,” among them oryx, fallow deer, red deer, axis deer, blackbuck, nilgai, giant eland, and many more, some sixty-five species in all. Inevitably, the animals escape, and already tens of thousands of nilgai and blackbuck from India run free in the Texas backcountry. Surreptitiously, America is in fact being restocked with megafauna. It is not the megafauna that roamed our plains and woodlands just a few thousand years ago, however, or even a surrogate, but one completely alien to our shores. Is this what we really want? It is a question we should be asking ourselves, for soon we shall be facing a fait accompli.

As for the restoration of a quasi-native megafauna, complete with lions and other large predators, I fear it is a pipe dream. Megafauna thrives on wide open plains, and most of our plains are occupied by farms and ranches. There is talk of creating a “buffalo commons” in parts of eastern Montana and the Dakotas, but elephants and lions aren’t part of the picture. Sadly, one must conclude that megafauna is a phenomenon of the frontier, and like it or not, the frontier has closed. Short of reorganizing our continent on a grand scale to make room for animals, I can’t see any way to restore what has been lost. More realistically, we can take steps to revive the fauna of America as it was when Europeans arrived. Bison and elk were then widespread in the east. Wolves and mountain lions were ubiquitous in fulfilling their crucial tasks as predators. Drawing on Flannery’s exhilarating book, we can still dream of a wild America, but it won’t ever be the way it was when the first Clovis hunters made their way south of the ice sheet and discovered a continent full of animals that knew no fear of humans.

This Issue

September 20, 2001