The ecological history of this great continent is the subject of Australian naturalist Tim Flannery's new book, The Eternal Frontier. Flannery, director of the South Australian Museum, covers the vast geological period from the demise of the dinosaurs to the contemporary era of global warming.

His narrative is framed by two events from North America's past that have decisively altered the basic patterns of life on Earth. The first occurred 65 million years ago, at the end of the Cretaceous Period, when a meteorite crashed near present-day Yucatan, on the southern boundary of North America. The impact of this wayward body, estimated to have been six miles in diameter, triggered a stupendous tidal wave, ignited vast conflagrations of the continent's forests, and darkened the sky with menacing clouds of pulverized rock and harmful gases for as long as six months. Many scientists, Flannery among them, believe that this cataclysm led to the extinction of the dinosaurs, which were unable to cope with this natural equivalent of "nuclear winter."

The second of North America's decisive events began as recently as the 1500s with the arrival of European settlers on its shores. These pioneers unknowingly brought killer diseases such as smallpox that wiped out millions of the indigenous peoples. The Europeans' other "contribution" was an aggressive hunger for land and profit, which Flannery views as the origin of today's global corporate economy.

These events, according to Flannery, were exceptional only in the extent of their impact. In many ways, the changes wrought by the Cretaceous extinction and the era of European expansion are typical of the entire range of North American history. Drawing inspiration from historian Frederick Jackson Turner's famous "frontier thesis," enunciated in 1893, which explained the dynamics of the United States' history up to that point in terms of the westward march of its pioneering citizens, Flannery projects a scenario of migrations and adaptations across the vast ages of North America's past.

To Flannery, North America is the classic example of frontier development. The wandering herds of animals that ventured across the prehistoric land bridge from Asia, the nomadic tribes of Native Americans, and the modern-day arrival of biological invaders like Dutch elm disease and other blights are as much a part of the frontier experience as was Daniel Boone.

The Eternal Frontier provides brilliant insight into the process of natural adaptation under adverse circumstances. For the ecosystems of North America have repeatedly had to adjust to more intense shifts in climate than have those of other continents - shifts that have had dramatic effects on the composition of its vegetation and animal life.

The author explains the contintent's climate problems on the basis of its unique shape. Wide at its northern boundary near the Arctic Circle, the continent narrows to a wedge as it extends southward. Fluctuations in weather patterns are intensified by the north-south orientation of its two major mountain ranges, which dominate the opposing coastal areas, leaving between them a vast central plain. North America is thus exposed to every vagary of climate in amplified form, from the onset of a new ice age to violent storms that sweep down from Canada to pound the southern grasslands and deserts.

During its most temperate period, the Miocene, 24 to 5 million years ago, North America experienced a time of bounty when its animal life grew in numbers and diversity. During this long era, the great elephant-like mastodons and gomphotheres, rhinos, fearsome running bears, and the ancestors of the horse (which migrated to Asia before it perished in North America) roamed the vast continent.

Flannery views this tendency toward diversity as another of North America's hallmarks. The multiplication of tribal groups, cultures, and distinct languages that followed the arrival of the first human immigrants from Asia 13,000 years ago further illustrates the continent as a place of plenty.

Flannery's assessment of the ecological impact of the first human inhabitants of North America is likely to be one of the most controversial aspects of his book. Although sympathetic to the later suffering of Native Americans at the hands of European settlers, Flannery does not portray them as paragons of ecological virtue or insight. For example, he maintains that the spear points of the Clovis people, the earliest of the groups to migrate from Asia, accounted for the extinction of the mammoths, giant sloths, and other mammalian species that had managed to survive the last ice age. The Clovis hunters' killing spree lasted roughly three centuries.

While this is no doubt true, Flannery's rather breathless account of the subsequent Native American cultures and civilizations does not do justice to their accomplishments. Although many of the Mayan city-states and the Toltec civilization of Mexico flourished only briefly, their long-term contributions were significant. Many of the world's staple crops, such as maize, were first cultivated by Native American peoples.

The next group of immigrants, particularly the English-speaking settlers, launched another orgy of frontier destruction, which certainly deserves Flannery's criticism. Beginning with the Jamestown settlements in 1607, it reached a climax with the slaughter of the bison herds in the 1880s and of the Sioux Indians at Wounded Knee in 1890. Along with these atrocities, the European domination of North America created an entire culture of heedless waste, including of underground water sources and the very soil itself.

Flannery's narrative makes sobering reading. Perhaps surprisingly, the author is optimistic, believing that the peoples of North America will respond to the looming environmental challenges left in the wake of European-Americans’ sense of "manifest destiny." If that optimism proves justified, then the response of North America's inhabitants will mark the opening of a new kind of frontier, a harmonious adaptation to the environment of a continent where change is truly evidence of life.