The Eternal Frontier

The most surprising thing Flannery does is refuse to treat North America's animals as native. Over deep time, almost everything came from somewhere else. The continent spent tens of millions of years connected, at one edge or another, to Asia and to South America, and its history is largely a record of what crossed over. When the sea levels dropped or land bridges rose, animals poured in; when the connections closed, whatever had arrived was left to adapt or vanish on its own.

The Bering land bridge is the recurring gateway to the west. Across it, at various times, came the ancestors of horses in one direction and, much later, mammoths and bison and eventually humans in the other. Flannery enjoys the irony that horses actually evolved in North America, spread to Asia, and then died out at home — so that the animal Europeans later reintroduced was, in the deepest sense, a returning native. The traffic ran both ways, and the continent was as much an exporter of species as an importer.

If animals give the book its drama, plants give it its architecture. Flannery devotes real attention to the slow rise of North America's forests, and especially to its deciduous trees — the oaks, maples, hickories and their relatives that shed their leaves each autumn. In the warm, wet world of the early recovery, subtropical and evergreen forests spread widely. As the global climate cooled and grew more seasonal over tens of millions of years, a new strategy emerged: dropping leaves to survive a cold, dark season and flushing them again in spring.

That strategy turned out to be quietly powerful. A deciduous forest is not a passive backdrop. It cycles enormous quantities of water and carbon, feeds the soil with a yearly fall of leaf litter, and shapes the climate around it as much as the climate shapes the trees. Flannery describes the great eastern hardwood forests as an ecosystem the continent effectively invented for itself — a response to seasonality that then became a driver of the conditions animals had to live in.

Step back from the individual episodes and Flannery's larger argument comes into focus. His title is not decoration. He means that North America has functioned, across its whole post-impact history, as a frontier — a place defined less by what lives there than by its permanent openness to what might arrive next. Other landmasses spent long stretches isolated, evolving in private. North America kept its doors ajar, gathering immigrants from Asia and South America, absorbing invasions, weathering extinctions, and reinventing itself after each disturbance.

A frontier, in this sense, is not a line on a map but a condition. It is what you get when a place is never allowed to finish becoming itself. The asteroid set the pattern by clearing the stage; the land bridges kept it going by delivering new players; the ice ages reshuffled everything one more time. Each upheaval that might have ruined a settled continent instead gave this one another chance to recombine. Flannery's continent thrives precisely because it is unstable — because collision and arrival are its normal state, not its exception.

Sixty-five million years after the sky fell over the Yucatán, the continent that took the worst of that blow is unrecognizable — and, in Flannery's reading, entirely consi