He begins sixty-five million years ago, with the catastrophic impact of an asteroid, ten kilometres wide, and travelling at a speed of twenty-five kilometres per second which abruptly brought to an end the Mesozoic age of dinosaurs. This stupendous collision which occurred in Chicxulub, northern Mexico, formed a vast crater and dissipated the energy equivalent to one hundred million megatons of high explosive. Flannery describes this event as if he were a CNN reporter flying high over the scene and seeing in real time the effects of a cauterising heat and shock wave directed northwards 7,000 kilometres across most of North America, flattening, burning, and destroying most of the trees and animal life. Whereas the continent itself took the full effect of the initial impact, its final effect was more insidious, with dust thrown up into the upper atmosphere, shutting out the sun’s rays, and causing a long dark winter, leading to global extinctions of entire plant and animal communities. It was only in the southern hemisphere that there was survival of the ‘vegetable dinosaurs’, namely the Huon pines of south-west Tasmania, the kauris, and araucarias of the rainforests of New Zealand and New Caledonia, and the few Wollemi pines growing in a remote canyon in the Blue Mountains west of Sydney. Now, with the dinosaurs gone, there was an evolutionary opportunity for a radiation of the flowering plants and mammals; and as Flannery laconically puts it, ‘the Cenozoic era was at hand’.

Catastrophism as a decisive agent in biological change is once more scientifically respectable, as is dynamism within the geological process. The latter has been due to the theory of continental drift and plate tectonics, originally proposed at the beginning of the twentieth century by the German Alfred Wegener, and after being subject to seventy years of academic derision, suddenly in the late 1960s became the new paradigm. This allows a gripping sense of narrative to re-enter the geological discourse, and it is in this vein that Flannery takes us with erudition and passion through the vast transformations of landscape and of faunas and floras that have characterised the biological history of the North American continent over the past fifty million years. A problem with such a narrative, of course, is that unlike at Chicxulub, most of these changes, from a human perspective, are almost infinite in scale; and too breezy a storyline may sometimes mask as much as it reveals.

Flannery has two principles to unify his tale. The first is one of geographical configuration: that the North American continent is shaped like a gigantic wedge with an open mouth, 6,500 kilometres wide in the sub-Arctic and its apex a few degrees away from the equator. This is flanked by two great north–south mountain ranges, the Appalachians and the newer Rockies, which can swiftly funnel vast quantities of cold air southwards into the heart of the continent, and vice versa northwards from the Gulf of Mexico. This he refers to as the ‘climatic trumpet’, which acted as a huge amplifier of any climatic variation, so that North America, more than any other continent, has tended to oscillate between two modes – a tropical and a frigid one, with virtual replacements of biotas on a continental scale at each transition. The second principle is one of crude size. The most fundamental law of biogeography states that the diversity of species on any land mass is directly proportional to its area, and, in addition, the evolutionary trajectory of a species is found within the core of its geographical range. Thus the question that Flannery calls the ‘heart of the matter’ is whether North America has exported its superior fauna; or, conversely, has it been a net recipient of animals that first evolved elsewhere?

A marked and sudden cooling of the earth some thirty-three million years ago due to the severing of the last tectonic link between Tasmania and the Antarctic, which allowed the uninterrupted circulation of the Antarctic circumpolar current, turned the planet from a tropical glass house into an ice house, the effects on the faunas being accentuated in North America, which saw the beginnings of marked seasonality and the first expansion of deciduous forests. In addition, tectonically, both India and Europe join onto Asia; the new accretions giving a decisive increase to the size of the Eurasian continent, which from then on became the ‘ecological superpower’. North America, dwarfed in size, and climatically robbed of a significant amount of its biological productivity, became a recipient world of waves of new immigrants. Here is biogeography on a heroic scale, and it explains the ‘eternal frontier’ in the title.

The second half of the book seeks to apply some of these biological processes to analyse the two major waves of human immigrants to North America. The first occurred about 12,000 years ago, at the very end of the last Glacial period, with the sudden incursion of hunting people using, with devastating effect, a technology of deadly flaked-stone projectile points. Called the Clovis culture, after the find site in New Mexico, these people hunted herds of mammoth on the plains with such effect that they initiated a rapid cascading collapse of the late Pleistocene megafauna, beginning with the mammoth and the mastodon, and within a few hundred years moving down to the giant bison and so on. Such was the speed of human advance that they had reached the Straits of Magellan 10,000 years ago. The scale of the extinction of the targeted large animals was unprecedented, and Flannery’s view is that no event, except the asteroid extinction, had left such an impoverished ecosystem. In a search for cause, Flannery strongly supports the hypothesis of a human agency. He draws interesting parallels with the situation in Australia, where it appears that there was also a massive episode in the extinction of giant marsupials sometime slightly more than 40,000 years ago and intriguingly close to the time of arrival of the ancestral Aborigines.

The legacy of Columbus brought European man to the New World, with Eurasian domestic animals and infectious diseases which so affected the recipient cultures, often with devastating effect. For Flannery, his historical analysis of the process of colonisation of North America, owes much to the historian Frederick Jackson Turner’s seminal essay ‘The Frontier in American History’, written in 1893, whereby ‘the wilderness masters the colonist’ and in a process of cultural stripping ‘the immigrants were Americanised, liberated, and fused into a mixed race, English in neither nationality nor characteristics’. The frontier itself across the great American plains was bountiful in its legacy of soil, a product of the glaciers which the geologist Louis Agassiz had called ‘God’s great plough’. There was an explosion of productivity and of population, which laid the economic basis of today’s economic superpower. Yet, in the sudden mass extinctions of bison and of passenger pigeon, in the speed by which Oklahoma wheatlands degenerated into dust bowls, there is also a niggling worry about the long-term stability of these ecological systems; that, in Turner’s words, when the frontier closes, ‘the stubborn American environment (will be) there with its imperious summons to accept its conditions’.

Flannery suggests that America’s pre-eminence as today’s dominant power has been won at a great cost – eighty per cent of the continent’s wildlife, more than half its timber, the damming of most of its rivers, and a depletion of many of its soils. This is not some clichéd green political jeremiad, but is a critique strongly rooted on biological principles. This is a book both entertaining and thought provoking. It was worth spending a year at Harvard, in the halls of Agassiz and Peabody, to have written it.