11. The Americas
Their arrival in the Americas in the late Pleistocene brought humans into a world with a layout very different from that of Australia. Whereas Australia is an island, South America is permanently joined to North America in the present geological epoch, and North America in turn is joined to Asia during ice ages. Just as striking is the contrast in climatic range. Australia is confined to temperate and tropical bands within a single hemisphere; North America extends far into the Arctic, while the tip of South America comes within some hundreds of miles of Antarctica. But the advantages in intercontinental competition are not all to the Americas. Although the Americas are far richer than Australia in terms of the number of climatic bands they contain, none of these bands has a length comparable to those of Eurasia. This matters, because innovations—most obviously domesticated plants, but other things too—spread more easily within climatic bands than between them. Land may stretch continuously for some 8,500 miles from Alaska to the Tierra del Fuego; but from east to west, the dimensions of the Americas are far less impressive—around 3,000 miles where each continent is at its widest. This distinctive layout does not in itself tell us much about the habitat humans would find when they reached the Americas, but it does yield a first approximation. We can already start to wonder how human societies would fare in such an environment.
In the Americas, just as in the Old World, we find a broad tropical band that includes large expanses of rainforest (though there would have been much less of it in the more arid conditions of the last ice age). The distribution of the tropical band between the two continents is, however, very unequal. South America gets the lion's share, since this continent extends on both sides of the equator and is at its widest in the tropics; whereas only the southernmost part of North America lies within the tropical band, and at this latitude the continent is at its narrowest.
North and south of the tropics, each continent has its temperate band; this may be forest, grassland, or desert, depending on the rainfall. In North America the temperate belt falls where the continent is at its widest, and we encounter substantial amounts of all three kinds of terrain: forest in the east, grassland in the middle, and desert in the southwest. The makeup of temperate South America is similar, but the continent is much narrower at these latitudes (though somewhat wider in an ice age).
Beyond the temperate bands lie the Arctic and Antarctic regions of the two continents. North America has a large amount of land as far north as the Arctic Circle, and this territory is similar in character to the Arctic regions of the Old World: as one goes north, a belt of Arctic forest—the taiga, in Old World parlance—gives way to a bare, open wilderness—the tundra. South America, by contrast, stops well short of the Antarctic Circle, and possesses only small areas of taiga and tundra.
The single most important thing missing from this sketch is the mountains. Unlike Australia, the Americas have their share of geologically recent mountain chains. These run from north to south, against the grain of the climatic bands, and are located along the western side of each continent (where the seafloor is being sub-ducted under the edges of the American continental plates). In the north the highlands are broad but not particularly elevated; the best- known component of the system is the Rocky Mountains. In the south the Andes tend to be both narrower and higher—but where they form two parallel ranges with a plateau in between, they provide a kind of corridor running north and south. These highland areas confuse our simple picture of climatic bands, but they matter enormously.