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.
