From Alfred Crosby, Ecological Imperialism: The Biological Expansion of Europe, 900-1900.

Chapter One:
Pangaea revisited, the Neolithic reconsidered
GOD SAID, 'Let the waters under heaven be gathered into one place, so that dry land may appear'; and so it was. God called the dry land earth, and the gathering of the waters he called seas; and God saw that it was good.
-Genesis I: 9-10
 

THREE SLENDER THINGS THAT BEST SUPPORT THE WORLD: the slender stream of milk from the cow's dug into the Pail; the slender blade of green corn upon the ground; the slender thread over the hand of a skilled woman.
- The Triads of Ireland (ninth century)
 
I.

IT IS NECESSARY TO BEGIN at the beginning in considering the Neo-Europes (areas like the United States, which have climates similar to Europe and which were taken over by Europeans), and that means not in 1492 or 1788 but about 200 million years ago, when a series of geological events began that brought these lands to their present locations. Two hundred million years ago, when dinosaurs were still lolling about, all the continents were jammed together in one great supercontinent that the geologists call Pangaea., It stretched over scores of degrees of latitude, and so we can assume that it had some variations in climate; but with only one land mass, there would not have been much variety among its life forms. One continent meant one arena for competition, and so only one set of winners in the Darwinian struggle for survival and reproduction. Reptiles, including all the dinosaurs, were the dominant kinds of land animals in Pangaea - and, therefore, the world - for three times as long as mammals have held that position since, and yet reptiles diversified into only two-thirds as many orders.

About 180 million years ago Pangaea began to break up like some immense tabular iceberg rotting in the heat of the Gulf Stream. First it split into two supercontinents, and then into smaller units that became, in time, the continents we know. The process was more complicated than we can describe here (indeed, more complicated than geologists completely understand as yet), but, in broad terms, Pangaea broke up along lines of intense seismic activity that later became undersea ridges. The most thoroughly examined of these is the Mid-Atlantic Ridge that boils and bubbles from the Greenland Sea to Spiess Seamount, twenty degrees of latitude and twenty of longitude southwest of Cape Town, South Africa. From this and other ancient drowned cordillera, lava poured (and in many cases still pours), generating new ocean floor and carrying continents on either side of a given ridge farther and farther from each other. Where these floors, moving away from the ridges that spawned them, back into each other, they curl downward into the earth's mantle, grinding and grating, sometimes scuffing continental mountain ranges up to the skies, sometimes creating underwater trenches, the deepest features on the surface of the planet. Geologists, who sometimes have a stony insensitivity to nuances, call this awesomely vast and eon-consuming activity "continental drift."

When mammals succeeded dinosaurs as the globe's dominant land animals and began to diversify into their myriad orders over the past few score million years, the separations of the continents seem to have been at their extremes, certainly more so than today, and there were large inland seas partitioning South America and Eurasia into two subcontinents each. On these fragments of Pangaea, life forms developed independently, and in many cases uniquely. This helps to account for the remarkable degrees to which mammals diversified and the speed at which they did so.

Continental drift largely accounts for the differences, often extreme, between the flora and fauna of Europe and those of the Neo-Europes. A European traveler sailing to any of the Neo-Europes must cross one or more of these undersea ridges and trenches. Europe and the Neo-Europes have not been part of the same continental mass for many millions of years (except for ephemeral Arctic connections between North America and Eurasia), years during which the ancestors of American buffalos, Eurasian cattle, and Australian kangaroos shambled and hopped down diverging paths of evolution. To cross these undersea seams is to step from one of those paths to another, almost to step into another world. (There are seams that are not under water and do not separate continents, but let us ignore them for brevity's sake.)

When Pangaea first split into northern and southern supercontinents, only North America of all the Neo- Europes was in the same supercontinent with Europe, and so the two have shared the same latitude and have had anciently similar histories. The floral and faunal differences between Europe and North America are less striking than the differences between either of them and the other Neo-Europes. Even so, their differences were enough to take the breath away from the Finnish naturalist, Peter Kalm, in Philadelphia and fresh off the boat from Europe in 1748:

I found that I was now come into a new world. Whenever I looked to the ground I found everywhere such plants as I had never seen before. When I saw a tree, I was forced to stop and ask those who accompanied me, how it was called ... I was seized with terror at the thought of ranging to many new and unknown parts of natural history.

Biogeographers have properly designated North America and Eurasia, including Europe, as different biological provinces or subregions. After all, Nero threw Christians to the lions, not to the cougars. As for the other Neo-Europes, there is no doubt about their deserving biogeographical categories separate from that of Europe. All three, for instance, have large - some of them man-sized - flightless birds.

The breakup of Pangaea and the decentralization of the processes of evolution began 180 or 200 million years ago. For almost all the time since, except for a few instances counter to the dominant trend (e.g., the periodic rejoining of North America and Eurasia by reappearances of the Bering land connection, and consequent intermixing of biota), centrifugal forces have prevailed in the evolution of life forms. This trend, prevalent since some of our distant mammalian ancestors made a living by stealing dinosaur eggs, halted about half a millennium ago (a tiny fraction of a single tick on the geological clock), and centripetal forces have dominated since. The breakup of Pangaea was a matter of geology and the stately tempo of continental drift. Our current reconstitution of Pangaea by means of ships and aircraft is a matter of human culture and the careening, accelerating, breakneck beat of technology. To tell that tale, we have to go back not 200 million years, fortunately, by only a million or three.

II.

The most adaptable and therefore most widely distributed of today's large land animals are human beings, and this has been true of the members of the species Homo sapiens and their hominid predecessors for a very long time - long from their point of view. Other creatures had to wait for specific genetic changes to enable them to migrate into areas radically different from those of their ancestors - had to wait for incisors to lengthen into daggers before they could compete successfully with hyenas on the veldt, or had to wait for hair to thicken into fur before they could live in the north - but not humans nor hominids. They made not a specific but rather a generalized genetic change: They developed bigger and better brains wired for the use of language and for manipulation of tools.

That growth of nerve tissue crammed into the treasure box of the skull began several million years ago, and as it did, the hominid became increasingly capable of "culture." Culture is a system of storing and altering patterns of behavior not in the molecules of the genetic code but in the cells of the brain. That change made the members of the genus Homo nature's foremost specialists in adaptability. It was as if the fisherman in the fairy tale to whom the princely flounder granted three wishes had asked first of all for all the wishes he would ever want.

These plump-brained apes made use of their new skills of adaptability to migrate out of their ancestral home (probably Africa) and across dry Pangaean seams into Eurasia. Ever since, hominids and humans have migrated; they seemingly have tried to occupy every crack, cranny, and niche above the low-tide line. Our ancestors (Homo erectus), with brains, on average, hundreds of cubic centimeters smaller than ours, increased in numbers, migrated through out the Old World tropics, and, by 750,000 years ago, moved into the northern temperate zone, taking up residence in Europe and China. By about 100,000 years ago, the human brain was as large as it is today, which is probably as large as it ever will be. We may or may not have folded in a few convolutions since, but there is no doubt that the actual physical development of the brain in our species was completed by 40,000 years ago, when Homo sapiens (the wise man!) appeared, face daubed with whatever raw pigments nature provided in the neighborhood, and a sharpened or stone-tipped stick clenched in his fist.

Humans were in occupation of the Old World from Europe and Siberia to the southern tip of Africa and the islands of the East Indies. Yet there were whole continents and myriads of islands we had not explored or settled. We had not yet passed over one of the expanding, deep-water seams of Pangaea.

III.

These early humans were about to do something of the same magnitude as moving from earth to another planet. They were about to leave a world - the riven core of Pangaea, Eurasia plus Africa - of life forms with which their ancestors had lived for millions of years and go to worlds where neither humans nor hominids nor apes of any kind had ever existed, worlds dominated by plants, animals, and microlife whose forms had often diverged sharply from the patterns of life in the Old World.

The new worlds were North and South America and Australia. (To get to New Zealand, a land mammal had to be a bat or an excellent sailor, and Homo sapiens arrived there tardily.) Members of the genus Homo had been in the East Indies for most of the time that the genus had existed; the waters between those islands were warm and the straits narrow, and the shallow strait between New Guinea and Australia becomes dry land during an ice age. Members of our species turned south and walked into Australia some 40,000 years ago, giving that continent its first large placental mammal. The second, the dog called the dingo, arrived some 8,000 years ago or even more recently. (These dates and others cited in this chapter are matters of controversy in which we need not involve ourselves. We are interested in sequences, not absolute dates.)

There is evidence that a number of species and even genera of Australian marsupials and reptiles, creatures considerably larger than those of historical times, disappeared at roughly the same time that humans spread through that continent. The temptation is to promote chronological coincidence to the level of proof and to blame those extinctions on the invaders, though it strains credulity to claim that Stone Age human hunters alone killed off Australia's giants. They may have had assistance from diseases that came south with them from the East Indies. They did have fire, which Aborigines in historical times have used to burn over vast areas of the continent annually, and in ancient times this practice conceivably could have altered the habitats of the giants sufficiently to make life and reproduction impossible.

Getting from the East Indies to Australia was a matter of crossing a few warm, narrow straits; getting to the Americas was something more difficult. The problem was not the cold, foggy, dangerous waters of the Bering Strait; indeed, that strait has been a broad highway of tundra for much of the time since the arrival of members of the genus Homo in Siberia. The problem was the hostility of the climate in the high latitudes. There were few human beings in Siberia to follow the herds of caribou and such across Beringia to Alaska, and once in Alaska the early migrant human fetched up against a continental ice cap that occupied much of America north of Mexico. There were warm periods that opened a corridor south from Alaska to Alberta and beyond, but all in all the pedestrian's passage from Asia to the lush grasslands and forests of North America was a miserably difficult one.

Humans probably did not arrive south of the North American ice cap until long after they set foot in Australia, but in the New World, as in Australia, there seems to have been a coincidence between the arrival of human big-game hunters and the extinction of many species of large mammals: mammoths, mastodons, giant ground sloths, giant buffalos, and horses, for example. Some individuals of these giant species were incontestably killed by humans - we have found stone spear points between the ribs of fossil mammoths - but most experts are reluctant to attribute the extinction of whole species to these human hunters. Again, humans may have been only part of a wave of invading species, including parasites and pathogens, that attacked the native fauna. But why would the latter concentrate on the larger mammals? Why and how would anything but humans kill mostly those animals representing the largest meals? Homo sapiens found a hunter's paradise in Australia and the Americas. All three continents were chock-full of toothsome herbivores utterly inexperienced in defending themselves against human aggressors, providing the newcomers with seemingly inexhaustible quantities of protein, fat, hide, and bone. Expansion of Homo sapiens into Australia and the Americas must have led to a very large increase in the total number of humans on the earth. The Americas and Australia were Edens to which God added Adam and Eve very tardily. "There can be no repetition of this," wrote Francois Bordes in The Old Stone Age, "until man lands on a hospitable planet belonging to another star."

IV.

Some 10,000 years ago all the larger ice caps melted, excepting those in Antarctica and Greenland, and the oceans rose to approximately their present levels, inundating the plains that had connected Australia with New Guinea, and Alaska with Siberia, and isolating the avantgarde of humanity in their new homelands. From that time until the Europeans made a common practice of sailing across the seams of Pangaea, these peoples lived and developed in complete or nearly complete isolation. One of the momentary respites from divergent evolution since the breakup of Pangaea had come to an end, and for the next few millennia genetic drift and, for the first time, cultural drift were in perfect consonance with continental drift.

V.

Then humanity made its next giant lunge, not a matter of geographical migration but of cultural mutation: the Neolithic Revolution or, more accurately, Revolutions. According to classic definition, the Neolithic Recolution began when humans started to grind and polish rather than chip their stone tools into final form, and it ended as they learned to smelt metal in quantity and work it into tools that stayed sharp longer and were more durable than their stone equivalents. In between, the story goes, humans invented agriculture, domesticated all the animals of our barnyard and meadow, learned to write, built cities, and created civilization. The complete story would be a great deal more complicated, but this definition will serve for our purposes.

The technological avant-garde of humanity, the peoples of the crossroads of the Old World, the Middle East, moved down the road toward becoming what we are today more rapidly than any others. The geographical avant-garde of humanity, the pioneers isolated in Australia and the Americas, had different histories. The Australian Aborigines kept to their Paleolithic ways; they did not smelt metals or build cities. When Captain Cook and the Australians of Botany Bay looked at each other in the eighteenth century, they did so from opposite sides of the Neolithic Revolution.

The peoples of the New World had their own Neolithic Revolution or Revolutions, most spectacularly in MesoAmerica and Andean America, but theirs, relative to that in the Old World, began slowly, accelerated tardily, and spread as though the Western Hemisphere were somehow less hospitable to the techniques and arts of civilization than the Eastern. When the conquistadores arrived with iron and steel, the peoples of the high Amerindian cultures were still in the early stages of metallurgy. They used metals for ornaments and idols, not for tools.

Why was the New World so tardily civilized? Perhaps because the long axis of the Americas runs north and south, and so the Amerindian food plants on which all New World civilizations depended had to spread through sharply differing climates, unlike the staple crops of the Old World, which by and large spread east and west through regions of roughly similar climates. Perhaps because American farmers required a very long time to transform their most important staple, maize, from what was at first a niggardly sort of plant into the richly productive food source that Europeans first encountered in the 1490s. In contrast, wheat, initially the most important Old World cultivar, was already highly productive when it was first exploited. The first maize could not support large urban populations; the first wheat could, and so Old World civilization bounded a thousand years ahead of that in the New World.

This kind of speculation, even if correct, does not explain why the American Neolithic Revolution was so inferior to that in the Old World in the matter of domesticating animals. Amerindians were better at this than Aborigines, who domesticated only the dog, but they were amateurs compared with peoples of the Eastern Hemisphere. Compare the American assemblage of livestock (dogs, llamas, guinea pigs, and some fowl) with that in the Old World: dogs, cats, cattle, horses, pigs, sheep, goats, reindeer, water buffalo, chickens, geese, ducks, honeybees, and more. Why such a contrast? It is not likely that the Eastern Hemisphere's wild animals were intrinsically more tamable than those in the Western Hemisphere. Indeed, the ancestor of our cattle, the Old World aurochs, seems to have been as unlikely a candidate for domestication as the North American buffalo. Some scholars believe that Amerindians placed a prohibitively high valuation on animals, considering them fellow creatures equal to or even superior to humans, not as potential servants. New World gods, in contrast to Old World gods (at least, one of the more widely publicized of them), did not give humans "dominion over the fish of the sea and over the fowl of the air, and over every living thing that moveth upon the earth."

Or perhaps the contrast between the Old World and New World Neolithic Revolutions was simply a matter of timing. Mark Nathan Cohen, in his book The Food Crisis in Prehistory, Overpopulation and the Origins of Agriculture, posits population pressure as the true driving force behind the migration of Paleolithic humanity out of Africa and to the rest of the habitable continents. He also credits the pressure of numbers for the beginnings of agriculture. His thesis, brutally abbreviated and simplified, is this: When the Australian and American pioneers reached their final frontiers and looked out on waters that led only to Antarctica, the world behind them was filling up with hunters and gatherers. There was nowhere else for the surplus population to go, and there were about as many people on the earth as could be supported by means of Paleolithic technology. Homo sapiens needed, not for the only time in the history of the species, to become either celibate or clever. Predictably, the species chose the later course.

All over the globe, east and west, people began to shift from dependence on the herds of large animals (many of which were in sharp decline) to exploitation of smaller animals and of plants. Gatherers became more important and hunters less so, and of necessity humanity produced its greatest practical botanists and zoologists of all time. Where conditions were particularly suitable - where, for instance, wild wheat grew in solid stands and included strains with ears that did not shatter and scatter wastefully when harvested with flint sickles - the jigsaw pieces of domestication came together, and gatherers became farmers. It would seem probable that the sense of population pressure, the primum mobile, was greater in the more ancient centers of human occupation (i.e., those of the Old World) than on the frontiers and that this may account for the faster acceleration of the Neolithic Revolution in the Old World relative to the New World.