Let's proceed continent-by-continent. As our first continental comparison, let's consider the collision of the Old World and the New World that began with Christopher Columbus's voyage in A.D. 1492, because the proximate factors involved in that outcome are well understood. I'll now give you a summary and interpretation of the histories of North America, South America, Europe, and Asia from my perspective as a biogeographer and evolutionary biologist � all that in ten minutes; 2_ minutes per continent. Here we go:

Most of us are familiar with the stories of how a few hundred Spaniards under Cort�s and Pizarro overthrew the Aztec and Inca Empires. The populations of each of those empires numbered tens of millions. We're also familiar with the gruesome details of how other Europeans conquered other parts of the New World. The result is that Europeans came to settle and dominate most of the New World, while the Native American population declined drastically from its level as of A.D. 1492. Why did it happen that way? Why didn't it instead happen that the Emperors Montezuma or Atahuallpa led the Aztecs or Incas to conquer Europe?

The proximate reasons are obvious. Invading Europeans had steel swords, guns, and horses, while Native Americans had only stone and wooden weapons and no animals that could be ridden. Those military advantages repeatedly enabled troops of a few dozen mounted Spaniards to defeat Indian armies numbering in the thousands.

Nevertheless, steel swords, guns, and horses weren't the sole proximate factors behind the European conquest of the New World. Infectious diseases introduced with Europeans, like smallpox and measles, spread from one Indian tribe to another, far in advance of Europeans themselves, and killed an estimated 95% of the New World's Indian population. Those diseases were endemic in Europe, and Europeans had had time to develop both genetic and immune resistance to them, but Indians initially had no such resistance. That role played by infectious diseases in the European conquest of the New World was duplicated in many other parts of the world, including Aboriginal Australia, southern Africa, and many Pacific islands.

Finally, there is still another set of proximate factors to consider. How is it that Pizarro and Cort�s reached the New World at all, before Aztec and Inca conquistadors could reach Europe? That outcome depended partly on technology in the form of oceangoing ships. Europeans had such ships, while the Aztecs and Incas did not. Also, those European ships were backed by the centralized political organization that enabled Spain and other European countries to build and staff the ships. Equally crucial was the role of European writing in permitting the quick spread of accurate detailed information, including maps, sailing directions, and accounts by earlier explorers, back to Europe, to motivate later explorers.

So far, we've identified a series of proximate factors behind European colonization of the New World: namely, ships, political organization, and writing that brought Europeans to the New World; European germs that killed most Indians before they could reach the battlefield; and guns, steel swords, and horses that gave Europeans a big advantage on the battlefield. Now, let's try to push the chain of causation back further. Why did these proximate advantages go to the Old World rather than to the New World? Theoretically, Native Americans might have been the ones to develop steel swords and guns first, to develop oceangoing ships and empires and writing first, to be mounted on domestic animals more terrifying than horses, and to bear germs worse than smallpox.

The part of that question that's easiest to answer concerns the reasons why Eurasia evolved the nastiest germs. It's striking that Native Americans evolved no devastating epidemic diseases to give to Europeans, in return for the many devastating epidemic diseases that Indians received from the Old World. There are two straightforward reasons for this gross imbalance. First, most of our familiar epidemic diseases can sustain themselves only in large dense human populations concentrated into villages and cities, which arose much earlier in the Old World than in the New World. Second, recent studies of microbes, by molecular biologists, have shown that most human epidemic diseases evolved from similar epidemic diseases of the dense populations of Old World domestic animals with which we came into close contact. For example, measles and TB evolved from diseases of our cattle, influenza from a disease of pigs, and smallpox possibly from a disease of camels. The Americas had very few native domesticated animal species from which humans could acquire such diseases.

Let's now push the chain of reasoning back one step further. Why were there far more species of domesticated animals in Eurasia than in the Americas? The Americas harbor over a thousand native wild mammal species, so you might initially suppose that the Americas offered plenty of starting material for domestication.

In fact, only a tiny fraction of wild mammal species has been successfully domesticated, because domestication requires that a wild animal fulfill many prerequisites: the animal has to have a diet that humans can supply; a rapid growth rate; a willingness to breed in captivity; a tractable disposition; a social structure involving submissive behavior towards dominant animals and humans; and lack of a tendency to panic when fenced in. Thousands of years ago, humans domesticated every possible large wild mammal species fulfilling all those criteria and worth domesticating, with the result that there have been no valuable additions of domestic animals in recent times, despite the efforts of modern science.

Eurasia ended up with the most domesticated animal species in part because it's the world's largest land mass and offered the most wild species to begin with. That preexisting difference was magnified 13,000 years ago at the end of the last Ice Age, when most of the large mammal species of North and South America became extinct, perhaps exterminated by the first arriving Indians. As a result, Native Americans inherited far fewer species of big wild mammals than did Eurasians, leaving them only with the llama and alpaca as a domesticate. Differences between the Old and New Worlds in domesticated plants, especially in large-seeded cereals, are qualitatively similar to t hese differences in domesticated mammals, though the difference is not so extreme.

Another reason for the higher local diversity of domesticated plants and animals in Eurasia than in the Americas is that Eurasia's main axis is east/west, whereas the main axis of the Americas is north/south. Eurasia's east/west axis meant that species domesticated in one part of Eurasia could easily spread thousands of miles at the same latitude, encountering the same day-length and climate to which they were already adapted. As a result, chickens and citrus fruit domesticated in Southeast Asia quickly spread westward to Europe; horses domesticated in the Ukraine quickly spread eastward to China; and the sheep, goats, cattle, wheat, and barley of the Fertile Crescent quickly spread both west and east.

In contrast, the north/south axis of the Americas meant that species domesticated in one area couldn't spread far without encountering day-lengths and climates to which they were not adapted. As a result, the turkey never spread from its site of domestication in Mexico to the Andes; llamas and alpacas never spread from the Andes to Mexico, so that the Indian civilizations of Central and North America remained entirely without pack animals; and it took thousands of years for the corn that evolved in Mexico's climate to become modified into a corn adapted to the short growing season and seasonally changing day-length of North America.

Eurasia's domesticated plants and animals were important for several other reasons besides letting Europeans develop nasty germs. Domesticated plants and animals yield far more calories per acre than do wild habitats, in which most species are inedible to humans. As a result, population densities of farmers and herders are typically ten to a hundred times greater than those of hunter/gatherers. That fact alone explains why farmers and herders everywhere in the world have been able to push hunter/gatherers out of land suitable for farming and herding. Domestic animals revolutionized land transport. They also revolutionized agriculture, by letting one farmer plough and manure much more land than the farmer could till or manure by the farmer's own efforts. Also, hunter/gatherer societies tend to be egalitarian and to have no political organization beyond the level of the band or tribe, whereas the food surpluses and storage made possible by agriculture permitted the development of stratified, politically centralized societies with governing elites. Those food surpluses also accelerated the development of technology, by supporting craftspeople who didn't raise their own food and who could instead devote themselves to developing metallurgy, writing, swords, and guns.

Thus, we began by identifying a series of proximate explanations � guns, germs, and so on � for the conquest of the Americas by Europeans. Those proximate factors seem to me ultimately traceable in large part to the Old World's greater number of domesticated plants, much greater number of domesticated animals, and east/west axis. The chain of causation is most direct in explaining the Old World's advantages of horses and nasty germs. But domesticated plants and animals also led more indirectly to Eurasia's advantage in guns, swords, oceangoing ships, political organization, and writing, all of which were products of the large, dense, sedentary, stratified societies made possible by agriculture.