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The largest-scale human population shift of the past millenium has been the European conquest of the Americas and of Australia, formerly settled by other peoples. Why did conquest go in that direction rather than in the reverse direction? It was largely an accident of biogeography: Europeans inherited the most useful suite of wild plant and animal species suitable for. domestication, on which subsequent technological and political development depended.

Some of the most obvious features of our daily lives pose the hardest questions for scientists. If you look around you at most locations in the US or Australia, most of the people you see will be of European ancestry. At the same locations 500 years ago, everyone without exception would have been an American Indian in the US, or a native (aboriginal) Australian in Australia. Why is it that Europeans came to replace most of the native population of North America and Australia, instead of Indians or native Australians coming to replace most of the original population of Europe?

This question can be rephrased to ask: why was the ancient rate of technological and political development fastest in Eurasia, slower in the Americas (and in Africa south of the Sahara), and slowest in Australia? For example, in 1492 much of the population of Eurasia used iron tools, had writing and agriculture, had large centralized states with ocean-going ships, and was on the verge of industrialization. The Americas had agriculture, only a few large centralized states, writing in only one area, no ocean-going ships or iron tools, and were technologically and politically a few thousand years behind Eurasia. Australia lacked agriculture, writing, states, and ships, was still in a pre-first-contact condition, and used stone tools comparable to ones made over ten thousand years earlier in Eurasia. It was those technological and political differencesnot the biological differences determining the outcome of competition among animal populationsthat permitted Europeans to expand to other continents.

Nineteenth-century Europeans had a simple, racist answer to such questions. They concluded that they acquired their cultural head start through being inherently more intelligent, and that they therefore had a manifest destiny to conquer, displace, or kill 'inferior' peoples. The trouble with this answer is that it was not just loathsome and arrogant, but also wrong. It is obvious that people differ enormously in the knowledge they acquire, depending on their circumstances as they grow up. But no convincing evidence of genetic differences in mental ability among peoples has been found, despite much effort.

Because of this legacy of racist explanations, the whole subject of human differences in level of civilization still reeks of racism. Yet there are obvious reasons why the subject begs to be properly explained. Those technological differences led to great tragedies in the past 500 years, and their legacies of colonialism and conquest still powerfully shape our world today. Until we can come up with a convincing alternative explanation, the suspicion that racist genetic theories might be true will linger.

In this chapter I shall argue that continental differences in level of civilization arose from geography's effect on the development of our cultural hallmarks, not from human genetics. Continents differed in the resources on which civilization dependedespecially, in the wild animal and plant species that proved useful for domestication. Continents also differed in the ease with which domesticated species could spread fr<&m one area to another. Even today, Americans and Europeans are painfully aware how distant geographical features, like the Persian Gulf or the Isthmus of Panama, affect our lives. But geography and biogeography have been moulding human lives even more profoundly, for hundreds of thousands of years. Why do I emphasize plant and animal species? As the biologist J. B. S. Haldane remarked, 'Civilization is based, not only on men, but on plants and animals. Agriculture and herding, though they also brought the disadvantages discussed in Chapter Ten, still made it possible to feed far more people per square mile of land than could live on the wild foods available in that same area. Storable food surpluses grown by some individuals permitted other individuals to devote themselves to metallurgy, manufacturing, writingand to serving in full-time professional armies. Domestic animals provided not only meat and milk to feed people, but also wool and hides to clothe people, and power to transport people and goods. Animals also provided power to pull ploughs and carts, and thus to increase agricultural productivity greatly over that previously attainable by human muscle power alone.

As a result, the world's human population rose from about ten million around 10,000 BC, when we were all still hunter-gatherers, to over five billion today. Dense populations were prerequisite to the rise of centralized states. Dense populations also promoted the evolution of infectious diseases, to which exposed populations then evolved some resistance but other populations did not. All these factors determined who colonized and conquered whom. Europeans' conquest of America and Australia was due not to their better genes but to their worse germs (especially smallpox), more advanced technology (including weapons and ships), information storage through writing, and political organizationall stemming ultimately from continental differences in geography.

Let's start with the differences in domestic animals. By around 4000 BC western Eurasia already had its 'Big Five' domestic livestock that continue to dominate today: sheep, goats, pigs, cows, and horses. Eastern Asians domesticated four other cattle species that locally replace cows: yaks, water buffalo, gaur, and banteng. As already mentioned, these animals provided food, power, and clothing, while the horse was also of incalculable military value. (It was both the tank, the truck, and the jeep of warfare until the Nineteenth Century.) Why did American Indians not reap similar benefits by domesticating the corresponding native American mammal species, such as mountain sheep, mountain goats, peccaries, bison, and tapirs? Why did Indians mounted on tapirs, and native Australians mounted on kangaroos, not invade and terrorize Eurasia? The answer is that, even today, it has proved possible to domesticate only a tiny fraction of the world's wild mammal species. This becomes clear when one considers all the attempts that failed. Innumerable species reached the necessary first step of being kept captive as tame pets. In New Guinea villages I routinely find tamed possums and kangaroos, while I saw tamed monkeys and weasels in Amazonian Indian villages. Ancient Egyptians had tamed gazelles, antelopes, cranes, and even hyenas and possibly giraffes. Romans were terrorized by the tamed African elephants with which Hannibal crossed the Alps (not Asian elephants, the tame elephant species in circuses today).

But all these incipient efforts at domestication failed. Since the domestication of horses around 4000 BC and reindeer a few thousand years later, no large European mammal has been added to our repertoire of successful domesticates. Thus, our few modern species of domestic mammals were quickly winnowed from hundreds of others that had been tried and abandoned.

Why have efforts at domesticating most animal species failed? It turns out that a wild animal must possess a whole suite of unusual characteristics for domestication to succeed. Firstly, in most cases it must be a social species living in herds. A herd's subordinate individuals have instinctive submissive behaviours that they display towards dominant individuals, and that they can transfer towards humans. Asian mouflon sheep (the ancestors of domestic sheep) have such behaviour but North American bighorn sheep do nota crucial difference that prevented Indians from domesticating the latter. Except for cats and ferrets, solitary territorial species have not been domesticated.

Secondly, species such as gazelles and many deer and antelopes, which instantly take flight at signs of danger instead of standing their ground, prove too nervous to manage. Our failure to domesticate deer is especially striking, since there are few other wild animals with which humans have been so closely associated for tens of thousands of years. Although deer have always been intensively hunted and often tamed, reindeer alone among the world's forty-one deer species were successfully domesticated. Territorial behaviour, flight reflexes, or both eliminated the other forty species as candidates. Only reindeer had the necessary tolerance of intruders and gregarious, non-territorial behaviour.

Finally, domestication requires being able to breed an animal in captivity. As zoos often discover to their dismay, captive animals that are docile and healthy may nevertheless refuse to breed in cages. You yourself would not want to-carry out a lengthy courtship and copulate under the watchful eyes of others; many animals do not want to either. This problem has derailed persistent attempts to domesticate some potentially very valuable animals. For example, the finest wool in the world comes from the vicuna, a small camel species native to the Andes. But neither the Incas nor modern ranchers have ever been able to domesticate it, and wool must still be obtained by capturing wild vicunas. Many potentates, from ancient Assyrian kings to nineteenth-century Indian maharajahs, have tamed cheetahs, the world's swiftest land mammal, for hunting. However, every prince's cheetah had to be captured from the wild, and not even zoos were able to breed them until 1960. Collectively, these reasons help explain why Eurasians succeeded in domesticating the Big Five but not other closely related species, and why American Indians did not domesticate bison, peccaries, tapirs, and mountain sheep or goats. The military value of the horse is especially interesting in illustrating what seemingly slight differences make one species uniquely prized, another useless. Horses belong to the group of mammals termed Perissodactyla, which consists of the hoofed mammals with an odd number of toes: horses, tapirs, and rhinoceroses. Of the seventeen living species of Perissodactyla, all four tapirs and all five rhinos, plus five of the eight wild horse species, have never been domesticated. Africans or Indians mounted on rhinos or tapirs would have trampled any European invaders, but it never happened. A sixth wild horse relative, the wild ass of Africa, gave rise to domestic donkeys, which proved splendid as pack animals but useless as military chargers. The seventh wild horse relative, the onager of western Asia, may have been used to pull wagons for some centuries after 3000 BC. But all accounts of the onager blast its vile disposition with adjectives like 'bad-tempered', 'irascible', 'unapproachable', 'unchangeable', and 'inherently intractable'. The vicious beasts had to be kept muzzled to prevent them from biting their attendants. When domesticated horses reached the Middle East around 2300 BC, onagers were finally kicked onto the scrapheap of failed domesticates. Horses revolutionized warfare in a way that no other animal, not even elephants or camels, ever rivalled. Soon after their domestication, they may have enabled herdsmen speaking the first Indo-European languages to begin the expansion that would eventually stamp their languages on much of the world (Chapter Fifteen). A few millenia later, hitched to battle chariots, horses became the unstoppable Sherman tanks of ancient war. After the invention of saddles and stirrups, they enabled Attila the Hun to devastate the

Roman Empire, Genghis Khan to conquer an empire from Russia to China, and military kingdoms to arise in West Africa. A few dozen horses helped Cortes and Pizarro, leading only a few hundred Spaniards each, to overthrow the two most populous and advanced New World states, the Aztec and Inca empires. With futile Polish cavalry charges against Hitler's invading armies in September 1939, the military importance of this most universally prized of all domestic animals finally came to an end after 6,000 years. Ironically, relatives of the horses that Cortes and Pizarro rode had formerly been native to the New World. Had those horses survived, Montezuma and Atahuallpa might have shattered the conquistadores with cavalry charges of their own. But, in a cruel twist of fate, America's horses had become extinct long before that, along with eighty or ninety per cent of the other large animal species of the Americas and Australia. It happened around the time that the first human settlersancestors of modern Indians and native Australiansreached those continents. The Americas lost not only their horses but also other potentially domestic-stable species like large camels, ground sloths, and elephants. Australia lost all its giant kangaroos, giant wombats, and rhinoceros-like diprotodonts. Australia and North America ended up with no domesti-catable mammal species at all, unless Indian dogs were derived from

North American wolves. South America was left with only the guinea-pig (used for food), alpaca (used for wool), and llama (used as a pack animal, but too small to carry a rider). As a result, domestic mammals made no contribution to the protein needs of native Australians and Americans except in the Andes, where their contribution was still much slighter than in the Old World. No native American or Australian mammal ever pulled a plough, cart, or war chariot, gave milk, or bore a rider. The civilizations of the New World limped forward on human muscle power alone, while those of the Old World ran on the power of animal muscle, wind, and water. Scientists still debate whether the prehistoric extinctions of most large American and Australian mammals were due to climatic factors or were caused by the first human settlers themselves (Chapters Seventeen to Nineteen). Whichever was the case, the extinctions may have virtually ensured that the descendants of those first settlers would be conquered over 10,000 years later by people from Eurasia and Africa, the continents that retained most of their large mammal species. Do similar arguments apply to plants? Some parallels jump out immediately. As true of animals, only a tiny fraction of all wild plant species have proved suitable for domestication. For example, plant species in which a single hermaphroditic individual can pollinate itself (like wheat) were domesticated earlier and more easily than cross-pollinated species (like rye). The reason is that self-pollinating varieties are easier to select and then maintain as true strains, since they are not continually mixing with their wild relatives. As another example, although acorns of many oak species were a major food source in prehistoric Europe and North America, no oak has ever been domesticated, perhaps because squirrels remained much better than humans at selecting and planting acorns. For every domesticated plant that we still use today, many others were tried in the past and discarded. (What living American has eaten sumpweed, which Indians in the eastern US domesticated for its seeds by around 2000 BC?)

Such considerations help explain the slow rate of human technological development in Australia. That continent's relative poverty in wild plants appropriate for domestication, as in appropriate wild animals, undoubtedly contributed to the failure of aboriginal Australians to develop agriculture. But it is not so obvious why agriculture in the Americas lagged behind that in the Old World. After all, many food plants now of worldwide importance were domesticated in the New World: corn, potatoes, tomatoes, and squash, to name just a few. The answer to this puzzle requires closer scrutiny of corn, the New World's most important crop. Corn is a cerealthat is, a grass with edible starchy seeds, like barley kernels or wheat grains. Cereals still provide most of the calories consumed by the human race. While all civilizations have depended on cereals, different native cereals have been domesticated by different civilizations: for instance, wheat, barley, oats, and rye in the Near East and Europe; rice, foxtail millet, and broomcorn millet in China and Southeast Asia; sorghum, pearl millet, and finger millet in sub-Saharan Africa; but only corn in the New World. Soon after Columbus discovered America, corn was brought back to Europe by early explorers and spread around the globe, and it now exceeds all other crops except wheat in world acreage planted. Why, then, did corn not enable American Indian civilizations to develop as fast as the Old World civilizations fed by wheat and other cereals?

It turns out that corn was a much bigger pain in the neck to domesticate and grow, and gave an inferior product. Those will be fighting words to all of you who, like me, love hot, buttered corn-on-the-cob. Throughout my childhood, I looked forward to late summer as the season to stop at roadside stands and pick out the best-looking fresh ears. Corn is the most important crop in the US today, worth twenty-two billion dollars to us and fifty billion dollars to the world. But before you charge me with slander, please hear me out on the differences between corn and other cereals. The Old World had over a dozen wild grasses that were easy to domesticate and grow. Their large seeds, favoured by the Near East's highly seasonal climate, made their value obvious to incipient farmers. They were easy to harvest en masse with a sickle, easy to grind, easy to prepare for cooking, and easy to sow. Another subtle advantage was first recognized by University of Wisconsin botanist Hugh Iltis: we did not have to figure out for ourselves that they could be stored, since wild rodents in the Near East already made caches of up to sixty pounds of those wild grass seeds.

The Old World grains were already productive in the wild, and one can still harvest up to 700 pounds of grain per acre from wild wheat growing naturally on hillsides in the Near East. In a few weeks a family could harvest enough to feed itself for a year. Even before wheat and barley were domesticated, there were sedentary villages in Palestine that had already invented sickles, mortars and pestles, and storage pits, and that were supporting themselves on wild grains. Domestication of wheat and barley was not a conscious act. It was not the case that several hunter-gatherers sat down one day, mourned the extinction of big game animals, discussed which particular wheat plants were best, planted the seeds of those plants, and thereby became farmers the next year. Instead, as I mentioned in Chapter Ten, the process we call domesticationthe changes in wild plants under cultivationwas an unintended by-product of people preferring some wild plants over others, and hence accidentally spreading seeds of the preferred plants. In the case of wild cereals, people naturally preferred to harvest ones with big seeds, ones whose seeds were easy to remove from the seed-coverings, and ones with firm non-shattering stalks that held all the seeds together. It took only a few mutations, favoured by this unconscious human selection, to produce the large-seeded, non-shattering cereal varieties that we refer to as domesticated rather than wild. By around 8000 BC, wheat and barley remains from archaeological digs at ancient Near Eastern village sites are beginning to show these changes. The development of bread wheats, other domestic varieties, and intentional sowing soon followed. Gradually, fewer remains of wild foods are found at the sites. By 6000 BC, crop cultivation had been integrated with animal herding into a complete food production system in the Near East. For better or worse (in some major respects worse, as I argued in Chapter Ten), people were no longer hunter-gatherers but farmers and herders, en route to being civilized.

Now contrast these relatively straightforward Old World developments with what happened in the New World. The parts of the Americas where farming began lacked the Near East's highly seasonal climate, and so lacked large-seeded grasses that were already productive in the wild. North American and Mexican Indians did start to domesticate three small-seeded wild grasses called maygrass, little barley, and a wild millet, but these were displaced by the arrival of corn and then of European cereals. Instead, the ancestor of corn was a Mexican wild grass that did have the advantage of big seeds but in other respects hardly seemed like a promising food plant: annual teosinte.

Teosinte ears look so different from corn ears that scientists argued about teosinte's precise role in corn's ancestry till recently, and even now some scientists remain unconvinced. No other crop underwent such drastic changes on domestication as did teosinte. It has only six to twelve kernels per ear, and they are inedible, because they are enclosed in stone-hard cases. One can chew teosinte stalks like sugar cane, as Mexican farmers still do. But no one uses its seeds today, and there is no indication that anyone did prehistorically either.

Hugh Iltis identified the key step in teosinte's becoming useful: a permanent sex change! In teosinte the side branches end in a tassel composed of male flowers; in corn they end in a female structure, the ear. Although that sounds like a drastic difference, it is really a simple hormonally-controlled change that could have been started by a fungus, virus, or change in climate. Once some flowers on the tassel had changed sex to female, they would have produced edible naked grains likely to catch the attention of hungry hunter-gatherers. The tassel's central branch would then have been the beginning of a corn cob. Early Mexican archaeological sites have yielded remains of tiny ears, barely an inch-and-a-half long and much like the tiny ears of our 'Tom Thumb' corn variety.

With that abrupt sex change, teosinte (alias corn) was now finally on the road to domestication. However, in contrast to the case with Near Eastern cereals, thousands of years of development still lay ahead before high-yield corns capable of sustaining villages or cities resulted. The final product was still much more difficult for Indian farmers to manage than were the cereals of Old World farmers. Corn ears had to be harvested individually by hand, rather than en masse with a sickle; the cobs had to be shucked; the kernels did not fall off but had to be scraped or bitten off; and sowing the seeds involved planting them individually, rather than scattering them en masse. The result was still poorer nutritionally than Old World cereals: lower protein content, deficiencies of nutritionally important amino acids, deficiency of the vitamin niacin (tending to cause the disease pellagra), and need for alkali treatment of the grain to partially overcome these deficiencies.

In short, characteristics of the New World's staple food crop made its potential value much harder to discern in the wild plant, harder to develop by domestication, and harder to extract even after domestication. Much of the lag between New World and Old World civilization may have been due to those peculiarities of one plant.

So far, I have discussed geography's biogeographic role, in providing the local wild animal and plant species suitable for domestication. But there is another major role of geography that deserves mention. Each civilization has depended not only on its own food plants domesticated locally, but also on other food plants that arrived after having been first domesticated elsewhere. The predominantly north/south axis of the New World made such diffusion of food plants difficult; the predominantly east/west axis of the Old World made it easy (see map overleaf). Today, we take plant diffusion so much for granted that we seldom stop to think where our foods originated. A typical American or European meal might consist of chicken (of Southeast Asian origin) with corn (from Mexico) or potatoes (from the southern Andes), seasoned with pepper (from India), accompanied by a piece of bread (from Near Eastern wheat) and butter (from Near Eastern cattle), and washed down by a cup of coffee (from Ethiopia). But this diffusion of valued plants and animals did not begin just in modern times: it has been happening for thousands of years.

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