Steppe cultures
It is thought that the earliest were the people called Scythians, between the rivers Volga and Irtysh. Horses had already been used for centuries (by the Hittites, for instance) to draw chariots, but the great new step was to saddle and train them for fast riding. This new technique, and the nomadic way of life, spread all over Central Asia in the course of the first millennium b.c. Meanwhile, in the deserts of Arabia, other herders had learned to ride camels. In about 1100 b.c., as we read in the Book of Judges, the hosts of Midian descended on Israel, camel-mounted; and early in the first millennium a.d. camel nomadism began to spread into North Africa.
The most striking difference between nomad and hydraulic societies was in numbers. The area of China within the Great Wall has been calculated as 1,532,795 square miles. The combined area of Outer Mongolia (now an independent state) and Inner Mongolia (now part of China), dominated until recently by the pastoral way of life, has been calculated as 914,100 square miles, or about three fifths the area of China. Yet the population of the two Mongolias in the 1930s, not counting recent farmer migrants from China, has been estimated as roughly 2,000,000, while that of China itself was estimated at the same time as roughly 450,000,000. So the number of Chinese per square mile was about 150 times the number of Mongols. The difference had persisted throughout history.
The life of the nomads was based entirely on their herds, which provided meat, milk produce, and hides and fleeces for clothing, harness, and even the tents that served them as homes. To support the large herds, they grazed them for a time on one area, and then moved to another. Just as the forest farmers were always moving on to fresh woods, steppe nomads were always in search of pastures new. They needed even more land than the forest farmers, and their populations were even less dense. On the steppe, as in the forest, numbers had to be kept low if the system was to work.
A moderate amount of grazing is often good for the grasses and clovers that are most nutritious and palatable for stock. Periodic removal of their leaves stimulates them to grow again, more vigorously than before. Grazing animals have preferred them for millennia, so they are now best fitted to respond to the stimulus. Under moderate grazing, they often spread at the expense of less nutritious plants. If the number of stock on a given area is just right, the good grass on any given part of it is stimulated and also periodically rested. But if this stocking rate rises too high, the whole area will be continuously grazed, and good grasses will have no chance to rest and grow lush. They will begin to die out, and be replaced by the less nutritious plants they formerly crowded out. The land will be overgrazed.
The effects of overgrazing have been observed in recent years in many countries of the dry belt (such as Iraq, Israel, and the Yemen), but they have been most thoroughly studied in the dry western states of the U.S.A., where much overgrazing went on in the 19th and early 20th centuries a.d. The sequence of events can be illustrated by studies of ranges in southern Idaho. If grassland is grazed at a moderate rate, it is plentifully covered with nutritious perennial grasses that go on growing year after year. With heavier grazing, the perennials are killed off, and replaced by tougher but less nutritious annual grasses that die every year and grow again from seeds. If the grazing pressure increases further, the annuals are killed before they can set seed, and are replaced by unpalatable shrubs, notably the sage-brushes called Artemisia, growing in scattered clumps over otherwise bare, dry, easily eroded desert.
As less-nutritious plants take over, the animals need a larger area to get the same amount of nourishment. In Idaho, when perennials cover 50 per cent of a site, one cow can be supported on 3 acres; when they cover 25 per cent, she needs 5 acres; when they cover only 5 per cent, she needs 15 acres; and when the land turns to desert, no cattle can be raised there at all. So overgrazing is a vicious cycle. Too many animals overgraze the land till it supports fewer than it did to begin with, so the overgrazing gets worse; as more land turns to desert, the rest is grazed more heavily than ever.
As long as nomads and their herds were few enough over a given area, they could graze each part of it just the right amount before going on to the next. But the balance was more precarious for them than for forest farmers because of the effects of drought. Drought has effects rather like those of overgrazing. A few good years, and herds began to increase. A few bad years, and the effects of overgrazing were added to the effects of drought: the combination was deadly.
The distribution of springs for watering the animals was another critical factor. On the Moghan steppe in northwestern Iran, where transhumant herders today bring their flocks down from the hills in winter, areas around springs and streams are always overgrazed. As long as there was plenty of room, the nomads could solve their problems by migrating long distances at times of crisis. But the nomads became so specialized for moving over long distances that they lost many opportunities for insurance against bad years. Their migratory way of life discouraged staying in one spot long enough to raise a food crop for themselves or a fodder crop like hay that could be stored for difficult periods, or even to dig a well. They became more and more dependent on natural supplies of water and grass.
The hair-trigger balance was easily tipped. Stock was the only wealth of the nomads. (It is the main status symbol in many dry parts of Africa today.) So there was a constant temptation to increase the numbers of stock beyond safe limits. We have seen what happened to the forest farmers when they increased their numbers or tried to produce a large surplus. When the numbers of the nomads or their herds rose too far, the result could be more dramatic: events on the steppe always moved fast.
A survey in 1962 of the Karamoja district of Uganda casts some light on what must have happened when stock became too numerous. Because of modern veterinary control of cattle diseases in Karamoja, the population of cattle in this district had risen steeply during the last few years. According to the report: "As pasture deteriorates from overgrazing, tribes begin to jostle. . . . With population growth, inter-sectional conflict has increased, because groups, deprived of their traditional grazings, move elsewhere to survive." As a result, there had been serious deterioration of law and order.
Karamoja is a limited district, where a civilized modern government frowns on range wars. In the vast open steppe, with no such restriction, disputes over pasture and water took a different course. Tribes, jostled by neighbours, jostled others in turn, in a chain reaction spreading across continents. If other conditions favoured it (especially conditions on the borders of the hydraulic states), the result could be a forcible gathering of tribes into great empires, exactly as happened between hydraulic states disputing water rights. But the empires of the steppe formed a great deal faster. The conquered did not have to be reorganized and integrated into an alien bureaucracy. They just joined the horde. So a small group could grow by combining all its neighbours into one great army, through a snowball process in which each conquered group swelled the conquerors' numbers
The rulers of these hordes allotted pastures, migration routes, and watering rights to their subjects. But they also had to be expert generals. The horde needed unified command, and nomad emperors were even more absolute than the hydraulic kings. Just as those kings found themselves equipped with mass labour, which they turned to any use they could, so the steppe emperors found themselves at the head of formidable armies, spoiling for action and eager to invade the lands of the farmers, and convert as much as possible to pasture. They descended on the hydraulic societies like locusts, with equally destructive results. There was nothing stable about the restless hordes, and they could build nothing permanent. They subsided again into fragments when the force of their explosion was spent. But the explosion itself happened again and again. Outside the great inert hydraulic empires, the nomads were always poised like a time-bomb, waiting to go off next time they fused into a horde. This fusion reaction supplied the final ingredient of human history in the dry belt.
Despite their far smaller total population, the nomads attacked the teeming millions of civilization with devastating effect.  Their terrific mobility enabled them to concentrate hundreds of thousands of men at a single point, where they suddenly appeared in overwhelming force. To terrified peasants and city folk, they seemed innumerable swarms, like the Midianites, who "came up with their cattle and their tents, and they came as grasshoppers for multitude; for both they and their camels were without number: and they entered into the land to destroy it."
While some herders moved in on civilized societies and became meat-eating rulers otherwise utterly remote from their ancestors, others began to develop a new kind of society in the open steppe or desert. Transhumance had prepared them for a wandering life, and in due course they took the next logical step, and became true nomads, migrating over much greater distances in search of water and grass. Instead of just commuting between summer and winter pastures, they now roamed over whole continents. Cut off from any close contact with civilization, other than a mixture of trading and raiding, these wild nomads of the steppe evolved a mode of life different in many ways from that of the hydraulic societies.
The transition to full nomadism was made possible by taming and harnessing fast riding animals, the horse and the camel. Between 1000 and 900 b.c., horse-riding nomads began to appear on the steppes of Central Asia; it is