This Fleeting World

Acceleration: The Agrarian Era 47
immunities may have caused some of these collapses; overexploitation of
the land, which could undermine the productive basis of entire civilizations,
may have caused others. In southern Mesopotamia toward the end
of the third millennium bce, populations fell sharply, probably as a result
of over-irrigation, which created soils too salty to be farmed productively.
Archaeologists can trace the progress of salinization during the late second
millennium through the increasing use of barley, a more salt-tolerant
grain than wheat.
Agriculture, Cities, and
Empires: 500 bce–1000 ce
Most of the long trends that began after 3000 bce continued during the
period from 500 bce to 1000 ce. Global populations rose (although they
did so slowly during the middle of this period), the power, size, and
number of states increased, and so did the extent of exchange networks.
As agriculture spread, cities and states appeared in once-peripheral regions
in northwestern Europe, sub-Saharan Africa, southern India, and
southern China. Increasingly, agrarian civilizations encroached on regions
inhabited by foragers, independent peasants, and pastoralists. Similar processes
occurred in the Americas but with a time lag of approximately two
thousand years.
Afro-Eurasia
The Achaemenid empire, created in Persia (modern Iran) during the sixth
century bce, marked a significant increase in state power because the empire
controlled a region five times as large as the greatest of its predecessors.
During the next fifteen hundred years empires on this scale became
the norm. They included the Han dynasty in China (206 bce–220 ce), the
Roman empire in the Mediterranean (27 bce–476 ce), and the Mauryan
empire (c. 324–c. 200 bce) in India. The Muslim Abbasid empire, which
ruled much of Persia and Mesopotamia from 749/750 ce (eventually collapsing
only in 1258), controlled a slightly larger area than its Achaemenid
predecessors. Contacts also flourished between imperial states. During
the sixth century bce Cyrus I, the founder of the Achaemenid empire,
invaded parts of modern central Asia. When the Chinese emperor, Han
Wudi, invaded the same region three centuries later, the separate agrarian
civilizations of the Mediterranean world and eastern Asia came into closer