Lecture 35: Long Trends—Disease and Malthusian Cycles
be constantly replenished by immigration from rural areas. Diseases spread
rapidly in ¿ lthy city streets; human and animal wastes accumulated in public
places and waterways; rivers were treated as sewers and dumps; and city air
was often polluted by ¿ res and manufacturing processes such as smelting or
tanning. The expansion of exchange networks, another important driver of
growth in this era, also encouraged the spread of disease. As William McNeill
showed in his classic study Plagues and Peoples, diseases spread along trade
routes, along with goods and ideas. As exchange networks expanded, they
spread diseases to new regions. Indeed, he argues that both the Roman and
Han empires may have declined, in part, because of the spread of devastating
plagues along Eurasia’s expanding trade routes.While humans are very good at ¿ nding new ways of exploiting their
environments, they are not as good at identifying the limits of ecological
exploitation. Time and again, Agrarian civilizations grew so rapidly that they
began to overuse their forests, their rivers, and their crop lands to the point
where entire civilizations collapsed. Estimates of populations in Mesopotamia
over 7,000 years show two periods of sudden decline. The sudden collapse
early in the 2nd millennium B.C.E. was almost certainly caused by over-
irrigation leading to salination and declining harvests. According to one
estimate, Mesopotamian populations fell from over 600,000 in 1900 B.C.E.
to about 270,000 by 1600 B.C.E., not to rise again for at least a millennium.
A similar pattern of growth and decline would be repeated again early in the
second millennium C.E.These and other factors meant that, alongside the positive feedback cycles
that drove innovation and growth, there were also negative feedback cycles
that inhibited growth. This balance shaped the fundamental rhythms of
historical change throughout the later Agrarian era. A typical cycle began
with innovations that stimulated population growth, which in turn stimulated
growth in other sectors. Eventually, though, populations began to press
against ecological constraints. Shortages appeared, as did growing evidence
of malnutrition, which encouraged disease. States increasingly began to
¿ ght over scarce resources; and ¿ nally, through warfare, disease, or famine,
populations crashed. These rhythms are very clear on a graph of Eurasian
populations over the last 2,000 years.