This Fleeting World

Acceleration: The Agrarian Era 35
As agriculture spread and became more productive, it supported larger,
denser, and more interconnected communities. Within these communities
population pressure and increasing exchanges of information generated
a steady trickle of innovations in building, warfare, record keeping,
transportation and commerce, and science and the arts. These innovations
stimulated further demographic growth in a powerful feedback cycle that
explains why change was so much more rapid during the agrarian era than
during the era of foragers. Yet innovation was rarely fast enough to keep up
with population growth. This lag explains why, on the scale of decades or
even centuries, all agrarian societies experienced cycles of expansion and
collapse that obscured the underlying trend toward growth. These cycles underlay
the more visible patterns of political rise and fall, commercial boom
and bust, and cultural efflorescence (blooming) and decay that have so fascinated
historians. (Such patterns of growth and decline can be described as
“Malthusian cycles,” after Thomas Malthus, the nineteenth-century English
economist who argued that human populations will always rise faster than
the supply of food, leading to periods of famine and sudden decline.)
Epidemic Diseases
Population growth could be slowed by epidemic diseases as well as by low
productivity. Foraging communities were largely free of epidemic diseases
because they were small and mobile, but farming communities created
more favorable environments for pathogens (causative agents of disease).
Close contact with livestock allowed pathogens to move from animals
to humans, accumulations of rubbish provided fertile breeding grounds
for diseases and pests, and large communities provided the abundant
reserves of potential victims that epidemic diseases need to flourish and
spread. Thus, as populations grew and exchanges between communities
multiplied, diseases traveled more freely from region to region. Just as
people had begun to hitch rides on domesticated animals, diseases began
to hitch rides on people. The impact of diseases took the form of a series
of epidemiological decrescendos that began with catastrophic epidemics,
followed by less disastrous outbreaks as immune systems in region after
region adapted to the new diseases.
As the historian William McNeill has shown, long-range epidemiological
exchanges within the Afro-Eurasian world zone immunized the populations
of this zone against a wide range of diseases to which populations in