Biological Exchange and Biological Invasion in World History
By J.R. McNeil
Paper given at the 19th International Congress of Historical Sciences (2000)
Introduction: For millions of years, most species stayed home. Geographic barriers, such as oceans and mountain chains, inhibited migrations and divided the earth into distinct biogeographical provinces. Only birds, bats, and flying insects bucked the trend consistently. A few other species did so occasionally, thanks to sea-level changes and land bridges, or a chance voyage on driftwood. Natural evolution took place, for most species, in separated biogeographical provinces, in effect in parallel universes.
This long phase in the earth’s history, and the history of life, ended when human beings began their long-distance migrations. Deep in prehistory people, or hominids at any rate, walked throughout Africa and Eurasia, occasionally bringing a plant, seed, insect or microbe to a place it would not easily have gotten on its own. But it was not until plant and animal domestication, some 10-12,000 years ago, that people began to do this on purpose and therefore more frequently. Most of the plants and animals susceptible to domestication were found in Eurasia, and the East-West axis of that continent eased the spread of those plants and animals sensitive to climate conditions, especially to day length. So, it is safe to say, the greatest degree of homogenization in flora and fauna took place within the Eurasian (and North African) land mass. The suites of domesticated plants and animals on which agriculture and herding rest were spread almost instantaneously by the standards of the past, although in fact it took a few millennia. This process no doubt proved highly disruptive biologically, as local biogeographic provinces were invaded by alien creatures that humanity worked hard to spread. It also proved highly disruptive historically, obliterating peoples who did not adapt to the changing biogeography, the changing disease regimes, and the changed political situations brought on by the spread of farmers, herders, and eventually of civilization and, more specifically, of states.
Out of this turmoil of Afro-Eurasian biological exchange emerged the great civilizations familiar to historians. They all based their societies on a handful of plants and animals, not identical sets, but strongly intersecting sets, from China to the Mediterranean.
This process of biological homogenization within Afro-Eurasia had its limits. The links between North Africa, say, and East Asia before 500 BC were slender indeed. Varying topography and climate also checked the spread of certain species. The process presumably accelerated when inter-regional contacts flourished, and when large states created favorable conditions for the movement of goods and people. The era of the Han and Roman empires, for example, when the trans-Asian silk road was a well-beaten path, unleashed a small flood of biological exchanges. The Mediterranean acquired cherries at this time; sorghum made its way from East Africa to India to China, while grapes, camels and donkeys also arrived in China from southwest Asia and North Africa.