At the outset of the Middle Ages the basic power sources for most tasks were humans and animals. During the Middle Ages the harnessing of animals underwent changes that increased productivity by as much as 200 percent. Farming altered the natural environment to the benefit of humankind. Anything that improves the efficiency or productivity of agriculture benefits the entire civilization. During the medieval period, when 90 to 95 percent of the European population tilled the soil, farmers made the most important advances in agricultural technology of any era before the Industrial and Chemical Revolutions. These advances were predicated on the more efficient use of animal power.

Oxen versus Horses. The two main draft animals are oxen and horses. Oxen are strong and relatively easy to maintain, while horses are even stronger—and faster—but they are much more expensive to maintain...