When viewed across the span of the 20th century, the effect that mechanization has had on farm productivity—and on society itself—is profound. At the end of the 19th century it took, for example, 35 to 40 hours of planting and harvesting labor to produce 100 bushels of corn. A hundred years later producing the same amount of corn took only 2 hours and 45 minutes—and the farmers could ride in air-conditioned comfort, listening to music while they worked. And as fewer and fewer workers were needed on farms, much of the developed world has experienced a sea-change shift from rural to metropolitan living.

Throughout most of its long history, agriculture—particularly the growing of crops—was a matter of human sweat and draft animal labor. Oxen, horses, and mules pulled plows to prepare the soil for seed and hauled wagons filled with the harvest—up to 20 percent of which went to feed the animals themselves. The rest of the chores required backbreaking manual labor: planting the seed; tilling, or cultivating, to keep down weeds; and ultimately reaping the harvest, itself a complex and arduous task of cutting, collecting, bundling, threshing, and loading. From early on people with an inventive flair—perhaps deserving the title of the first engineers—developed tools to ease farming burdens. Still, even as late as the 19th century, farming and hard labor remained virtually synonymous, and productivity hadn't shifted much across the centuries.

At the turn of the 20th century the introduction of the internal combustion engine set the stage for dramatic changes. Right at the center of that stage was the tractor. It's not just a figure of speech to say that tractors drove the mechanization revolution. Tractors pulled plows. They hauled loads and livestock. Perhaps most importantly, tractors towed and powered the new planters, cultivators, reapers, pickers, threshers, combine harvesters, mowers, and balers that farm equipment companies kept coming out with every season. These vehicles ultimately became so useful and resourceful that farmers took to calling them simply GPs, for general purpose. But they weren't always so highly regarded. Early versions, powered by bulky steam engines, were behemoths, some weighing nearly 20 tons. Lumbering along on steel wheels, they were often mired in wet and muddy fields—practically worthless. Then in 1902 a pair of engineers named Charles Hart and Charles Parr introduced a tractor powered by an internal combustion engine that ran on gasoline. It was smaller and lighter than its steam-driven predecessors, could pull plows and operate threshing machines, and ran all day on a single tank of fuel. Hart and Parr's company was the first devoted exclusively to making tractors, a term they are also credited with introducing. Previously, tractors had been known as "traction engines."