These events were largely made possible by the release of enormous amounts of cheap energy from fossil fuels starting in the mid-nineteenth century. Oil, coal, and natural gas, produced by natural processes over scores of millions of years, represent far more concentrated forms of energy than any of the sources previously available to humanity (food crops, human and animal muscles, and simple windmills or water mills) and, with even basic technology, are comparatively easy to access. With this abundant energy available to drive production processes, it became possible to increase rates of extraction of other natural resources—as, for example, chain saws and powered trawlers could harvest timber and fish at rates previously unimaginable. Meanwhile, fuel-fed tractors enabled a relatively small number of farmers to support many specialists in industrial or commercial enterprises, leading to massive urbanization in nearly every country. Modern chemistry (largely based on organic compounds derived from fossil fuels) led also to modern pharmaceuticals—which, together with improved sanitation (likewise dependent on cheap energy), enabled longer life spans and growing populations.