Another theme is to measure the displacement of natural ecosystems of forest and grasslands by plowed cropland that supports an increasing human population. Only about five million people existed worldwide preagri-culture, subsisting on hunting and gathering of wild animals and plants. Humans existed like any other wild animals in the biological world. Postagriculture, the human population grew slowly, but as people's mastery of food production technology developed (such as irrigation, weed control by hoe and plow, and planting crops in monocultures) and the number of crop plants increased, the world population climbed to an estimated 130 million people by the time of Christ, a twenty-five-fold increase from the Paleolithic pre-agriculture estimate. By 1650 the world population had reached a half billion, and half of these people were in settled urbanized villages, towns, and cities and not engaged in agriculture to produce their own food. All of the major food crops and domestic farm animals known today were known and used worldwide. The only significant crops added since 1650 are industrial crops such as rubber.
Since the middle of the nineteenth century the population has increased from one billion to six billion, an increase that would not have been possible without increases in agricultural yields. Through breeding, plus the use of fossil fuels to plant, fertilize, and protect crops, the average yield of all plants and productivity per unit area has increased ten- to fiftyfold. At present humans produce and consume over a twenty-year period as much food crop rotation alternating crops from year to year in a particular field crop rotation alternating crops from year to year in a particular field
as was produced in the eight thousand years between the development of agriculture and the sixteenth century. Nonetheless, of the six billion people in the world, over one billion are estimated to be malnourished, and half of these are seriously underfed, mostly due to poverty and the diminished af-fordability of agricultural products. An estimated fifty thousand to eighty thousand starve to death or are fatally compromised each day—a majority are children, in part because they are growing rapidly and do not get enough essential materials such as vitamin A or quality protein.
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