Over time, the North American diet has evolved through stages that are strikingly similar to those experienced by other societies today considered technologically advanced. Roughly speaking, all of these societies have obtained their food first by foraging, followed by hunting and collecting stages; then rudimentary forms of food production (horticulture and animal husbandry) have followed. The latter then have evolved into intensive agriculture and other forms of food production and, finally, to large-scale mechanized agriculture, including both plant and animal production. With these changes have come periodic fluctuations in dietary adequacy. Now these countries are moving into the postindustrial stage of biotechnology, which will bring further changes in dietary adequacy. Americans have unique dietary patterns and food preferences caused by historical development of the nation and cultural diversity. Over the century, Americans have steadily increased their overall consumption of fat while substituting vegetable for animal fat. Essentially, the consumption of animal fat has declined since 1940, and the consumption of vegetable oils has increased steadily since 1909, overtaking animal fats in 1950. There has been a steady increase in vegetable consumption since 1909. Total fresh vegetable consumption has increased 42 percent since 1970 alone. As with fruits, the most common reason given for increasing fresh vegetable consumption are concerns about health and nutrition (Smith 99). Although overall vegetable consumption has increased over the century, there has been a large decline in potato consumption (73 percent of all vegetables in 1909 versus only 30 percent in 1987). Part of this decline undoubtedly can be attributed to the increased popularity of pasta and other wheat products. For example, per capita consumption of pasta products rose from 9 pounds in 1970-74 to 13 pounds in 1990. Even so, potatoes, along with iceberg lettuce, are the two most popular vegetables in America. In 1990, they constituted 37 percent and 17 percent of fresh vegetable consumption, respectively (Smith 96). The history of American food begins with the North American Indians, who descended from Asians. They are thought to have crossed the Bering Straits from Siberia at least 20,000 years ago, as the fourth Pleistocene glaciation receded. These people were food foragers who searched for plant and animal foods without the aid of special techniques (Goldbeck and Goldbeck 58). Thus far, they had discovered neither tools nor agriculture. It is believed that they and their descendants, the Paleo-Indians (13000-8000 B.C.), continued to use this most rudimentary method of obtaining food for several thousand years (Goldbeck and Goldbeck 59). Milk was not part of their diet, since they had no domestic milk-giving animals. Their calcium intake, largely from fish and animal bones, plus a small amount from plant sources, probably was marginally adequate. Although their diet probably was reasonably well balanced, undoubtedly it often was inadequate in amount. Because of the limited technology for obtaining what already constituted an uncertain food supply, life must have been precarious. As is true even for t