During the Holocene, the present geological epoch, an increasing portion of humans began to manipulate the reproduction of plants and animals in a series of environmental practices known as agriculture. No other ecological relationship sustains as many humans as farming; no other has transformed the landscape to the same extent. The domestication of plants by American Indians followed the end of the last glacial maximum (the Ice Age). About eight thousand years ago, the first domesticated maize and squash arrived from central Mexico, spreading to every region and as far north as the subarctic boreal forest. The incursion of Europeans into North America set off widespread deforestation, soil depletion, and the spread of settlement, followed by the introduction of industrial machines and chemicals. A series of institutions sponsored publically funded research into fertilizers and insecticides. By the late 19th century, writers and activists criticized the technological transformation of farming as destructive to the environment and rural society. During the 20th century, wind erosion contributed to the depopulation of much of the Great Plains. Vast projects in environmental engineering transformed deserts into highly productive regions of intensive fruit and vegetable production. Throughout much of the 19th and 20th centuries, access to land remained limited to whites, with American Indians, African Americans, Latinas/os, Chinese, and peoples of other ethnicities attempting to gain farms or hold on to the land they owned. Two broad periods describe the history of agriculture and the environment in that portion of North America that became the United States. In the first, the environment dominated, forcing humans to adapt during the end of thousands of years of extreme climate variability. In the second, institutional and technological change became more significant, though the environment remained a constant factor against which American agriculture took shape. A related historical pattern within this shift was the capitalist transformation of the United States. For thousands of years, households sustained themselves and exchanged some of what they produced for money. But during the 19th century among a majority of American farmers, commodities took over the entire purpose of agriculture, transforming environments to reflect commercial opportunity.