About 10,000 years ago, after nearly 4 million years of human evolution and over 100,000 years of successful foraging for food, human beings, although isolated, nearly simultaneously developed a subsistence strategy that involved domesticated plants and animals. Why? Some scholars seek a single, universal explanation that would be valid for all cases of domestication. Thus, it has been argued that domestication is the outcome of population pressure, as the increasing hunting-and-gathering human population overwhelmed the existing food resources. Others point to climate change or famine, as the post-glacial climate got drier. Increasing archaeological research has made it clear, however, that the evidence in favor of any single-cause, universally applicable explanation is not strong. Some scholars have proposed universally applicable explanations that take several different phenomena into account. One such explanation, called the broad-spectrum foraging argument (the argument that humans employed a subsistence strategy based on obtaining a wide range of plants and animals), is based on a reconstruction of the environmental situation that followed the retreat of the most recent glaciers. The very large animals of the Ice Age began to die out and were replaced by increased numbers of smaller animals. As sea levels rose to cover the continental shelves, fish and shellfish became more plentiful in the warmer, shallower waters. The effects on plants were equally dramatic, as forests and woodlands expanded into new areas. Consequently, scholars argue, people had to change their diets from big-game hunting to broad-spectrum foraging for plants and animals by hunting, fishing, and gathering. This broadening of the economy is said to have led to a more secure subsistence base, the emergence of sedentary communities, and a growth in population. In turn, population growth pressured the resource base of the area, and people were forced to eat so-called third-choice, foods, particularly wild grain, which was difficult to harvest and process but which responded to human efforts to increase yields. Although the broad-spectrum foraging argument seems to describe plant domestication in the New World, the most recent evidence from ancient southwestern Asia does not support it. There is also evidence for the development of broad-spectrum foraging in Europe, but domestication did not follow. Rather, domesticated crops were brought into Europe by people from southwestern Asia – where the broad-spectrum revolution had not occurred. A very different argument comes from Barbara Bender, who argues that before farming began, there was competition between local groups to achieve dominance over each other through feasting and the expenditure of resources on ritual and exchange, engaging in a kind of prehistoric arms race. To meet increasing demands for food and other resources, land use was intensified, and the development of food production followed. This argument clearly emphasizes social factors, rather than environmental or technical factors, and takes a localized, regional approach. It is supported by ethnography (direct and systematic observations of a human culture) concerning competitive exchange activities, such as the potlatch (traditional celebrations in which groups gather and give gifts) of the indigenous inhabitants of the northwest coast of North America. These people were foragers in a rich environment that enabled them to settle in relatively permanent villages without farming or herding. Competition among neighboring groups led to ever-more elaborate forms of competitive exchange, with increasingly large amounts of food and other goods being given away at each subsequent potlatch. As suggestive as Bender's argument is, however, it is difficult to find evidence for competitive feasting in archaeological remains. Recently, archaeologists have avoided grand theories claiming that a single, universal process was responsible for domestication wherever it occurred. Many prefer to take a regional approach, searching for causes particular to one area that may or may not apply to other areas. Currently, the most powerful explanations seem to be multiple-strand theories that consider the combined local effects of climate, environment, population, technology, social organization, and diet on the emergence of domestication.