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In the early 1980s, a small but vocal segment of anthropologists and archaeologists attempted to demonstrate that contemporary groups usually identified as hunter-gatherers do not, in most cases, have a continuous history of hunting and gathering, and that in many cases their ancestors were agriculturalists and/or pastoralists[citation needed] who were pushed into marginal areas as a result of migrations, economic exploitation, and/or violent conflict (see, for example, the Kalahari Debate). The result of their effort has been the general acknowledgement that there has been complex interaction between hunter-gatherers and non-hunter-gatherers for millennia.[citation needed]

What kind of upset could force agriculturalists into being foragers?
migrations