Major changes took place community by community and spread first through local networks. Over time, ideas moved over larger distances, but, until five hundred years ago, there were some fundamental barriers to the movement of people, ideas, and technologies, including farming. Rising sea levels after the end of the last ice age severed links between Eurasia and the Americas, and there was hardly any communication between Eurasia and Australasia or with the islands of the western Pacific, some of which were settled as early as thirty thousand years ago. In effect, humans now lived in a number
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