The transition from hunting and gathering to settled agriculture took place over millennia in northern China, where millet was the mainstay, and the Yangtze River Valley, where rice dominated. Plant and animal domestication, known as the Neolithic Revolution, stimulated population growth and the expansion into territories occupied still by hunter-gatherers. By 2000 BC, early Neolithic groups were reaching Southeast Asia, and their settlements contrast sharply with those of their hunter-gatherer contemporaries. In some sites, there is growing evidence for integration between the two populations, while plant cultivation and the raising of domestic pigs, dogs, and cattle were combined with fishing, collecting, and hunting. The Neolithic was a seminal period in Southeast Asia, laying the foundations for profound social changes made possible through the wealth generated by agricultural surpluses.