Among Eastern Woodland peoples and most other Archaic cultures, agriculture supplemented but did not replace hunter-gatherer subsistence strategies. Reliance on wild animals and plants required most Archaic groups to remain small and mobile. But beginning about 4000 BP, distinctive southwestern cultures began to depend on agriculture and to build permanent settlements. Later, around 2500 BP, Woodland peoples in the vast Mississippi valley began to construct burial mounds and other earthworks that suggest the existence of social and political hierarchies that archaeologists term chiefdoms. Although the hunter-gatherer lifestyle never entirely disappeared, the development of agricultural settlements and chiefdoms represented important innovations to the Archaic way of life.