Accumulating archaeological and archival evidence provides the basis for a more comprehensive reconstruction of the later prehistory of southern Africa than was possible as recently as 1970, when it was generally thought that Bantu-speaking peoples arrived only two or three centuries ago, bringing with them grain horticulture and cattle-sheep-goat pastoralism. It is now known that cattle and sheep were widely kept, almost certainly by Khoisan-speaking peoples, beginning about 2,000 years ago; both the Khoe languages and Zhu I õasi contain indigenous vocabularies for stock keeping, indicating that some of these pastoralist-foragers must have been San speakers. About 500 years later, Bantu-speaking agro-pastoralists spread into many parts of the region, introducing sorghum, millet, and probably goats, as well as metallurgy; linguistic evidence suggests that speakers of Nguni and Sotho-Tswana Bantu languages obtained cattle from Khoisan peoples. Since that time, mixed economies that combine foraging, herding, and farming in varying proportions have been predominant. Nevertheless, it has not been uncommon for local groups of both Bantu and Khoisan speakers to rely exclusively on foraging when drought, disease, raiding, or political subjugation have made herding and farming impossible; San peoples in this condition have been the subjects of most ethnographic studies. Interregional and transcontinental trade is archaeologically documented from the ninth century, when iron and copper jewelry, glass beads from Asia, and cowrie shells from the Indian Ocean were widely distributed and reached even into the Kalahari and Ngamiland. The Portuguese Atlantic-coast trade penetrated from the Congo through Angola into Namibia and Botswana in the seventeenth century and was recorded a hundred years later by the first Europeans to enter those areas; these observers noted that many different groups of San speakers engaged with Bantu and Nama speakers in such trade, which followed ancient routes of communication and exchange. European hunter-merchants accelerated this trade, beginning in the 1840s. Throughout the 1880s, until the market collapsed, San speakers were major providers of ivory and ostrich feathers, for which the merchants paid with European goods. Horses and donkeys were introduced by these merchants; donkeys, especially, became important economic assets. Most of the San-speaking peoples became impoverished during the period from 1850 to 1920, but some of them (the Deti and a few Zhu ¦ õasi, for example) retained modest herds. Many, especially in southern and eastern Botswana, became serfs of Twsana patrons; a few became marginal foragers, partially dependent on their serf and client kin. Labor migration to South African gold mines, which began in the 1890s, increased dramatically for San speakers in the 1950s, when efforts to recruit them were intensified in order to augment insufficient Bantu labor. Different groups were variably affected: as many as 50 percent of Kwa men were absent from their villages at any given time, but only 10 percent of Zhu I õasi men ever went to the mines. Opportunities to work in the mines are no longer available to San speakers.