The San people (or Saan), also known as Bushmen or Basarwa – all considered pejorative to some degree – are members of various indigenous hunter-gatherer people of Southern Africa, whose territories span Botswana, Namibia, Angola, Zambia, Zimbabwe and South Africa. There is a significant linguistic difference between the northern people living between the Okavango River in Botswana and Etosha National Park in northwestern Namibia, extending up into southern Angola; the central people of most of Namibia and Botswana, extending into Zambia and Zimbabwe; and the southern people in the central Kalahari towards the Molopo River, who are the last remnant of the previously extensive indigenous San of South Africa. Starting in the 1950s, and lasting through the 1990s, the San switched to farming as a result of government-mandated modernisation programs. They have provided a wealth of information for the fields of anthropology and genetics, even as their lifestyles change. One broad study of African genetic diversity completed in 2009 found that the San people were among the five populations with the highest measured levels of genetic diversity among the 121 distinct African populations sampled. The San are one of fourteen known extant “ancestral population clusters” from which all known modern humans descend.