The origins of the Zimbabwe culture and Shona civilisation has been pushed back by new discoveries in SW Zimbabwe and NE Botswana, to the 11th century AD (around 1030). No doubt more archaeology will push origins even further into the past.
These are fairly arid regions nowadays so the suggestion is that climate was wetter in this part of Africa back then. This is necessary for cultivation to have taken place, as clearly occurred (sorghum and millet). However, Shona origins at some stage would have been further north as at some point they impinged on the Bushmen and Hottentot aboriginals of southern Africa. Did climate change cause the Shona to migrate south?