The Sahara desert is vast, large, dry, and empty of resources. This means people/armies wouldn't walk through there because it's basically suicide. This acts as a "natural barrier" because nobody would cross the desert in the first place.
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The negatives of cultural encounters in East Africa can be observed in different periods of time with different new cultures coming in the region. For example, the cultural encounters with the Arab Empire of the Middle Ages, it brought a lot of violence because of the attempt to impose a new religion on the local population, and that was not accepted, so there was bloodshed, as well as slave trade, since the Arabs were seeing the locals as lower race that is only good for physical work.
No, these rivers seem to be located in South America
One is that we are the perfect distance therefor it is not to hot for life or to cold for life on earth