<P> Since probably 3500 BCE , the Saharan and Sub-Saharan regions of Africa have been separated by the extremely harsh climate of the sparsely populated Sahara , forming an effective barrier interrupted by only the Nile in Sudan , though the Nile was blocked by the river 's cataracts . The Sahara pump theory explains how flora and fauna ( including Homo sapiens ) left Africa to penetrate the Middle East and beyond . African pluvial periods are associated with a Wet Sahara phase , during which larger lakes and more rivers existed . </P>
<P> Since probably 3500 BCE , the Saharan and Sub-Saharan regions of Africa have been separated by the extremely harsh climate of the sparsely populated Sahara , forming an effective barrier interrupted by only the Nile in Sudan , though the Nile was blocked by the river 's cataracts . The Sahara pump theory explains how flora and fauna ( including Homo sapiens ) left Africa to penetrate the Middle East and beyond . African pluvial periods are associated with a Wet Sahara phase , during which larger lakes and more rivers existed . </P>
what geographical feature limited contact between sub-saharan africa and the outside world
what geographical feature limited contact between sub-saharan africa and the outside world
the extremely harsh climate of the sparsely populated Sahara
the extremely harsh climate of the sparsely populated Sahara