Redefining the entry into North Africa by pastoralists from Asia, via Sinai, Mathilda (at http://mathildasanthropologyblog.wordpress.com/2010/05/15/eurasian-y-chromosome-R1b-in-africa/) we find they migrated there some 8000 years ago. In other words, in the window after the 6200-6000BC event, the most severe of the Holocene climate glitches. In all likelihood the migration was driven by that event and there is every reason to think the Sahara has the same Semitic root as Sarah. However, although DNA evidence is crystal clear the evidence is that they ended up in the Lake Chad region – which at that time was highly fertile. They also clearly interbred with Africans.