Shinfa

The Longhorns came to graze at the Atbara source Shinfa, northwestern Ethiopia. They were looking at 72kBC. They found the Toba layer.

The map is in conflict with what I see in Wiki. Atbara is not the Blue Nile. Atbara goes to... Atbara; the Blue Nile goes to Khartoum upstream. There've been dams, in the meantime. And anarchy in the region, namely Tigray; and in good parts of Sudan over the border.

Back to 72kBC: the Texans think that the Nilotes here experienced a drought. This led to easier fishin' ... at the truncated waterholes. So they taught themselves archery to go after smaller and quicker game they couldn't trap and weren't going to (what was left of) the waterholes.

Also suspected is migration. These are not our ancestors; for one, our ancestors may not have learnt archery (the New World had to learn this independently). So the Texans (must) speculate: migrations east of this watershed, where they haven't done digs.