I’ll add to this as I find resources for it. What reading I’ve done today suggests that Omotic predates agriculture, and that Cushitic split off from it during the Neolithic, as it has words for sheep/goat etc in it which arrive then. Quite interesting was a Blench book that pointed out North and South Omotic and Cushitic all have different roots for cattle, suggesting that pastoralism was foreign to them prior to the arrival of Asian neolithic domesticates, which doesn’t help the case for an early independant domestication of cattle in Africa. This also rather dents any claims to agriculture in the area prior to the split wth Cushitic, which dates to the arrival if ovicaprines (neolithic era, 7,500 bp or later).
I see Omotic clasified as Afro Asiatic, but there does seem to be a fair bit of dispute over this, with a couple of scholars pointing out it shares just as much in common with the other language groups around it. Difficult to tell.
The conclusion is..
My conclusion is that Omotic should be treated as an independent language family. No convincing alternative has ever been presented. Hayward (1995: 11) writes that «[i]t is, of course, a relief not to have Omotic as an isolate; we do not need a whole family of ‘Basques’ on our hands!» An alternative point of view is possible. Africa is the cradle of mankind. Why are there no language isolates on a continent where humans have lived since language was invented?