African helix

Last week I saw Ragsdale et al. about the deep-structure of African humanity before the great outmigration from the Horn. Here the NYT summary.

I have to notice that what this blog has been terming the Eemian is, here, "Marine Isotope Stage 5" aka MIS 5e. MIS 6 is the glacial before that, 195-123kBC. MIS 5 was nice for Neanders in Iran. Outside the tropics, warm periods, like Eemian and now, are nice for humans. Glacials are nasty; only sheltered "refugia" could outlast the long winters.

In the tropics, that depended on where you lived. Bad weather for the Sahara and, indeed, for Iran; means cooler so temperate weather for South Africa and Australia. These formed refugia for MIS 6 humans in Africa. Such had also held for MIS 3 and 2, which is our LGM I believe. The record starts from 280kBC.

Throughout all this time, Africa seems to have held two populations which mixed, but didn't mix much. These are termed "Stem1" and "Stem2" and both predate Neander-Denisovans; I'd say within the last million years. Stem1 in fact birthed those Neander-Denisovans. But then Stem1 stayed in (East?) Africa. Stem1's descendents here mingled with Stem2, at different levels in different times and (African) sites. We the East Africans are products of one such mingle, finally meeting up again with our Stem1 kin.

This accounts for the strange African genetic diversity which, earlier, had been ascribed to a "ghost population" of wildly divergent hominins; think, Neanders but black. I take it that the "ghost population" couldn't be constrained as well as the Denisovans were. "Stem2" is now to be considered the "ghost" population - no longer a ghost.

During MIS / Eemian, the populations in South Africa jostled enough that Stem1+Stem2 created the Nama, who speak Khoi-San. (Okay: the basal Nama. They have some Indian-Ocean ancestry and, lately, Europeans now.)

Of course Neander DNA itself will show up after the Cushites backwash into the Horn, with the Semites following. On the other side of Africa, Stem2 remained aloof from Stem1 until 23kBC; then, they created the Mende. This implies to the researchers that Stem2 was western. To me that means Stem1 was eastern and, further, that Neander-Denisovans had taken the same trip through the Horn that we did.