North African variability

Another point of interest in Dartnell: variability. Dartnell, 22 dates such events, in East Africa: 2.7-2.5 mya, 1.9-1.7, and 1.1 mya to 900kBC. All before anything mate-ably human. Dartnell sets an 800k period to these cycles.

This implies, for a fourth, the end of the Eemian 120kBCish. But "Eemian" didn't make the index. And by then the various humans around the Old World were smart enough to weather the changes. Perhaps.

Of the three spikes, Dartnell holds that these were the crucible which made humans. I quibble here that he's not looking at the Sahara, North Africa, nor West Africa. East Africa preserves the remains, and is more-or-less Qaeda-free; so East Africa's where he's got to look.

The middle span of variability falls in the ~2.3-1.46 mya Dry Sahara epoch - at least, as seen from Tenerife. But the other spans fall outside it. Either the Sahara was wet or else the westbound wind wasn't strong; and since we know that the Sahara depends on the monsoons (or not) we can rule out the latter.

Dartnell, 38 instead cites the Milankovitch Cycles. These have always been ongoing but 2.8 mya, the Atlantic / Pacific current got SERIOUSLY weakened when Panama was closed off. That in turn weakened the North Atlantic conveyor. So next time the cycle hit, 2.6 mya... ice age, in the Northern Hemisphere. Which was Variable Age in East Africa.

I propose in the Sahara it just took another 400ky or so to dry out Chad again.