A single southern exit of modern humans from Africa: Before or after Toba?

Abstract

During the Late Pleistocene, anatomically modern humans (AMH) dispersed out of Africa. They first spread north with game, across the Sahara to the Levant during the Eemian interglacial (c.125 ka); and there is some recent evidence they may also have spread via Arabia to the Far East at the same time. However, they failed to continue to Europe, then occupied by Neanderthals. The Sahara and Arabia then reverted to aridity, and AMH vanished from the fossil record after 92 ka, being later replaced in the Levant, again by Neanderthals. There is recent fragmentary evidence AMH may also have spread via Arabia to the Far East, but the archaeological record fades similarly, long before the Toba eruption. There is no evidence of surviving non-African DNA lineages, dating from anywhere near the Eemian, to contradict this narrative. Genetic evidence indicates that AMH successfully left Africa much later, as a single group, by the southern route to India. Since all non-African uniparental lineages date to this later exit, this appears to have been the only ultimately successful AMH exit. AMH reached the isolated Sahul continent at least by 48 ka and possibly by 60–50 ka. AMH only finally arrived in Europe from South Asia before 46 ka, probably linked to climatic amelioration during MIS-3.Dating the definitive exit without adequate fossil evidence relies on disputed genetic dates with wide confidence intervals. At the top end, independent genetic dates combined with ash-associated data from Malaysia and from India argue for a pre-Toba exit, although the consensus view is still largely post-Toba. Recent correction for non-linearity due to purifying selection and recalibration of the mtDNA clock has improved confidence in genetic dating, but reduced the age of the main exit lineage (L3, which effectively embodies its maximum estimate) to just below that of Toba (71.6 ka), although relevant 95% Confidence Intervals (CI: 57.1–86.6 ka) still include the Toba event. This weakening of the genetic argument puts more pressure on the archaeological evidence associated with Toba ash and the need for diagnostic, well-dated fossil data between 90–70 ka.