Climate & the Out of Africa migration(s)

Wet phases in the Sahara/Sahel region and human migration patterns in North Africa:

The carbon isotopic composition of individual plant leaf waxes (a proxy for C3 vs. C4 vegetation) in a marine sediment core collected from beneath the plume of Sahara-derived dust in northwest Africa reveals three periods during the past 192,000 years when the central Sahara/Sahel contained C3 plants (likely trees), indicating substantially wetter conditions than at present. Our data suggest that variability in the strength of Atlantic meridional overturning circulation (AMOC) is a main control on vegetation distribution in central North Africa, and we note expansions of C3 vegetation during the African Humid Period (early Holocene) and within Marine Isotope Stage (MIS) 3 (â50-45 ka) and MIS 5 (â120-110 ka). The wet periods within MIS 3 and 5 coincide with major human migration events out of sub-Saharan Africa. Our results thus suggest that changes in AMOC influenced North African climate and, at times, contributed to amenable conditions in the central Sahara/Sahel, allowing humans to cross this otherwise inhospitable region.

More details from the discussion:

The influence of AMOC on vegetation type in North Africa is of particular interest because this region may have played a key role in the dispersal of anatomically modern humans, which originated in sub-Saharan Africa at 195 ka...into Europe and Southwest Asia...A major dispersal period occurred between 130 and 100 ka...which coincides with a major expansion of C3 vegetation from 120-110 ka (Fig. 2), and thus wetter conditions in the Sahara region, supporting the hypothesis that the Sahara could have provided a dispersal route out of Africa...

The most depleted n-alkane 13Cvalues of the entire record are noted within MIS 3, from 50-45 ka (Fig. 2), suggesting a major expansion of C3 plants. Interestingly, this interval coincides with a second major dispersal period of hominins out of Africa, dated to 60-40 ka, mainly based on studies of mtDNA...Additionally, mtDNA evidence suggests that a back migration into Africa from southwestern Asia occurred at 45-40 ka, and it is thought that this event resulted from a climatic change that allowed humans to enter the Levant...Thus, as with the first out of Africa migration during MIS 5, our data suggest that a second period of hominin migration at 60-40 ka may have been facilitated by amenable climate conditions in the central Sahara....

Here's a chart which shows the correlation with the "Out of Africa" event:

Citation: Isla S. Castañeda, Stefan Mulitza, Enno SchefuÃ, Raquel A. Lopes dos Santos, Jaap S. Sinninghe Damsté, and Stefan Schouten, Wet phases in the Sahara/Sahel region and human migration patterns in North Africa, PNAS published online before print November 12, 2009, doi:10.1073/pnas.0905771106