This is an extremely important article for anyone wishing to understand the climate history of Africa during the time of the emergence of modern humans. I'm no climatologist, so the arguments about glaciation cycles, currents, monsoons, orbital cycles, etc. go mostly over my head. But, I did manage to glean some useful information from the article:
- Climate change in Africa was asynchronous; where one region may have experienced a wet phase, another one may have experienced a dry one. Moreover, its variations did not correspond perfectly with Marine Isotope Stages and glaciation cycles.
- The impact of wet and dry phases may be different by region. A wet phase in the Sahara means more vegetation and game, and more people; by contrast, a wet phase in a tropical region means more and denser rainforest, and more hardship.
- The middle regions of Africa were more shielded from climate change than either north or south Africa: human occupation appears more even; however, West and East Africa seem to have been following opposite trajectories: "Relative site abundances in tropical Africa increase during one period of aridity (~115-90 ka), and in East Africa decline during the comparable interval of humidity (~110-95 ka)."
Hominin dispersals to Eurasia via northern Africa have also been linked to humid intervals (Osbourne et al., 2008; Drake et al., 2011). From ~30-75 ka, North Africa experienced arid climate regimes. The Sahara expanded 65-50 ka and the demographic data imply abandonment of the interior of North Africa during this time (Fig. 15), suggesting that dispersals from North Africa occurred prior to 75 ka. This finding is at odds with hypotheses that posit a dispersal timing of ca. 60 ka (Mellars, 2006), or a dispersal origin in the South African Howiesons Poort industry at 65-60 ka (Marean, 2010), although a ca. 65-60 ka dispersal could well have occurred from East Africa over the Bab el-Mendeb, as humid conditions dominated the latter region between 80 and 65 ka. The timing of the humid intervals in North Africa (~135-115 ka and ~105-75 ka) corresponds with those in the Levant (54a) (Vaks et al., 2007), providing multiple opportunities for one or more earlier hominin dispersals out of Africa via the Sahara, or an early exodus from East Africa to Arabia over the Bab el Mendeb (e.g., Rose et al., 2011). This pattern does not rule out possible migrations from this region during either the earlier or the later interval through the Nile Valley. However, during the ~105-75 ka humid interval in North Africa, East Africa (where the sources of the Nile occur) was largely under an arid climate regime, except for a narrow window ~80 ka when both northern and eastern Africa were experiencing humid climates (Fig. 15), placing a more restricted temporal window on dispersal via the Nile Valley. We note that although only one out of Africa event is recorded in the modern human genome (e.g., Prugnolle et al., 2005), this fact is not inconsistent with earlier multiple migration and extinction events using any of these routes, with only a single lineage surviving to the present.It will probably take me a while to digest the information in this paper, but I'm definitely glad that someone thought to write it. For the time being I will try to sanity-check my own two deserts theory:
- The 135-115ka humid interval in the Sahara and the Levant could explain the origin and growth of a population that would spill over to Asia at this time. Jebel Faya may date to this period.
- ~115ka the onset of dryness would have been detrimental to populations in the Sahara-Arabia belt. It is at this time that site abundances increase in tropical Africa which is also becoming drier, and hence more conducive to human occupation (less dense rainforest); we can perhaps infer that the deepest human lineages got isolated from the rest of mankind as they entered tropical Africa, and even reached Southern Africa which was humid at this time.
- Post 105ka we see another wet phase in North Africa/Levant, which would again spill over to Asia; the Nubian Complex appears at around this time in Arabia. There are uncertainties of dating about the Skhul/Qafzeh modern humans, and they may date from this time, or the previous wet period (135-115ka).
- At the 70ka mark where something big may have happened, North Africa/Levant go to a dry period; this is perhaps a good time to see survival of a few lineages (mtDNA L3 and Y-chromosome BT and CT) and another exodus of modern humans from this area. An exodus from East Africa at this time would not make much sense because humans would go from a humid region to a dry one; it would be an awful time for Africans to be headed to Asia.
J Hum Evol. 2012 May;62(5):563-92. Epub 2012 Apr 17.
The environmental context for the origins of modern human diversity: a synthesis of regional variability in African climate 150,000-30,000 years ago.
Blome MW, Cohen AS, Tryon CA, Brooks AS, Russell J.
Abstract We synthesize African paleoclimate from 150 to 30 ka (thousand years ago) using 85 diverse datasets at a regional scale, testing for coherence with North Atlantic glacial/interglacial phases and northern and southern hemisphere insolation cycles. Two major determinants of circum-African climate variability over this time period are supported by principal components analysis: North Atlantic sea surface temperature (SST) variations and local insolation maxima. North Atlantic SSTs correlated with the variability found in most circum-African SST records, whereas the variability of the majority of terrestrial temperature and precipitation records is explained by local insolation maxima, particularly at times when solar radiation was intense and highly variable (e.g., 150-75 ka). We demonstrate that climates varied with latitude, such that periods of relatively increased aridity or humidity were asynchronous across the northern, eastern, tropical and southern portions of Africa. Comparisons of the archaeological, fossil, or genetic records with generalized patterns of environmental change based solely on northern hemisphere glacial/interglacial cycles are therefore imprecise. We compare our refined climatic framework to a database of 64 radiometrically-dated paleoanthropological sites to test hypotheses of demographic response to climatic change among African hominin populations during the 150-30 ka interval. We argue that at a continental scale, population and climate changes were asynchronous and likely occurred under different regimes of climate forcing, creating alternating opportunities for migration into adjacent regions. Our results suggest little relation between large scale demographic and climate change in southern Africa during this time span, but strongly support the hypothesis of hominin occupation of the Sahara during discrete humid intervals ~135-115 ka and 105-75 ka. Hominin populations in equatorial and eastern Africa may have been buffered from the extremes of climate change by locally steep altitudinal and rainfall gradients and the complex and variable effects of increased aridity on human habitat suitability in the tropics. Our data are consistent with hominin migrations out of Africa through varying exit points from ~140-80 ka.