Rock painting at Tassili n'Ajjer, Algeria
The sea level climate record tells us that there were glacial maximums 150 ky ago and 20 ky ago. Glacial maximums are associated with dryer and colder climates in most areas of the world, including Africa. There's evidence that during these colder and drier glacial periods, hunting strategies in Africa are shifted from a Generalized Grassland Model to a Seasonal Grassland Model that includes seasonal use of tactical hunting methods (link). Additionally, the sea level is at its lowest during glacial maximums and was approximately 120 meters lower than today both 150 ky ago and 20 ky ago.
We also know from the geological record that there were periods when it was much wetter in the Sahara and the Arabian peninsula than today. During the Abbassia Pluvial, North Africa experienced a wet and rainy period lasting from 120 ky ago to 90 ky ago. During the Mousterian Pluvial, which lasted from 50 to 30 ky ago, the Sahara was again a greener place than it is today. The Neolithic Subpluvial was the most recent period in which the Sahara and Arabian Peninsula were wetter.
Blue Marble 3000 World has a good simulation of the glacial-pluvial process at work since the Last Glacial Maximum. The effect of the Neolithic Subpluvial as it contributes toward a greener Sahara and Arabian Peninsula is beautifully rendered.
Lake Victoria disappears in about 15,000BC and reappears at about 12,000BC. Note the sudden greening of the Sudan and Arabian Peninsula at about 9000 BC, then another green sweep northward into the Maghreb in 7000BC. Between 9000BC and 8500BC, Lake Chad grows into a lake that is almost the size of the Caspian Sea. At about 4500BC, the pluvial system in the Sahara begins to collapse. Patches of grassland persist in the Southern Arabian peninsula, Niger, Southern Algeria including Tassili N'Ajjer and de l'Ahaggar National Parks, and the Sudan until 2000BC. Lake Chad drys up.
From the archaeological record, we know that each of these pluvial expansions into the Sahara and the Arabian Peninsula brought people northward. From the genetic record, it appears that the Neolithic Subpluvial also brought people southward (link).
For humans, who didn't have a macro view of the climate they were experiencing, glacial maximums would have necessitated a switch to tactical hunting methods. They would have become more dependant on migratory animals for their food(150 kya and 20kya). As the pluvial patterns swept northward, migratory animals and their hunters would have moved northward into the expanding grassland of the Sahara and the Arabian Peninsula (120-90 kya) and (50-30 kya). The collapse of these systems would strand some of the migratory hunters on the north side of the Sahara and Arabian peninsula.
More recently, the collapse of the Neolithic Subpluvial has "stranded" wayfarers on either side of the Arabian peninsula(link) and on both sides of the Sahara.
There's a cyclic process at work, pushing hunters toward a tactical migratory hunting pattern during dry glacial periods, followed by a transformation of the desert into an inviting grassland for migratory game and hunters, followed by a pluvial collapse, stranding and pushing some hunters northward, well beyond their African origin.
It's also interesting to point out that a few of these migratory hunters would have been able to take advantage of the lower Mediterranean sea level.
The cycle is aptly demonstrated by the distribution and phylogeny of Haplogroup E.
Figure 1 (Semino et al)
Phylogeny and frequency distributions of Hg E and its main subclades (panels A-G.) The numbering of mutations is according to the Y Chromosome Consortium (YCC) (YCC 2002, Jobling and Tyler-Smith 2003). To the left of the phylogeny, the ages (in 1,000 years) of the boxed mutations are reported with their SEs (Zhivotovsky et al. 2004). [See the paper for the author's further comments on this figure.]
Consider: E-M35 crosses a wet Sahara into the Mahgreb and into the Arabian peninsula at the end of the Mousterian Pluvial. Some descendants of E-M78 take advantage of a lower Mediterranean and cross from Tunisia into Sicily. Subsequently, E-M78 again cross the Adriatic into the Balkans during the Upper Paleolithic. Their E-V13 descendants establish themselves on the Balkan peninsula during the Mesolithic (link).
The climate driven cyclic Out of Africa process is most aptly demonstrated by Haplogroup E because it is more recent (Mousterian Pluvial) than other earlier Out of Africa migrations. However, the cyclic climate process likely applies to earlier Out of Africa migrations, including those of Haplogroups D and F.
North African Archaeological Sites:
Was North Africa the Launch Pad for Modern Human Migrations?
Late quarternary climatic reconstruction for the mahgreb (North Africa)
82,000-year-old shell beads from North Africa and implications for the origins of modern human behavior
African Archaeological Sites:
Hunter Gatherer Foraging Strategies in Tropical Grasslands: Model Building and Testing in the East African Middle and Late Stone Age
Related Posts from this site:
The Mediterranean Coastline During the LGM
Refining the eustatic sea-level curve since the Last Glacial Maximum using far and intermediate field sites
Evidence for African seasonal grassland tactical hunting during the Last Glacial Maximum
Eaten out of house and home: Homo Sapiens outruns and out hunts their Neandertal cousins