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December 31, 2010

Mapping the “Green Sahara”

Filed under: geography,Geology,Green Sahara,Human Evolution,Out-of-Africa,Sahara — Razib Khan @ 2:30 pm

Guelta d’Archei, Chad. Credit: Dario Menasce.

Everyone who is literate knows that the Sahara desert is the largest of its kind in the world. The chasm in cultural, biological, and physical geography is very noticeable. Northern Africa is part of the Palearctic zone, while the peoples north of the Sahara have long been part of the circum-Mediterranean population continuum. The primary continuous habitable corridor is that of the Nile valley. And yet scholars have long known that there has been variation in the climatic regime of the Sahara. The pharaohs of ancient Egypt seem to have hunted a wider range of fauna than is to be found in the deserts surrounding the current Nile valley, perhaps relics from a more humid period. Rock art in some regions of the desert indicate aquatic life, and species more characteristic of the savanna. And yet we should not think of the Sahara as a recent phenomenon; it does seem to be geologically ancient, despite periodic humid interregnums.

ResearchBlogging.orgA new paper in PNAS attempts to map the hydrography of the Sahara over the Holocene, as well as back to the Pleistocene. The ultimate aim seems to be to better frame the geographic constraints on the expansion of humanity from its African homeland, and refute a simple projection from the present to the past. In this case, it is the existence of the Nile as a verdant and habitable watercourse which connects the north and south, and bisects the continuous desert. Ancient watercourses and biogeography of the Sahara explain the peopling of the desert:

Evidence increasingly suggests that sub-Saharan Africa is at the center of human evolution and understanding routes of dispersal “out of Africa” is thus becoming increasingly important. The Sahara Desert is considered by many to be an obstacle to these dispersals and a Nile corridor route has been proposed to cross it. Here we provide evidence that the Sahara was not an effective barrier and indicate how both animals and humans populated it during past humid phases. Analysis of the zoogeography of the Sahara shows that more animals crossed via this route than used the Nile corridor. Furthermore, many of these species are aquatic. This dispersal was possible because during the Holocene humid period the region contained a series of linked lakes, rivers, and inland deltas comprising a large interlinked waterway, channeling water and animals into and across the Sahara, thus facilitating these dispersals. This system was last active in the early Holocene when many species appear to have occupied the entire Sahara. However, species that require deep water did not reach northern regions because of weak hydrological connections. Human dispersals were influenced by this distribution; Nilo-Saharan speakers hunting aquatic fauna with barbed bone points occupied the southern Sahara, while people hunting Savannah fauna with the bow and arrow spread southward. The dating of lacustrine sediments show that the “green Sahara” also existed during the last interglacial (∼125 ka) and provided green corridors that could have formed dispersal routes at a likely time for the migration of modern humans out of Africa.

This paper was written before the Denisovan admixture results shifted the necessity to genuflect so explicitly to Out of Africa. But its results are interesting nonetheless, since they don’t depend too deeply on a paleoanthropological model. Rather, by surveying biogeogeography and geologic data they produce a sense of how the Sahara exhibited climatic flux over the past 100,000 years as a function of time and space. The latter is important because the Sahara is not an amorphous sandy waste. Rather, it exhibits a great deal of topographical variation:

Credit: T L Miles

In the Tibesti mountains the highest peaks are ~11,000 feet above sea level (3,400 meters). Because of the aridity of the Sahara in general even these elevations does not induce sufficient precipitation to produce a “green mountain” effect, common in other arid parts of northern Africa and Arabia. But in a regime of slightly only higher precipitation and milder temperatures (remove 3 degrees fahrenheit per 1,000 feet against latitude controlled sea level temperature) one can imagine the Tibesti having been much more biologically productive in the past. Consider this from the Tassili n’Ajjer region of southern Algeria:

Because of the altitude and the water-holding properties of the sandstone, the vegetation is somewhat richer than the surrounding desert; it includes a very scattered woodland of the endangered endemic species Saharan Cypress and Saharan Myrtle in the higher eastern half of the range.

The range is also noted for its prehistoric rock paintings and other ancient archaeological sites, dating from neolithic times when the local climate was much moister, with savannah rather than desert. The art depicts herds of cattle, large wild animals including crocodiles, and human activities such as hunting and dancing….

The main thrust of the paper seems to be to refute the common assumption that an eternal Nile served as the north-south corridor for African fauna, including humans. Here is the reason:

Reanalysis of the Saharan zoogeography…suggests that many animals, including water-dependent creatures such as fish and amphibians, dispersed across the Sahara recently. For example, 25 North African animal species have a spatial distribution with population centers both north and south of the Sahara and small relict populations in central regions. This distribution suggests a trans-Saharan dispersal in the past, with subsequent local isolation of central Saharan populations during the more recent arid phase. If a diverse range of species (including fish) can cross the Sahara, it is impossible to envisage the Sahara functioning as barrier to hominin dispersal. The zoogeography of the Nile suggests that it was a much less effective corridor…Only nine animal species that occupy the Nile corridor today are also found both north and south of the Sahara….

There are also isolated pieces of evidence which refute a Nile-only model: Saharan oases which have endemic species of crocodiles. The existence of “desert crocodile” populations is a signal of a more well-watered past, with a subsequent retreat into isolated oases (some of these populations did go extinct in the 20th century though). In some ways this is a problem. Simple models make simple predictions, and are easier to test. But if simple models are false, that is an even greater problem.

Here are the figures which outline the primary results from geology and biogeography:

There are two primary inferences made in regards to humans:

1) The Holocene inference seems to be that Nilo-Saharan populations have their origins in the societies which expanded north and south along the liminal zone of the Sahara. The authors argue that Nilo-Saharan populations on isolated oases in the northern Sahara are relics from the past expansion in the early Holocene. This sounds plausible, but it would be nice to explore this in more depth via linguistic and genetic analysis. With the rise of the camel and Islam a trans-Saharan trade in humans may have resulted in a great deal of trans-location of whole populations from one area to another. Concurrent with the Nilo-Saharans who pushed north the authors also suggest that savanna hunters moved south. I am not clear who these people are from the paper, and the mapping between archaeology and linguistics here seems more tentative.

2) A deep history inference also seems to be that trans-Sahara population movements were feasible in a period around 120-100 years BP, but not 50-60 years BP. The distinction here matters because the latter is a relatively young age for the Out of Africa migration, while the former is an older one. If the latter view is correct then the only plausible route of migration is probably the coastal fringe of the Horn of Africa. If the former view is correct then a whole host of possibilities confront us, because the hydrography of the Sahara may have been constrained, but there were several avenues of migration.

In regards to #2, a clement phase, and then resealing of the genetic barrier, may align well with recent models which posit a non-trivial period of separation between Africans and non-Africans after the Out of Africa migration. In other words early modern humans may have followed the pattern of many species, with  an expansion into, and beyond, the Sahara, and then a subsequent separation of two populations by a resurgent desert. The difference is that the daughter population isolated on the far side of the desert eventually “broke out” from the margins of the African homeland to the rest of the world.

Citation: Drake NA, Blench RM, Armitage SJ, Bristow CS, & White KH (2010). Ancient watercourses and biogeography of the Sahara explain the peopling of the desert. Proceedings of the National Academy of Sciences of the United States of America PMID: 21187416

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