November 30, 2009

Wet phases in Sahara and migrations in North Africa

This is an open access article.

PNAS doi:10.1073/pnas.0905771106

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

Isla S. Castañeda et al.


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 (≈50–45 ka) and MIS 5 (≈120–110 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.



Eric said...

terryt said...

And that's just the "substantially wetter conditions than at present", with trees. Presumably at less than 'substantially wetter conditions' grassland would have allowed humans 'to cross this otherwise inhospitable region' and move along the north coast into the Levant. So where does this leave the Bab el Mandeb theory?

eurologist said...

where does this leave the Bab el Mandeb theory?

This paper just confirms what others have published in terms of wetter intervals for the region. Remember that for a successful expansion you need two components: a passable corridor (sufficient food and fresh water), and areas of bounty in which the population can grow quickly.

And for either one above, the people have to be adapted or adapt to actually be able to use the resources. For example, even in those two wet times the Levant was considerable colder than today (and extremely cold for anyone coming out of Africa), with almost impassable dry regions in much of Egypt and the Sinai. In addition, as archeological records seem to confirm, the Levant even then could not support a large population, and - like the Iran/Iraq region - hosted powerful competing Neanderthals that had a much better historic grasp of the region, climate, and resources.

Sure, it is remotely possible that AMHs survived the first period locally in North Africa or in small pockets in the Levant/Iran - but there is no record of that - quite the contrary. On the flip side, the coastal route is in compararison a super highway to much more steady resources and orders of magnitude larger possible populations in the East.

Importantly, the wetter climates during both periods also would have allowed much larger population densities in Ethiopia and surroundings, and would have eased otherwise existing migration difficulties along the Arabian shores. In other words, the coastal route - which is easier in the first place - also and crucially so benefited from the improved climate.

Now, the only persistent problem I see is that all logical pathways point to the second interval as that taken by AMHs from northern India/Pakistan via Afghanistan westward (and perhaps also NE to the Altai) - which requires a significant population in India/Pakistan just before. All well and good, but unless they got there in no time, they must have actually originated from the first phase to make it there in time and multiply, accordingly. In other words, I would like to see more convincing evidence of AMHs and their growth in that region in the 100,000 to 50,000 year time frame...

terryt said...

"In other words, the coastal route - which is easier in the first place -"


eurologist said...

Yes, the coastal route did not have hundreds of miles of stretches without fresh water or food supply even during wet intervals (which still were much colder than today, and much dryer than today in much of the Levant and in surrounding regions). It also was not occupied by Neanderthals who would know the region, climate, and its resources.

terryt said...

Let's not forget the name, 'Bab al Mandab'. According to Wikipedia:

"The strait derives its name from the dangers attending its navigation".

Presumably the dangers were just as great in ancient times. The tidal flow would be very strong. So, easily crossed during the Paleolithic? Perhaps tides didn't exist in those days.

Next, the coast of the Hadramawt. How about this picture of the Yemen coast:

Are you saying humans had no trouble walking along such a coastline? 'On the flip side, the coastal route is in compararison a super highway'? And don't go claiming lowered sea level opened a four-lane highway along the coast. It didn't. The coast in the region plunges steeply to depths well below any postulated sea level drop. The next step, the Makran coastline, has much the same difficulties.

So, compared to the difficulties of that route a few Neanderthals would be nothing. Besides which, compared to the southern coastal route, a route from the Eastern Nile Delta along the Gaza coast to the Yisrael River valley would be a Sunday stroll. The Yisrael Valley would allow easy access to the (now) dry Jordanian and Syrian region. But in times of relative moistness of the Sahara we could assume the region was grassland during the same periods. And a little cold never worried the near-naked Tasmanian Aborigines, at more than 40 degrees south, when early Europeans first met them.

"unless they got there in no time, they must have actually originated from the first phase to make it there in time and multiply".

It's obvious to me that you do see the biggest (of many) problems with the postualed southern coastal route.