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| Engraving of Giraffes near Gobero in Niger, ca. 8,000 yrs old, witness ancient green times in the desert.
Photo: Mike Hettwer, 2006, www.hettwer.com |
Today, we think of the Sahara Desert in North Africa as a hot, very dry place where only highly specialises species can live and where humans can only survive by careful preparation including a supply of water.
Having said that though, my experience of the Tunisian part of it, one unseasonably cold Spring about ten years ago, is that the Sahara is a cold, slightly damp place with a fine drizzle where the last thing you want to be dressed for is Summer!
Now, however, an international team including researchers from Germany, South Korea, the Netherlands and the USA, led by Cécile Blanchet of the GFZ German Research Centre for Geosciences, Helmholtz Centre, Potsdam, Germany has shown that it was not always the hot, dry place most people find today.
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| Overview of the area of study in Northern Africa and off the coast of Libya. The old river courses and the place where the sediment core was taken can also be seen. Graphic: Axel Timmermann |
From examining sediment in the Gulf of Sirte in the Mediterranean, deposited by (now dry) rivers that flowed north out of the desert, they have shown that there were several periods of 'greening' when the area supported an abundant ecosystem of plant and animal life, interspersed with periods of drastic climate change, over the past 160,000 years.
Key to the research were the 10-meter-long cores drilled out of the sediment. These contain rock fragments and plant remains transported from the nearby African continent. They are also full of shells of microorganisms that grew in seawater.
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| Photo of the surface of a one-meter long section of the sediment core used for this work (64PE349-8). Black sediments are typical layers in the Mediterranean called sapropels. These layers contain the sediments transported by the extinct rivers. Photo: C. Blanchet and R. Tjallingii |
The PFZ News release explains:
Using a method called “piston coring”, the scientists were able to recover 10-meters long columns of marine mud. “One can imagine a giant hollow cylinder being pushed into the seafloor”, says co-author Anne Osborne from GEOMAR, who was onboard the research ship. “The marine mud layers contain rock fragments and plant remains transported from the nearby African continent. They are also full of shells of microorganisms that grew in seawater. Together, these sediment particles can tell us the story of past climatic changes”, explains Blanchet.For me, the interesting thing about this research is how it confirms the 'Sahara Pump' theory, which explains the migration of species, including humans, across what is today an almost impenetrable barrier. This pump was hypothesised to be several just such 'green' intervals, during which species could spread north, only to have to retreat again when the dry periods followed, leaving some of them 'stranded' in the coastal strip on the southern shore of the Mediterranean and from there able to migrate around the Levant and up into Eurasia.
“By combining the sediment analyses with results from our computer simulation, we can now precisely understand the climatic processes at work to explain the drastic changes in North African environments over the past 160,000 years”, adds co-author Tobias Friedrich from the University of Hawai’i.
From previous work, it was already known that several rivers episodically flowed across the region, which today is one of the driest areas on Earth. The team's unprecedented reconstruction continuously covers the last 160,000 years. It offers a comprehensive picture of when and why there was sufficient rainfall in the Central Sahara to reactivate these rivers. “We found that it is the slight changes in the Earth’s orbit and the waxing and waning of polar ice sheets that paced the alternation of humid phases with high precipitation and long periods of almost complete aridity”, explains Blanchet.
The fertile periods generally lasted five thousand years and humidity spread over North Africa up to the Mediterranean coast. For the people of that time, this resulted in drastic changes in living conditions, which probably led to large migratory movements in North Africa. “With our work we have added some essential jigsaw pieces to the picture of past Saharan landscape changes that help to better understand human evolution and migration history”, says Blanchet. “The combination of sediment data with computer-simulation results was crucial to understand what controlled the past succession of humid and arid phases in North Africa. This is particularly important because it is expected that this region will experience intense droughts as a consequence of human-induced climate change.”
This gives humans an alternative rout out of Africa to the one which either came up the Nile valley, or crossed the Red Sea into Arabia and then either up the southern edge of Arabia to Iran, India, etc, or up the Eastern shore of the Red Sea into the Middle East, Iraq and Asia Minor.
It is entirely possible that the early modern Homo sapiens whose 300,000 year-old remains were found in Morocco recently came out of Africa by this route, although this research only covers the last 160,000 years.
As I said at the time of that discovery:
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A composite reconstruction of what its discovers believe is the earliest known Homo sapiens fossil (from Jebel Irhoud), based on scans of multiple specimens. The virtual imprint of the braincase (blue) indicates that brain shape, and possibly brain function, evolved within the H. sapiens lineage, the scientists say. Credit: Philipp Gunz, MPI EVA Leipzig |
So what exactly is new here?The team's research was published a couple of days ago in Nature Geoscience, sadly behind a paywall.
Well, we previously thought that modern humans, H. sapiens, emerged as a distinct species around 200,000 years ago in East Africa (with South Africa flagged as a possibility), so this puts the date back by about 100,000 years. It also shows that H. sapiens may have been widespread in Africa and was present at least in the far North West, north of the Sahara.
The animal remains found with them show they had a diet of big game such as gazelle and zebra - which also begs the question, what were sub-Saharan species doing in Morocco? The answer to that is also the answer to the question of how humans from East and maybe South Africa came to be north of the Sahara. The answer could well be the so-called Sahara Pump. I mentioned this in my recent book, What Makes You So Special? The idea is based on the archaeological evidence that North Africa experienced a slow cycle of very wet weather alternating with very dry weather. During the wet period the Sahara became savanna with lakes and rivers and contiguous with the sub-Saharan savanna enabling sub-Saharan species to migrate north along, in this case, maybe with humans. In the dry periods the Sahara dried up isolating populations in the north.
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