Breaking down barriers: green deserts and windows for Homo sapiens dispersals

Imagine a place on the surface of our planet that we, Homo sapiens, could not reach.

We are found across the planet, from Inuit tribes in the freezing realms of the Arctic circle to the Bedouin tribes in the desert regions of the Middle East. If Elon Musk has his way we’ll be on Mars without giving the geological timescale a chance to blink its eye.

But it was only between 300,000 and 200,000 years ago that our species originated from an ancestral Homo species on the African continent1. So how is it that we are already looking at travelling to Mars? How and when did we make our first great expedition from Africa?

The modern-day Nefud Desert, Saudi Arabia. The area currently receives between 30 – 90 mm of rainfall a year, but lake sediments in inter-dune depressions give evidence of much wetter phases.
Image copyright Richard Clark-Wilson.

Models for our dispersal from Africa

The earliest Homo sapiens fossils outside of Africa are found in the Mediterranean Levant at sites called Skhul and Qafzeh, and date to between 90,000 and 120,000 years old2. But there is debate as to whether this represents our first successful dispersal.

One group of theories argue that it was a failed dispersal, and that we successfully left Africa for the first time much later, approximately ~60,000 years ago3. These are called ‘Late Dispersal Models’.

Recently, however, evidence is building to suggest that we left Africa earlier than this, between ~75,000 and 130,000 years ago. These are called ‘Early Onset Dispersal Models’ and support the idea that the fossils found in Skhul and Qafzeh were part of a wider, successful dispersal4,5.

Green deserts: the northern route from Africa into Arabia

A map of the Saharo-Arabian desert belt including the Saharan and Nefud Deserts, along with the Rub al Khali desert in southern Saudi Arabia. The approximate location of the Skhul/Qafzeh Homo sapiens fossil sites in the Levant, dated to between 90,000 – 120,000 years2, is highlighted by black dots within a dashed box.
Original map by Google, modified by Richard Clark-Wilson.

Whilst there are different routes that we may have taken from Africa, it is the northern inland route that is most fascinating. This took us from sub-Saharan Africa to Eurasia via the Saharan Desert and the Sinai Peninsula, before moving through the northern portion of the Arabian Peninsula and into the Nefud Desert.

You are stood at the foot of a hyper-arid desert in a tribe equipped with stone tools and not much else. Could you get across some of the most inhospitable environments in the world?

No, neither could they. Something must have changed.

What if I were to tell you that the Saharan and Nefud Deserts have not always been impassable deserts?

In fact, we have a lot of evidence to show that they weren’t.

Inside the deserts are sediments that could only have been laid down in lakes, rivers and soils6,7, whilst satellite imagery of the deserts has revealed numerous ancient river channels8,9. These are features that could only form in a much wetter climate than today.

It was the climate that changed. And it transformed the deserts into a useful corridor for human dispersal.

An example of lake sediments preserved in the Nefud Desert. They occur within inter-dune depressions but lie above the dune floor as inverted relief features.
Image by Simon Armitage, Palaeodeserts Project, used with permission.

How and when did these climatic changes occur?

The Earth’s climate is orbitally-driven by ‘Milankovitch Cycles’. These cycles include three parameters: (1) eccentricity (the way the Earth orbits the sun), (2) obliquity (how the Earth moves on its axis) and (3) precession (how the Earth wobbles on its axis). They occur on ~100,000, ~41,000 and ~21,000 thousand year cycles respectively, and combine to change the amount of energy – insolation – we receive from the sun.

This is important because insolation drives the monsoon systems. Greater insolation during the summer leads to the land heating up faster than the ocean, resulting in intense convection and a low-pressure system that draws in cool, moist air from the ocean, creating monsoonal rainfall.

As insolation drives this process, the northerly extent of African monsoon rainfall should be close to or at its greatest when summer insolation at 30°N is at a maximum10. This is the most likely time at which the Saharan and Nefud Deserts transformed into habitable landscapes, with grasslands, rivers and freshwater bodies able to support human populations.

Mean June insolation at 30°N over the past 150 ka11. Insolation controls the African monsoon extent, and the three insolation peaks between ~75,000 – 130,000 years (highlighted by black boxes) could have produced wetter phases within the Saharo-Arabian desert belt.
Graph produced by Richard Clark-Wilson using data from Berger, A. and Loutre, M.F. (1991)11

Multiple windows for Homo sapiens dispersals from Africa

Remember the Early Onset Dispersal Model with the idea that we left Africa between 75,000 and 130,000 years ago? Well, if we look at the insolation the earth received between this period there were three peaks. One at ~126,000 years ago, another at ~105,000 years ago, and another at ~85,000 years ago. These, in theory, could have produced multiple green phases in the Saharan and Nefud Deserts, and multiple windows for our ancestors to disperse across them.

This, combined with synchronous wet phases in the Sinai Peninsula, Levant, and Negev Desert caused by a southward shift in Mediterranean Westerly rainfall12, could have formed a green corridor that connected sub-Saharan Africa to the Arabian Peninsula, and provided our ancestors with a northern route out of Africa. Importantly, research by the Palaeodeserts team, using climate models, modelled surface freshwater availability and dated wetland sediments (e.g. lakes, rivers and soils), shows such a connection could have existed multiple times between 75,000 and 130,000 years ago13. This is the northern route out of Africa, and could have allowed early dispersers past the Mediterranean Levant, into the Arabian Peninsula and beyond.

The emerging palaeoenvironmental evidence supports Early Onset Dispersal Models, and suggests that orbitally-driven climate change presented multiple windows for Homo sapiens to disperse from Africa between ~75,000 and 130,000 years ago. Our ancestors had multiple opportunities to start their journey to Mars, but whether they successfully took them or not remains up for debate.

References

1. Richter, D., Grün, R., Joannes-Boyau, R., Steele, T.E., Amani, F., Rué, M., Fernandes, P., Raynal, J.P., Geraads, D., Ben-Ncer, A. and Hublin, J.J. (2017) ‘The age of the hominin fossils from Jebel Irhoud, Morocco, and the origins of the Middle Stone Age’, Nature, 546(7657), pp.293-296, doi: 10.1038/nature22335
2. Grün, R., Stringer, C., McDermott, F., Nathan, R., Porat, N., Robertson, S., Taylor, L., Mortimer, G., Eggins, S. and McCulloch, M. (2005) ‘U-series and ESR analyses of bones and teeth relating to the human burials from Skhul’, Journal of Human Evolution, 49(3), pp.316-334, doi: 10.1016/j.jhevol.2005.04.006
3. Mellars, P., Gori, K.C., Carr, M., Soares, P.A. and Richards, M.B., (2013) ‘Genetic and archaeological perspectives on the initial modern human colonization of southern Asia’, Proceedings of the National Academy of Sciences, 110(26), pp.10699-10704, , doi: 10.1073/pnas.1306043110
4. Groucutt, H.S., Petraglia, M.D., Bailey, G., Scerri, E.M., Parton, A., Clark‐Balzan, L., Jennings, R.P., Lewis, L., Blinkhorn, J., Drake, N.A. and Breeze, P.S. (2015) ‘Rethinking the dispersal of Homo sapiens out of Africa’, Evolutionary Anthropology: Issues, News, and Reviews, 24(4), pp.149-164, doi: 10.1002/evan.21455
5. Liu, W., Martinón-Torres, M., Cai, Y.J., Xing, S., Tong, H.W., Pei, S.W., Sier, M.J., Wu, X.H., Edwards, R.L., Cheng, H. and Li, Y.Y., (2015) ‘The earliest unequivocally modern humans in southern China’, Nature, 526(7575), pp.696-699, doi:10.1038/nature15696
6. Petraglia, M.D., Alsharekh, A.M., Crassard, R., Drake, N.A., Groucutt, H., Parker, A.G. and Roberts, R.G. (2011) ‘Middle Paleolithic occupation on a marine isotope stage 5 lakeshore in the Nefud Desert, Saudi Arabia’, Quaternary Science Reviews, 30(13), pp.1555-1559, doi: 10.1016/j.quascirev.2011.04.006
7. Rosenberg, T.M., Preusser, F., Risberg, J., Plikk, A., Kadi, K.A., Matter, A. and Fleitmann, D. (2013) ‘Middle and Late Pleistocene humid periods recorded in palaeolake deposits of the Nafud desert, Saudi Arabia’, Quaternary Science Reviews, 70, pp.109-123, doi: h10.1016/j.quascirev.2013.03.017
8. Breeze, P.S., Drake, N.A., Groucutt, H.S., Parton, A., Jennings, R.P., White, T.S., Clark-Balzan, L., Shipton, C., Scerri, E.M., Stimpson, C.M. and Crassard, R. (2015) ‘Remote sensing and GIS techniques for reconstructing Arabian palaeohydrology and identifying archaeological sites’, Quaternary International, 382, pp. 98-119, doi: 10.1016/j.quaint.2015.01.022
9. Drake, N.A., Blench, R.M., Armitage, S.J., Bristow, C.S. and White, K.H. (2011) ‘Ancient watercourses and biogeography of the Sahara explain the peopling of the desert’, Proceedings of the National Academy of Sciences, 108(2), pp.458-462, doi: 10.1073/pnas.1012231108
10. Kutzbach, J.E., (1981) ‘Monsoon climate of the early Holocene: climate experiment with the earth’s orbital parameters for 9000 years ago’, Science, 214(4516), pp.59-61.
11. Berger, A. and Loutre, M.F. (1991) ‘Insolation values for the climate of the last 10 million years’, Quaternary Science Reviews, 10(4), pp.297-317, doi: 10.1016/0277-3791(91)90033-Q
12. Parton, A., White, T.S., Parker, A.G., Breeze, P.S., Jennings, R., Groucutt, H.S. and Petraglia, M.D. (2015) ‘Orbital-scale climate variability in Arabia as a potential motor for human dispersals’, Quaternary International, 382, pp.82-97, doi: 10.1016/j.quaint.2015.01.005
13. Breeze, P.S., Groucutt, H.S., Drake, N.A., White, T.S., Jennings, R.P. and Petraglia, M.D. (2016) ‘Palaeohydrological corridors for hominin dispersals in the Middle East∼ 250–70,000 years ago’, Quaternary Science Reviews, 144, pp.155-185, doi: 10.1016/j.quascirev.2016.05.012

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Chronologies and palaeoenvironments of humid phases in the western Nefud Desert, Saudi Arabia over t...


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