How a wobbly earth and green Sahara led to the spread of humans

It’s an interesting tale, and it takes place in the hot, arid Sahara Desert in Africa. Actually, it takes place in the warm, wet and green Sahara. The Sahara Desert was not always the hyper-arid, dune-swept region it is now. It has been lush-green, damp and full of rivers in the past (about 230 times in the last 8 million years, every 21,000 years or so, to be more precise). During these green periods, vegetated corridors created distribution channels for various species – including humans. So, how did the Sahara change so much? What led to the climatic variations that changed the desert into a savannah?

It all comes down to the Earth’s dance in the Milky Way: the past changes occurring in the Sahara are linked to cyclic variations in the Earth’s orbit. To understand this, one needs to know how our Earth moves. We all know that the Earth rotates every 24 hours (causing day and night) and that it undergoes a 365-day revolution around the sun (causing seasons), but did you know that every 100,000 years, the shape of Earth’s orbit shifts between circular and oval (a phenomenon known as eccentricity)? And that every 41,000 years the tilt of Earth’s axis changes (termed obliquity) between 22.1 and 24.5 degrees? These changes in the eccentricity and obliquity cycles are responsible for driving the ice ages of the past 2.4 million years. Furthermore, the Earth is not a perfect sphere; it bulges at the Equator and is influenced by the sun’s gravity, moon, and planets. Just like a spinning top, it sometimes wobbles during its rotation. This is called ‘precession’ and is defined as the slow change in the direction of Earth’s rotational axis, and it varies on a 21,000-year timescale. There are no prizes for guessing then that the 21,000-year precession cycle is linked to the 21,000-year timing of the wet, green Sahara periods.

How does a wobbly Earth make a desert go green?

Firstly, one needs to understand what climate systems influence North Africa. The region’s climate is governed by three different systems: the northern (Mediterranean and northernmost Atlantic) coastal belt of westerly rains that fall mainly during the late autumn to early spring, and the West and East African monsoons (WAM and EAM) which bring summer rains to the subtropical regions located west and east of the River Nile. When the Earth wobbles on its axis (precession) at different times during the cycle, the seasons will become more or less extreme in the northern or southern hemisphere. When the precession causes the Northern Hemisphere to be closer to the sun during the summer months, there is an increase in North African summer rainfall. This happens because of an increase in the amount of solar radiation in the tropics, which is the engine that fuels the monsoon system. This then results in the enhancement of East African Monsoon summer rainfall over the southern part of the Nile catchment and the Ethiopian highlands and also causes the intensification and enhanced northward penetration of the West African Monsoon summer rains over the present-day Sahara. During these times, the increased rainfall and solar radiation resulted in the greening of the Sahara, with swathes of savannah vegetation and abundant lakes and rivers. These are the so-called Green Sahara Periods (GSPs) or the (more prosaic) term “North African humid periods” (NAHPs).

However, the humid periods sometimes do not occur (they skip a beat). Using climate change modelling, Armstrong et al. (2023) found that these periods occurred during the ice ages when sizeable glacial ice sheets covered much of the polar regions. These vast ice sheets cooled the Earth’s atmosphere, offset precession’s warming influence and suppressed the expansion of the African monsoon system.

In summary: the Earth wobbles, temperatures go up in some places and down in others, the monsoon engine is revved up, and more rain falls in the Sahara, except for during the Ice Ages, which are driven by the eccentricity cycle (how circular Earth’s orbit is around the sun). During these times, the vast surface area of ice sheets causes cooling, which offsets the warming. This is one of the most exciting findings of Armstrong et al. (2023); it shows us how connected everything is. The desert is linked to the ice, and the vegetation is linked to the movement of the Earth.

How did we first learn about these wet periods in the Sahara?

Studies on pollen analysis and marine and lake sediments in the Sahara have shown us that there was far more vegetation during these periods than there is now. But we also know about these GSPs through the details of the rock art in the area. The humans living during those greening periods told the story of antelopes, crocodiles, hippos, and giraffes through the language of art. The details of the rock art in the World Heritage Site, Tassili n’Ajjer, located in south-east Algeria at the borders of Libya, Niger and Mali, contain some of the “most eloquent expression of relationships between humans and the environment, with more than 15,000 drawings and engravings testifying to climate changes, wildlife migrations, and the evolution of humankind on the edge of the Sahara. This art depicts water-dependent species like the hippopotamus, which have been extinct in the region for thousands of years.”

So how did the greening of the Sahara change humankind’s journey?

We need to keep the bigger picture in mind. The Sahara Desert takes up 9 million km², one-third of the African continent, and when dry, represents a significant barrier to the dispersal of species, including ancient hominid races. The theory is that these GSPs led to vegetated corridors, which then allowed changes in species’ distribution and evolution and may have facilitated the out-of-Africa migrations of ancient humans. These fertile phases presumably resulted in a significant expansion of human populations, which may, in turn, have increased the number of favourable genetic mutations which underpinned the speciation of hominin lineages.

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Combined with the environmental variability associated with GSPs coming and going, the humans probably arrived and left, which might have had an additional impact on human population dynamics as it might have split African and Asian populations. When GSPs ended, human groups were likely forced to retreat to already densely populated areas or to survive in regions with still water. There is a significant association between the currently known first and last appearance datums of the major hominin lineages, suites of technological behaviours, and dispersal events with the predicted intervals of prolonged high climate variability associated with precession cycles.

Therefore, a wobbly Earth led to periods of increased climate variability, prompting human adaptability and flexibility and leading to evolutionary change in the hominids. So, if you are feeling a little off-colour today, just cast your mind back millions of years and think about how much of our history is based on a wobble. We need the wobblies to keep us on our toes.

How a wobbly earth and green Sahara led to the spread of humans

How does a wobbly Earth make a desert go green?

How did we first learn about these wet periods in the Sahara?

So how did the greening of the Sahara change humankind’s journey?

Further reading