The Sahara in central Africa is the largest hot desert on Earth, its blistering sands practically lifeless. But just a few thousand years ago, it was a home to early human civilizations. Over the past decade, we've gotten a sharp picture of what happened to the people who lived in this lost landscape.
Photo by Mike Hettwer
Roughly the size of the United States, the Sahara stretches across several African and Middle Eastern countries, including Egypt, Chad, Sudan, Libya, Algeria and more.
Today, the desert is vast and forbidding. But a few brave archaeologists who traveled deep into the western reaches of the area over the past hundred years have discovered something incredible. There are enormous murals of paleolithic rock art stretching back at least 10,000 years, depicting everything from animals to the clothing people wore during village ceremonies.
Often the people in these paintings are drawn with very round heads, a characteristic Saharan style. Paintings that share this style probably come from peoples with similar cultural origins. As the centuries passed, the paintings become more sophisticated and we see images that show bronze age tools, people riding horses, and using clay pots. The question is, what happened to these people? How did they live in such arid conditions? Where did they go?
Stefan Kröpelin is an archaeologist from Germany who wanted to find out. He and his team ventured out into the unexplored desert every year for decades, looking for clues. They tracked the locations of these cave paintings, and along the way they began to discover signs of what the Sahara had been like thousands of years ago. In massive, dry valleys they found shells and fish skeletons. They found remnants of trees and traces of pollen.
They realized that what they were witnessing was a history of climate change in the region. A once-fertile land of rains and lakes had dried up into a Martian landscape in just over 10,000 years. And as the rains moved, so too did the people.
In a 2006 paper, Kröpelin and his team explain this chart of human occupation (click to enlarge). Red dots are areas of long-term human habitation, and white dots show transient human occupation. What they suggest is that after a long arid period during the last ice age, the Sahara began to experience heavy monsoons starting about 8500 years BCE. The whole region became a grassy savannah, full of edible plants and animals, and people moved from the Nile valley deep into the eastern Sahara. As the monsoons grew milder, about 7000 BCE, people moved south too. But then, about 5300 BCE, the monsoons began to dry up. That's when people began to cluster back around the Nile again.
Kröpelin and his team write:
Rainfall zones are delimited by best estimate isohyets on the basis of geological, archaeozoological, and archaeobotanical data. (A) During the Last Glacial Maximum and the terminal Pleistocene (20,000 to 8500 BC), the Saharan desert was void of any settlement outside of the Nile valley and extended about 400 km farther south than it does today. (B) With the abrupt arrival of monsoon rains at 8500 BC, the hyper-arid desert was replaced by savannah-like environments and swiftly inhabited by prehistoric settlers. During the early Holocene humid optimum, the southern Sahara and the Nile valley apparently were too moist and hazardous for appreciable human occupation. (C) After 7000 BC, human settlement became well established all over the Eastern Sahara, fostering the development of cattle pastoralism. (D) Retreating monsoon rains caused the onset of desiccation of the Egyptian Sahara at 5300 BC Prehistoric populations were forced to the Nile valley or ecological refuges and forced to exodus into the Sudanese Sahara where rainfall and surface water were still sufficient. The return of full desert conditions all over Egypt at about 3500 BC coincided with the initial stages of pharaonic civilization in the Nile valley.
Photo by Vyacheslav Argenberg
The question is how fast did this transformation happen? Some scientists have argued that the climate change was very abrupt, and that it only took about a century for those grassy hills and valley lakes to become the stark sand dunes of today's Sahara. But Kropelin and his team, in a 2008 paper, argued that it probably took more like five centuries or more. They based this hypothesis on evidence taken from core samples drilled out of an ancient lake in Chad, whose waters have been so undisturbed that the sediment on the bottom provides an almost picture-perfect record of the past 6,000 seasons. By looking at an extremely deep core sample, they can see layers of vegetation going back millennia.
What they found was that vegetation in the area died off slowly, over a long period of time. There was no abrupt shift in climate. And that's why humans in the area likely had plenty of time to migrate to the shores of the Nile, where ancient Egyptian civilization rose up to become a legendary power in the region.
It's likely some of our earliest examples of human civilization arose in a fertile land that no longer exists. In the Sahara, thousands of years ago, a complex human culture was displaced by climate change.
Annalee Newitz is the author of the book, Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction. Follow her on Twitter.