Lining Cape South Africa and its southern coast are long chains of caves that nearly 200,000 years ago were surrounded by lush landscapes and plentiful food.
During the ice age, which lasted between 195,000 and 123,000 years ago, these caves served as a refuge for a group of humans that some researchers believe were the only people who survived this ice age. This is called the sixth marine isotope stage, or MIS6. And in this coastal area, a lot of archaeological research has been done. Of less interest to archaeologists is the interior of South Africa, which was thought of as an uninhabited and uninhabitable place during at least two waves of ice ages, MIS3 and 2.
Now, a study shows that the area may have been more fertile and temperate during these two glacial periods than previously thought, and that the area likely hosted human groups living around a series of ancient fossils. The study, led by University of Michigan archaeologist Brian Stewart, provides a more comprehensive timeline of the ages and stages of these lakes, and shows human imprints throughout the region. The research, funded by the National Geographic Society, is published in the journal PNAS.
“There’s this ever-present assumption that human concentration centers have always been along the coast and that inland areas, especially the southern interior of the Karoo Desert, have been largely depopulated for long periods of time,” Stewart said. “The funny thing is one has only to go inside and walk around and notice that there is archeology everywhere.”
But to mark the area as worthy of archaeological interest, the researchers needed to prove that humans could have actually lived there. The research team, an international group that includes researchers from South Africa, the United Kingdom and France, examined a series of hyperemic expanses of land surrounded by higher ground. They showed that these areas, called “basins” in Afrikaans, are ancient lake beds, while the upland areas that surround them are erosional terrain and sedimentary deposits left over from their ancient shores.
This indicates that these periods of time were not as dry in this region as previously thought: continuous rainfall and moisture had to keep these lakes full. The milder climate required to maintain the lakes also meant that the landscape was also able to sustain both the flora and fauna needed to support the population.
Reconstruction of ancient excavations
Researchers have used a variety of techniques to date and reconstruct these ancient lakes, shorelines, and lake bottom sediments, and to recreate the landscapes of the area.
That includes radiocarbon dating and a technique called luminescence, which measures the radioactivity of tiny crystals of quartz or feldspar that haven’t seen daylight since sediment covered them tens of thousands of years ago. As they are buried, electrons from radioactive elements common to all deposits are trapped in these crystal arrays at a constant rate. By measuring the number of electrons accumulated in a sample and comparing it to the degree of background radioactivity, researchers can predict its age.
The researchers used these techniques to date plumes from sedimentary lake shores and lake-floor sediments, called lacustrine deposits, found surrounding and within a series of three basins spread over an area of about 100,000 square miles, about the size of Texas. They also aged freshwater mollusk shells that were found scattered throughout the area and embedded within sedimentary lake sediments.
The water gastropod, Tomichia ventricosa, was found in a pan called Swartkolkvloer, in a column of lacustrine deposits. The sediments and shells have been dated together to two time periods: about 39-55 thousand years before the present, and about 31-34 thousand years before the present.
In another pan called Grootvloer, the researchers found a freshwater mollusk called Unio caffer, which requires “perpetual fresh water” and the presence of fish to reproduce. The shells and deposits of lacustrine in this pan date from approximately 20,000 to 22,000 years before the present.
The researchers were also able to predict how much land the lakes would cover, as well as how deep they would be, based on the sediment height of the lakes that flank their coastlines. For example, they determined that the ancient crater at Swartkolkvloer was about 83 square miles across and 59 feet deep, while the old crater at Grootfloer was likely about 17 square miles and 62 feet deep. Another area called Alexandrefontein, about 300 miles northeast near the town of Kimberley, had a lake about 13.5 square miles in area and 48 feet deep.
Based on these criteria, the researchers knew that the lakes would have required a climate radically different from the climate found in the interior of southern Africa today. Using a hydrological model, they determined that evapotranspiration rates within the region were 20% to 25% lower than they are today, while precipitation was 200% higher in Swartkolkvloer and 88% higher in Alexanderfontein.
“In various parts of the interior, from about 60 to about 12,000 years ago, we can see that there were permanent phases of very large bodies of water circulating around what was thought to be an arid, inhospitable environment,” Stewart said. “This was a period when it was seen as dry and very cold in the winter. But we have this evidence that these large lakes existed during this time period.”
The researchers say that today’s regions with similar climates and rates of precipitation and evaporation are likely to be northern Botswana, Zambia and Zimbabwe. Based on this estimate and data found from a contemporary site called Equus Cave, researchers believe that large grazing and browsing mammals would have thrived in this area.
Complex prehistoric people
Dotted along the lakes are human artifacts such as triangular points, blades, ostrich shells, and other assemblages. Much of it dates back to the Mesolithic Age, which ranged between 280,000 years ago and about 25,000 years ago, and the Late Stone Age, which lasted from 25,000 years ago until European contact in the 15th century.
“There is Mesolithic archeology everywhere,” Stewart said. “You can’t drive anywhere and open the car door and not step on it. It’s amazing how much there is.”
Their findings, Stewart says, could expand the area where archaeologists believe humans have become more behaviorally sophisticated. Over the past decades, researchers have found evidence that humans living in coastal southern Africa are beginning to jump into complex thinking, showing some of the first signs of behavioral complexity. It was thought that both the stabilization of the climate and nutrients—including the rich omega fatty acids obtained from seafood—allowed humans here to become behaviorally modern.
The group’s work may challenge this notion by suggesting that favorable conditions for hunter-gatherers were not limited to the coast but that many regions may have been characterized with climate change, including the present inland desert.
“Our study at the basin level has implications on a much larger scale,” Stewart said. “It also includes some of these areas within this coastal and mountain belt that have dominated the archaeological narratives for a long time.” “Only the interior of this region is an area that has long been portrayed as hostile, and it appears it hasn’t been so in long periods of time — with the caveat that we need information on temperature drops to understand how humans deal with that.”
Stewart says the next steps will be to return to the basins to study archeology across the region to better understand how humans lived in the area.