A tale of a climate tipping point, and climate refugees, from southern African pre-history
The entrance to a nondescript cave on a south-facing slope of the Swartberg Mountains, overlooking the Little Karoo, holds in its earthen floor a history of dramatic climatic change, and hunter-communities who were climate refugees as a warming world inundated their foraging grounds with a rising ocean.
Written by: Leonie Joubert
Photographs by: Sam Reinders
To a trained eye, the paper-thin black lines in the otherwise rust-coloured earth wall, carved with architectural precision 3 m deep into the ground beneath the limestone-roofed cave outside the South African town of Oudtshoorn, are archaeological gold. They tell of generations of ancient fireplaces, of different kinds of social gatherings, and a dramatically changing environment that was shifting the face of diplomacy in the world of the hunter-gatherers who roamed here in the past 30,000 years.
The short, narrower lines were where ancient humans gathered to cook meals, warm themselves, and tell stories, explains archaeologist assistant professor Justin Pargeter during an annual six-week excavation at a cave whose secrets have been closely studied since the 1970s.
Above them, in a layer (2.5m), are longer, thicker seams of ash and charcoal that tell of great gatherings, where larger groups likely came together to feast over a diverse range of plant and animal foods, including the favoured, but now extinct, giant Cape zebra. This was most likely the hunter-gatherer’s equivalent of a dining hall where different groups came together to break bread, so to speak, most likely to hammer out some agreements about how to share the spoils from large game hunting during a time when environments and climates looked radically different to today.
They didn’t know it, but these hunter-gatherers lived through what we today call a climate tipping point.
‘They were essentially climate refugees fleeing a rapidly rising coast,’ explains Pargeter.
Boomplaas cave is not the tourist spectacle that the renowned Cango Caves are, about 3 km east of Boomplaas, with their cathedral-like domed ceilings and glistening stalactite pillars that can be if a person is tall. But Boomplaas is one of the most important archaeological sites on the subcontinent because it tells us about human resilience in the face of rapid and dramatic climate and environmental changes.
Pargeter squats in the trench, whose rim is stacked with sandbags to protect the delicate walls from damage as the excavation team steps carefully around the site.
‘The coastline was moving inland, creeping ever closer to its modern location roughly 80km from Boomplaas. We have a shark tooth from those layers,’ he says, pointing to the smooth carved wall packed full of remnants from ancient fires, tool making, and social networking, ‘there are also fragments of marine shell that are tokens of a rapidly changing coastal way of life.’
Pargeter is part of a long line of archaeologists who have steadily unravelled this cave’s secrets. This expedition’s work is the next chapter in a scholarly lineage whose story reveals a world where the original southern Africans were responding to the inexplicable encroachment of the sea on the vast plains of the southern Cape coastline where their forebears had long hunted down game, gathered plants, and collected omega-3 dense morsels from tidal pools down on the coast.
Today, the distance from Boomplaas cave to the nearest point on the coast is about 80 km, near the city of George. During the Last Glacial Maximum, from about 26,000 to 19,000 years ago, cooler global temperatures resulted in vast amounts of water being locked up in ice sheets, driving sea level down by as much as around 120 meters, and adding over 85,000 square kilometres of land to Africa’s southern tip. This new land — the Paleo Agulhas Plain — had savanna-type vegetation, with extensive grassland, an abundance of grazing animals, and, most significantly, large areas where different hunter-gatherer clans could collect food without stepping on each other’s toes too much.
But then something dramatic happened: in a relatively short period, from around 17,000 years ago, the ocean began to lurch inland. It happened in pulses over about 5,000 years.
‘We know from ice and sea cores from both the north and southern hemispheres, too, that within those 5,000 years, there were pulses where sea levels rose so rapidly that it was within one person’s lifetime, as much as 1.5 meters in 25 years.’
Climate refugees of the southern Cape
The plain flooded.
The glacial period was wrapping up. Earth was moving into a warmer interglacial phase. Half a planet away, ice sheets began to melt, shedding fresh water into the ocean, which filled up and swelled as it warmed. The ocean rose, inundating the Paleo Agulhas Plain. Where once the nearest beaches and rock pools were 160 km from Boomplaas, now they were just 80 km away.
‘An area the size of Ireland gets drowned,’ says Pargeter.
In a matter of generations, coastal foragers lost vital food sources. Rocky intertidal foraging zones — delicate yet rich ecosystems which can take thousands of years to establish — were diminished under the expanding ocean. Without these rocky intertidal foraging zones and with rapidly encroaching coastlines, hunter-gatherers would have been left with little to eat.
Pargeter’s scientific co-director, Professor Tyler Faith from the University of Utah has shown that across Africa at least 24 large mammal species went extinct around this time. In southern Africa, these included the giant Cape zebra, southern springbok, giant long-horned buffalo, and giant hartebeest.
This suggests ‘unthinkably dramatic changes’ to the vegetation and the animals that lived in and around this region, according to Faith.
‘The fossil mammals from Boomplaas reveal a dramatic transition from an exceptionally diverse, almost Serengeti-like animal community dominated by herds of large grazers to a poorer collection of fauna with relatively few small-bodied herbivores,’ he says.
Laying one line of archaeological and environmental evidence alongside another, like contour lines on a map, the shape of this complex natural history begins to rise into three dimensions.
Isotope studies have added an important contour line to this chart of the past.
Project co-director, Professor Brian Chase’s work reconstructing past environments has confirmed the significant impact of that climate change since the Last Glacial Maximum on regional environments. For his work, Chase studies isotopes, pollen and micro-charcoal found in the fossilised urine of rock hyraxes in the nearby Swartberg Mountains. Commonly known as dassies, these little creatures have the crouched pose of a rabbit and are about equal in size but are surprisingly closely related to the elephant. Colonies of dassies have urinated in the same place in the Swartberg Mountains for tens of thousands of years. This repeated behaviour produces layers of built-up fecal matter that trapped airborne materials such as micro-charcoal and pollen and provides a detailed isotopic record that, when sampled, is like studying an ice core to collect bubbles of ancient air trapped thousands or even millions of years ago to see what the atmosphere’s composition was back then.
Together, these give rare glimpses into long-term climate patterns and their effects on local vegetation dynamics.
Results indicate that the Last Glacial Maximum was a period of significantly wetter conditions, fostering the development of more extensive grassland capable of supporting the diversity of large animals found in the sediments at Boomplaas Cave. From 19,000 years ago, conditions rapidly became more arid, with the driest conditions on record occurring about 10,000 years ago, at the beginning of the current interglacial period. This change in climate resulted in dramatic changes to the environment, with grasses all but disappearing from the record, being replaced by dense fynbos shrubland and thicket vegetation. Isotopes of carbon found in bovid tooth fragments preserved in the cave’s rich animal assemblages also confirm the kind of vegetation that herbivores were eating, as the local vegetation changed from lush grasslands to the shrubby vegetation found here today.
But it is the seams of ancient ash and charcoal in the layers of the earthen floor that hold the secret of how communities responded to this dramatic environmental change as they lost much of the Agulhas Plain to a spreading ocean and to the dramatic changes to the game and vegetation they’d depended on for food. With more people pressed into a shrinking communal pantry, they gathered around the dinner table with groups they might otherwise not have socialised with. They came together to feast and negotiate how to share this shrinking world.
It is in the ostrich beads that a crucial thread of the story emerges.
Secrets in the ostrich beads
Back in the 1970s, archaeologist Dr Hilary Deacon came to the Swartberg Mountains in the southern Cape in search of sites where hunter-gatherer groups would likely have treated as home, in so far as wandering folk put down roots.
This limestone nook on a south-facing slope — 225 square metres — looked like just the spot. It had long been used as a sheep kraal by local farmers, and the floor was capped with a layer that was as hard as cement: decades of sheep droppings and the steady drip, drip, dripping of lime-loaded water seeping through the limestone roof. Once this crust had been chopped open, archaeologists could carve out about an 8 m by 5 m trench, going 5.5 metres down. What lay here was one of South Africa’s richest inland archives of people’s presence, going back more than 65,000 years, preserved in layers like a tree’s rings showing its growth history.
It is in that historical time of the climate tipping point, where conditions drifted from cool to warm and the ocean crept inland, that ostrich shell beads begin to dot the soil layer like pearls.
‘We’re in ostrich territory,’ explains Pargeter, ‘but it’s only much later in the archaeological record that we see rock art depicting ostriches.’ Before this time, people used ostrich eggs as a food source, and when climate change grew most extreme, they began making jewellery from these shells.’
But these are not jewels for ornamentation. They’re treasures that tell a much more complex social story.
‘This is when we see people seriously use eggshells, especially to make beads,’ says Pargeter. ‘They're not only making them to look pretty. They're making them build alliance, to form territories, and to signal belonging.’
Ethnographic accounts of modern-day Bushman living in the Kalahari show that, to this day, small hunter-gatherer groups use beads to build insurance networks in marginal environments.
‘These clans will have their own territories and access of the rights and resources to things like water.’
When different groups come together, they usually share information about river flow, which landscapes might be particularly abundant at a time of year, or how game is migrating.
‘People use these networks in a way that allows them to move through each other’s territories,’ Pargeter says.
The next piece in the puzzle is what the shell beads can tell us about how far these groups were travelling to gather in this great limestone banqueting hall.
Elemental to this is strontium.
Strontium occurs naturally in soils, and the kinds of strontium isotopes will vary across regions, depending on what geological, hydrological, or other environmental factors might make an area unique.
Plants draw nutrients from the soils, along with the unique isotopic markers present in the elements in the soil. Animals eat the plants, and, so too have their tissues stamped with the same isotopic markers.
The same applies to the ostrich eggshells the ancestral hunter-gatherers used to make beads. By looking at the strontium-87 to strontium-86 ratio in the different beads, archaeologists hope to determine where the eggshells came from initially and then work out how far the different visiting parties at Boomplaas may have travelled to attend these large gatherings.
Were groups travelling 10km or 100km to gather here at Boomplaas? This will be one of the next secrets that isotope analysis will reveal.