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Today, PLOS ONE is publishing a paper entitled “Tufas indicate prolonged periods of water availability linked to human occupation in the southern Kalahari” by a group of researchers from the Human Evolution Research Institute (HERI), based at The University of Cape Town. In this interview, PLOS ONE Associate Editor Katrien Janin (KJ) speaks to first author Jessica von der Meden (JV) about her experiences conducting this study.
KJ: “Your recent paper focuses on reconstructing the paleoclimate in the southern Kalahari, to evaluate the impact of environmental change on human evolution in this region. Can you explain the link between climate change and human evolution, and the role of southern Africa in the human evolutionary story?”
JV: Climate is a major driver of human evolution. There are debates about the extent to which climate influenced human evolution, but it is generally accepted that changing climatic conditions did influence early human movement, adaptations and behaviour, and there is growing evidence of this link. This doesn’t seem hard to imagine as we feel the effects of the changing climate even today, and at a time when humans were tethered to and dependent on the environment for survival, for example relying on fresh water and conditions conducive to hunting and foraging, it is likely that the climate played a prominent role in human evolution.
However, sites with both archaeological and geological records, underpinned by a reliable chronology, are needed to better understand how climate change impacted early humans. Datable archives of palaeoclimate, associated with well-preserved archaeological material are rare, particularly in arid interior regions. Ga-Mohana Hill is one such locality, providing a valuable opportunity to investigate the impact of climate change on human evolution. The climate system in South Africa is complex; unravelling how it was different in the past, and how this influenced human-environment interactions, is a major challenge, but it is important for understanding how our species adapted to changing climatic conditions, and what this can tell us about climate change in the future.
A time of particular interest in human evolution studies is the Middle Stone Age, during which early human populations developed behaviours characteristic of Homo sapiens, e.g. an ochre drawing at Blombos Cave, and collections of crystals at Ga-Mohana Hill. In South Africa, many archaeological sites dated to this period that preserve evidence of these behavioural advancements are situated along the southern Cape Coast, which is argued to have been a nexus for these behavioural developments, in part due to the favourable and stable climatic conditions that prevailed.
Archaeological sites with evidence for similarly advanced behaviours exist in the interior parts of southern Africa, and these sites are receiving renewed attention; however, the associated climate conditions are still poorly understood. Our research contributes important information to what is developing as a complex, multi-factorial picture of early human-environment interaction, and our results challenge the notion that humans only occupied arid regions when they were humid.
KJ: “What are tufas, and why are they such a good indicator of humidity levels and paleoclimatic conditions?”
JV: Tufas are rocks that form from ground waters that emerge at the surface as springs. These fresh spring waters are rich in dissolved calcium, typically sourced from carbonate bedrock, in this case 2.4 billion year old dolomites from the Palaeoproterozoic era.
Tufas are similar to stalagmites or stalactites that form from drip-waters in caves – the big difference is that tufas form from ground waters that emerge at the surface of the landscape, not inside a cave environment, and so they are exposed to light, dust and plant matter, making them slightly more complicated deposits.
Tufas form when particular climatic conditions are met, the most important being sufficient rainfall to recharge the underground aquifers. The groundwaters dissolve calcium from the dolomitic bedrock, and when the aquifers are full, these calcium-rich waters overflow. In addition to sufficient rainfall, higher levels of humidity, and moderate temperatures are necessary to maintain the conditions that are favourable for tufa formation (too hot and this would create too much evaporation, reducing the amount of water available; too cold and the levels of carbon dioxide in the soil through which the rain water infiltrates will be too low, making the waters less efficient at dissolving calcium from the bedrock). As such, the presence of relict tufas points to periods in the past when this balance of sufficient moisture, humidity and temperatures existed. Today, the tufas at Ga-Mohana Hill are mostly inactive as the area experiences a semi-arid, evaporative climate, with only little rainfall during the austral summer months (December – February).
Through field observations, we determined that the tufa deposits represent past periods of flowing water in the form of shallow streams, standing pools and waterfalls cascading down the hillside.
This means that in the past, Ga-Mohana Hill would have been an oasis of fresh water, likely supporting plant productivity, and providing a crucial resource for early human populations active in the area. Despite their complexity, tufas are amenable to dating, which is important for constraining the timing of this wetter environment.
In our study, we use the uranium-thorium dating method to obtain ages for the tufa deposits at Ga-Mohana Hill. Knowing the ages of the tufas allows us to constrain times in the past that fresh water was available on the landscape. We determined that there are at least five distinct episodes of tufa formation during the last 114 thousand years. Three of these times coincide with the timing of archaeological horizons, dated using optically stimulated luminescence (OSL) to approximately 105, 31, and 15 thousand years ago, which indicates contemporaneous human occupation and tufa formation.
KJ: “Working in the Kalahari must come with its challenges. Tells us about the logistics involved with conducting fieldwork in such a remote place. What does a typical fieldwork day look like, and what were your most memorable fieldwork moments?”
JV: Field work in the Kalahari is wonderful – it really is a special place and the landscape is beautiful, with big open skies and bare land that stretches as far as the eye can see.
I’ve been fortunate to conduct my field work with an experienced team of archaeologists, who are well-organised and efficient. Ga-Mohana Hill is also located close to a town, Kuruman, and so we have been lucky to enjoy relatively luxurious field accommodation at a local B&B.
A typical day of field work involves an early start and a substantial breakfast to get us through the day. We then drive to Ga-Mohana Hill, where we all pitch in to assist with carrying the equipment needed for the archaeological excavation up the hill to the rockshelter (it is then that I am thankful that geologists in the field only really need a hammer and a notebook!). The archaeologists set up their excavation, and I am often reminded not to walk too close to the excavation pit as I peer in with interest to see the archaeologists at work.
After examining the map and a discussion on the days plans, our survey team then embark on foot to explore the area. Sometimes this also involves visits to local farmers to request permission to survey their land for archaeological material. We traverse the hills, observing the geology, looking for secondary carbonates to sample, and identifying stone tools, which are photographed and georeferenced, but left in their place to preserve the material culture. I use a geological hammer and chisel to sample the tufas, but sometimes power tools are necessary to better extract samples, and in those instances I have fun wielding an angle grinder or diamond-tipped drill.
Around lunch time we find a spot in the shade to eat our melted cheese sandwiches, and then continue with our survey and sampling in the afternoon. If our survey is close to the rockshelter, we join the excavation team for mid-morning tea and biscuits. Despite being in a fairly remote location, we still enjoy some creature comforts! Most of the field seasons are conducted in winter, when the mornings are crisp and the days are warm and clear. We have conducted shorter field seasons during the summer months, and then an earlier start to beat the heat, and carrying enough water is essential.
One of my most memorable field moments was during a visit to Ga-Mohana Hill in January which is the height of summer and also the rainy month. We were there to collect rain and drip water from the rockshelter and surrounding areas. As we were walking up the steep hillside to the shelter, we heard rumbling and a large, low storm cloud appeared. We observed the clouds roll across the valley in front of us, and felt the first big warm drops of rain on our skin. We reached the shelter just as the cloud burst, and watched in awe from our vantage point as a large curtain of rain drenched the valley below us. The downpour didn’t last very long, and after a few minutes the storm clouds rolled on, with the rain curtain stalking across the landscape like a giant figure. The air felt extra clear, like it had been rinsed clean, and a sweet, warm smell floated up from the freshly wet earth. It was a beautiful moment.
KJ: “As you may know, PLOS is dedicated to advancing not just Open Access, but pushes the boundaries of “open” to create a more equitable system of scientific knowledge and understanding. Our global research inclusivity policy promotes not only interaction between researchers from all over the world, but also encourages local engagement where we conduct our research. Archaeology and anthropology have been historically vulnerable to ‘parachute research’, where researchers from other nations arrive at a country of interest and conduct research without consulting or crediting any of the local population. What are your thoughts on global research inclusivity, and how does this ethos fit in with your research?
JV: That’s absolutely right, and such parachute practices are very apparent in a place like South Africa, where we have a rich and abundant archaeological and geological heritage that has mostly been researched by foreigners. This was jarringly evident to me when I attended a Palaeoanthropology conference in Austin, Texas; the majority of posters and oral presentations on Stone Age archaeology were on sites from southern and eastern Africa, but the authors were American. I found this so strange, as I hadn’t quite grasped the uniqueness of our heritage and the extent to which this was being investigated by people from around the world, very few of whom enter into collaborations with researchers at local institutions. This system robs local researchers of the opportunity to work on artefacts and collections in their own country, and it excludes the local population from being involved in the process, as foreigners generally don’t know how (or can’t be bothered) to engage with local communities. This creates a division and mistrust between scientists and local communities, who are the true custodians of the heritage. The research also suffers because local knowledge, customs and practices are not taken into account, and so interpretations are made through a narrow and foreign lens, without consideration of local perspectives. As such, the local population are unaware of the scientific publications produced, and are excluded from the knowledge and the conversation.
The authors of this study are a diverse interdisciplinary team with researchers from South Africa, Australia and North America. The lead archaeologists, Dr Jayne Wilkins (Canadian) and Dr Ben Schoville (American) are now based in Australia at Griffith University and the University of Queensland respectively, but both spent time at the University of Cape Town (UCT) in South Africa, where they trained South African students and continue to involve them in field work and projects in the Northern Cape. They also maintain a close collaboration with Dr Robyn Pickering, a South African geologist at UCT, who conceptualised the tufa study and facilitated my training on U-Th methods. Through her, I had the opportunity to visit the Isotope Geochemistry Group at the University of Melbourne, where Prof Jon Woodhead and Dr John Hellstrom trained me in analysing the tufas using laser ablation and U-Th dating, with help from Dr Alan Greig, Dr Helen Green and Dr Rieneke Weij. It is through global collaborations such as this, where a diverse range of expertise, knowledge and perspectives are shared and combined, that inclusive, quality research can be produced.
In conducting our research at Ga-Mohana Hill, it was important for us to involve the local community as much as possible. We engaged with the Baga Motlhware Traditional Council to speak with them about the work we were interested in conducting and to request permission to carry it out at Ga-Mohana Hill, which is a place of spiritual and ritual significance.
To respect the ritual significance, I took a low impact approach, sampling the tufas carefully with targeted methods (e.g. using custom-made core barrels attached to a hand-held drill) and in unobtrusive locations, taking care to leave very little trace. Also, the archaeological excavations are back-filled and covered at the end of each season, so that no trace is left. These protocols were established shortly after we began investigations at Ga-Mohana after discussions with local community members about the best way to respect local traditions. The project is always working toward improving our understanding of the ways in which we can better engage with and involve the local community.
KJ: “You are part of the Human Evolution Research Institute (HERI –https://www.heriuct.co.za/). Can you tell us more about that? And how do you think institutes like HERI help to address the important issue of research inclusivity?”
JV: HERI is doing important work in bringing attention to palaeoscience research in South Africa, and the people behind it. Through financial aid and media engagement, HERI provides support to researchers, particularly African womxn and people of colour, to promote transformation and the inclusion of diverse skills, backgrounds and perspectives in the palaeosciences. I am grateful to HERI for supporting my research and granting me opportunities to kickstart my career.
About the author:
Jessica von der Meden is a PhD candidate at the University of Cape Town, South Africa, interested in Quaternary geology and palaeoclimates related to human evolution. She is working on the occurrence, formation and dating of tufa (secondary fresh water carbonate deposits) at the archaeological site of Ga-Mohana Hill in the southern Kalahari. She is first author of Tufas indicate prolonged periods of water availability linked to human occupation in the southern Kalahari