Chew Bahir on the border of Kenya and Ethiopia is a tough and merciless place. It is hot as hell, up to 40°C with strong ruthless tropical sun. Humans are well adapted to this climate. We walk on two legs reducing the area the sunlight falls on, and sweating allows us to keep our body temperature cool even in these inhospitable landscapes. We have lost most of our hair to stay cool and the little we have left is on our head to protect our fragile brain from the intense heat of the sun. Our large brain facilitates our survival even in some of the most extreme climates on this planet from the Arctic to the Sahara. But 10,000 years ago the Chew Bahir basin was a large freshwater lake. How did a lake the size of Luxembourg fill what is now a very arid desert? How long has there been such a lake? Is there a rule in time that determines this shift between deserts and lakes?
During the last decades researchers from all over the world aimed at answering these questions to understand the causal relationships between climate change and human evolution in the cradle of human kind (Cohen et al. 2016). A couple of years ago, a large group of scientists drilled three-hundred meters in depth into half a million years old lake sediments and achieved through chemical indicators cycles of humidity and wetness in Eastern Africa. The last cycle, the so called African Humid Period, lasted from 15000 to 5000 years before present. During that time, the Chew Bahir basin was flooded with a large freshwater lake, with Stone Age people settlements at its shore, as archaeological records indicate. It is imaginable that fishing was a favourable way of surviving before cereals and livestock supplied our daily diet. But what happened when these lakes disappeared? And how fast did humans need to adopt to this incredible environmental transition from a desert to a lake and vice versa?
We answered this question by building up a computer model of the basin, which follows a simple bookkeeping principle (Blodgett et al. 1997). All the water that rains into the lakes’ drainage area gets subtracted by the evaporation above land and water. Clearly, the surplus or the lack is the changing water volume of such a lake. Using this model, we can estimate the precipitation that is necessary to fill the lake by understanding that a lake needs a lot of water just to compensate for the increased evaporation above the water area. Furthermore, only humid months have surplus water that can flow through the streams towards the lake. So, if having another seasonality pattern in ancient times, we need to take this into account and calculate the compensated difference between today’s actual evaporation in arid months and a potential evaporation on land during a supposed additional rainy season (Junginger and Trauth 2013). To achieve the effect of paleo-vegetation, we used results from other scientists, who quantified the increased humidity that shifts landscapes from grasslands towards forests and trees with higher transpiration. Overall, in numbers, this means Chew Bahir would need between +20.6 and 28.6% increased precipitation in a long-term trend to get flooded and only decades to dry out and to shift from a large freshwater lake to an inhospitable desert basin floor.
This must have been an enormous change for a human being that has settled during the Neolithics at the lakes shore. Imagine that within less than a generation the lake, which provided you with all indispensable to life, disappeared. And just within one or two decades, the water, which has been your living fundament, disappeared. What would you do? Maybe look up to the green forests at the mountain peaks of the Ethiopian Highlands? Foerster et al 2016 investigated increased settling activity in the wet Highlands during these times of dryness, and through the basin-model we can estimate that this climate-change-caused migration happened within just a generation. This could have led to a cascade of migration pressure and cultural remixing that could have forced evolutionary and cultural adaption in the specific landscape of the East African Rift System, amplifying environmental change (Trauth et al. 2010). Maybe this landscape prepared and gave us the necessary tool kits to survive all over the world. Maybe our cradle has been a tough school as well.
The last 15,000 or even 300,000 years of homo sapiens history is a short time period compared to the 3.5 million years of human evolving history in Eastern Africa, as fossils of Australopithecus afarensis in Ethiopia indicate. During the last 3.5 million years the landscape of Eastern Africa moulded through tectonism. The last 15,000 years implied various cycles of enormous environmental changes that forced cultural adaption in humanity’s daily live. It’s not a provable, but maybe a reasonable hypothesis, that every single time this landscape shifted, the smartest way of surviving got the opportunity to dominate in this new landscape and to spread her or his new tool kit through sex, genes and cultural exchange. Huge water body forced mixing, separating and remixing of populations in a challenging non-constant landscape that could have been the recipe for our awesome human adaptability. Let’s use our awesomeness wisely in this modern global and fast changing environment.
Blodgett, T. A., Isacks, B. L., & Lenters, J. D. (1997). Constraints on the origin of paleolake expansions in the central Andes. Earth Interactions, 1(1), 1-28.
Cohen, A., Campisano, C., Arrowsmith, R., Asrat, A., Behrensmeyer, A. K., Deino, A., … & Lamb, H. (2016). The Hominin Sites and Paleolakes Drilling Project: inferring the environmental context of human evolution from eastern African rift lake deposits. Scientific Drilling.
Foerster, V., Vogelsang, R., Junginger, A., Asrat, A., Lamb, H. F., Schaebitz, F., & Trauth, M. H. (2016). Reply to the comment on” Environmental change and human occupation of southern Ethiopia and northern Kenya during the last 20,000 years. Quaternary Science Reviews 129: 333-340″. Quaternary Science Reviews, 141, 130-133.
Junginger, A., & Trauth, M. H. (2013). Hydrological constraints of paleo-Lake Suguta in the Northern Kenya Rift during the African humid period (15–5 ka BP). Global and planetary change, 111, 174-188.
Trauth, M. H., Maslin, M. A., Deino, A. L., Junginger, A., Lesoloyia, M., Odada, E. O., … & Tiedemann, R. (2010). Human evolution in a variable environment: the amplifier lakes of Eastern Africa. Quaternary Science Reviews, 29(23-24), 2981-2988.