Sediment samples from a paleolake in southern Ethiopia show the environmental conditions under which Homo sapiens lived for hundreds of thousands of years – and when climate stress put a strain on scattered population. Were early modern humans climate refugees?

By Eva Schissler

It was a feast for the mosquitoes when scientists and engineers from all over the world invaded the region of Lake Chew Bahir in southern Ethiopia. In this landscape with its sparse vegetation, such fat prey usually is rare. Now and then, Hamar people, who raise cattle, pass through the area. But apart from that, the dried-up lake that was once five times the size of Lake Constance resembles a lunar landscape. Except for the mosquitoes that lurk in cracks in the ground and pounce on new arrivals at nightfall. “We taped our trousers and sleeves shut, and yet we were covered in mosquito bites,” said Dr Verena Förster from the Institute of Geography Education.

The international team spent six weeks on site in November 2014, drilling two adjacent holes in the lake floor with heavy equipment day and night. In about a hundred individual drilling runs, they removed sediment-filled core liners up to three metres long per hole to a total depth of 280 metres. The sedimentary facies under the lake floor, which is now dry and covered with a salt crust, is an optimal climate archive because materials such as sediments, pollen and the remains of plants and animals have continuously accumulated in this deep basin over thousands of years. This has made it possible to reconstruct the climate history of the past 620,000 years.

Holes – The aim of scientific drilling is to obtain a continuous sediment core. To prevent important information from being lost at the intersections, at least two parallel cores are extracted in which these transitions overlap between the individual liners. If the sedimentation rate is high enough, the combination of both cores provides an extremely high resolution in which even individual decades emerge over the millennia.

The paleolake is also located in close proximity to a well-known fossil site: Omo Kibish, where some of the world’s earliest remains of Homo sapiens were found. With the help of these sediment cores, a precise picture of the dry and wet phases in the region emerged over the entire investigation period. Despite numerous fossil finds of hominids – modern humans and older human species – in East Africa, the regional environmental conditions during the Pleistocene, the last ice age, had not yet been sufficiently clarified.

Pollen, sediments and ancient DNA

Förster was assigned to the night shift most of the time. In this project, research involved real physical exertion: mosquitoes at night, sleeping in tents during the day in scorching heat. Nevertheless, she exudes great enthusiasm when she talks about her work: “Actually, I had studied to become a teacher, but my first drilling expedition as a student in Patagonia fascinated me and I knew I had to commit myself to science.”

The expedition extracted a total of three and a half tonnes of ancient lake sediments. The sediment cores were then packed light- and airtight and shipped to the LacCore/CSDCO Institute in Minneapolis, which specializes in sediment cores from paleolakes. In 2015, experts from all over the world travelled to the so-called ‘core opening party’. “Actually, it was anything but a party, it was a lot of work,” said Förster. In order not to contaminate the material, the cores were opened and sampled under strictly sterile conditions and sometimes even in darkness. The individual samples then went to specialized laboratories: at the University of Potsdam, for example, a team examined DNA preserved in the sediment, while at other institutions fossil remains of diatoms were evaluated. As a geographer, Förster is interested in the sediment – its grain sizes and geochemical and mineralogical composition. These so-called proxies can serve as reliable indicators of climatic conditions such as drought or temperature.

After years of meticulous work by a wide range of disciplines, an overall picture emerged from the individual findings. The main publication on the research expedition appeared in October 2022.

Modern humans enter the scene

The researchers identified three major phases, in each of which different climatic phenomena influenced human population dynamics. In the oldest phase, from 620,000 to 275,000 years ago, there was a relatively stable climate with probably favourable living conditions for the hominid groups, interrupted by a series of short, abrupt and extreme dry spells. During these phases, landscapes with large, deep lakes disappeared and it was even drier than today.

At this time, great diversity was evident in early populations. Archaic species like Homo erectus were still around, but new ones evolved as well. The periods of climatic stress caused many populations to thin out considerably and possibly become separated from each other. Later, however, they were able to reunite thanks to expanding potential habitats.

The biological but also cultural exchange that now set in may have been a decisive engine for progress. “We must remember that biological adaptation to new conditions takes many thousands of years. A climate change over decades or centuries can only be countered by decisions: the way of life has to adapt or people have to migrate to a more hospitable place,” Förster explained.
In the second phase from 275,000 to 60,000 years ago, considerable climate fluctuations occurred, which periodically led to repeated changes in human habitats: from lush vegetation with deep freshwater lakes to extremely arid landscapes in which lakes dried up into small salty puddles. Climate fluctuations also increased.

Climate fluctuations – In addition to climate events such as volcanic eruptions, the variability of greenhouse gases or possible impacts on the vegetation, the position of the Earth in relation to the Sun influences the climate. Important are the course of rotation, the inclination of the Earth’s axis, and so-called precession, a kind of wobbling of the Earth’s rotation axis. These three parameters, which influence the angle of incidence and the intensity of solar radiation, and thus the climate, recur in fixed cycles that are clearly reflected in the findings of the research project.

Homo sapiens entered the scene during this phase, and during the transition from the first to the second phase, humans developed new technologies and specialized tools, such as lighter and more delicate stone wedges. Finds also show that special material for tools was transported over long distances, suggesting that people exchanged ideas over greater geographical distances.

“These social and technological advances suggest that people planned their actions and solved problems better,” said the scientist. Whether climate led to these new developments or whether they were a reaction to them is a classic chicken and egg problem. Förster said: “We can’t establish causalities, but our new climate data has allowed us to directly compare human developments in the region with climatic conditions for the first time.”

The third phase, from about 60,000 to 10,000 years before today, saw the most extreme climate fluctuations, including the driest phase in the entire record. This phase may have accelerated the cultural change of the population. Whether it triggered their migration from the increasingly inhospitable region to Europe, or whether the short pulses of wet phases enabled human migration along a corridor to Europe in the first place, has not been definitively resolved.

Another piece is added to the puzzle

Comparable studies also exist for West Africa, for example from Lake Bosumtwi in Ghana, or for Europe from Lake Ohrid between North Macedonia and Albania – the oldest lake on our continent. A comparison of the East African data with the findings from other climate archives and from further studies provides a bigger picture for certain periods of time. Each drill core is like another piece in a puzzle. “A wet phase from our data overlaps with a wet phase in the climate history of the eastern Mediterranean, which could have favoured the spread of humans to Europe. Because only during such a phase would it have been possible to cross the Sahara at all,” Förster explained.

Understanding the relationship between climate and human development – that is paramount for Förster also in view of the current threats posed by climate change. At the same time, she emphasized that the climate is a complex system that cannot be predicted according to simple causalities. It is subject to cosmic cycles as well as unforeseeable occurrences. "If a large chunk of ice crashes into the sea at the North Pole, this will also have an impact in the tropics. However, we cannot predict the effects precisely."

Nevertheless, data from the research project will also be incorporated into climate models: “We have often seen a strong acceleration of climate pulses before a tipping point in the climate system. This could be an indication that such an event is imminent.” In future research, she therefore wants to focus on extreme drought on the one hand, and on phases in which the climate tipped, such as at the end of the African wet phase, on the other. Together with colleagues, she wants to look for clues as to whether the climate system can recover on its own and avert the tipping point.

However, until she can plunge back into the field in Kenya or Ethiopia, the teacher training students in Cologne will benefit from the research expedition. Some came along to Ethiopia, others can now examine the collected materials under the microscope or work out how highly complex research results can also be made accessible in the classroom. Moreover, they learn what a climate proxy is or how the Earth’s rotation influences the climate. Förster is convinced, “It can be a life-changing experience to be involved in such a major project. And I can hardly imagine a more exciting topic than the climate.”
 

The research is part of the Hominin Sites and Paleolakes Drilling Project (HSPDP). To assess the effects of different time scales and magnitudes of climate change on the living conditions of early humans, this project collected cores from five lake archives of climate change over the last 3.5 million years. All five drill locations in Kenya and Ethiopia are in close proximity to important paleoanthropological sites from various stages of human evolution.
Under the HSPDP, the project was funded by the International Continental Scientific Drilling Program (ICDP), the German Research Foundation (DFG), the Natural Environment Research Council (NERC), the National Science Foundation (NSF) and DFG Collaborative Research Centre 806 (CRC 806) ‘Our Way to Europe’. The CRC 806 brought together scientists from the Universities of Cologne, Bonn, and Aachen from 2009 to 2021.