Photo credits: Prof. Martin Trauth, University of Potsdam
Although it is generally accepted that climate change has fueled the evolution of our species in Africa, the exact nature of this climate change and its effects are not well understood. Glacial-interglacial cycles have a profound effect on the patterns of climate change in many parts of the world and have also been believed to regulate environmental changes in Africa during the critical phase of human evolution over the past ~ 1 million years. It is believed that the ecosystem changes caused by these glacial cycles stimulated the evolution and spread of early humans.
A paper published in Proceedings of the National Academy of Sciences of the United States of America (PNAS) this week challenges that view. Dr. Kaboth-Bahr and an international group of multidisciplinary staff identified ancient El Niño-like weather patterns as drivers of major climate changes in Africa. This enabled the group to reevaluate the existing climatic framework of human evolution.
Walking with the rain
Dr. Kaboth-Bahr and her colleagues have integrated 11 climate archives from all over Africa from the past 620,000 years to create a comprehensive spatial picture of when and where wet or dry conditions prevailed on the continent. “We were surprised to find a pronounced climatic east-west ‘rocking movement’, which is very similar to the pattern of the weather phenomena of El Niño and which today has a profound effect on the distribution of precipitation in Africa,” explains Dr. Kaboth-Bahr, who led the study.
The authors conclude that the effects of the tropical Pacific Ocean on the so-called “Walker Circulation” – a belt of convection cells along the equator that influences precipitation and drought in the tropics – were the main drivers of this climate crisis. The data clearly shows that the humid and arid regions have shifted on time scales of about 100,000 years between the east and west of the African continent, with each of the climatic shifts being accompanied by major changes in flora and mammalian fauna.
“This alternation between dry and wet periods seems to have determined the spread and development of vegetation and mammals in East and West Africa,” explains Dr. Kaboth-Bahr. “The resulting environmental patchwork was also likely a critical component of human evolution and early demographics.”
Scientists point out that while climate change was certainly not the only factor driving early human evolution, the new study offers a new perspective on the close link between environmental variability and the origins of our early ancestors.
“We see many species of pan-African mammals whose distribution patterns correspond to the patterns we have identified and whose evolutionary history seems to be articulated with the wet-dry oscillations between East and West Africa,” adds Dr. Eleanor Scerri, one of the co-authors, added. and evolutionary archaeologist at the Max Planck Institute for the History of Man in Germany. “These animals maintain the signals of the environment in which humans evolved, and it is likely that our human ancestors in Africa were similarly divided because they were exposed to the same environmental pressures.”
Ecotones: the transition regions between different environmental zones
The scientists’ work suggests that a swing-like rainfall pattern alternating between East and West Africa likely led to the creation of critically important ecotonal regions – the buffer zones between different ecological zones such as grasslands and forests.
“Ecotones offered diverse, resource-rich and stable environmental conditions that are believed to be important for people in the early modern era,” adds Dr. Kaboth-Bahr added. “You certainly seem to have been important to other faunal communities.”
For the scientists, this suggests that the interior regions of Africa may have been crucial in promoting long-term population continuity. “We see the archaeological signatures of early members of our species all over Africa,” says Dr. Scerri, “but innovations come and go and are often reinvented, suggesting that our deep population history has a constant sawtooth pattern of local population growth and collapse. Ecotonal regions may have provided areas for longer-term stock continuity and ensured that the larger human population stayed alive, even if the local population often died out. “
“Reassessing these patterns of stagnation, change and extinction through a new climatic framework will provide new insights into the deep human past,” says Dr. Kaboth Bahr. “This does not mean that humans were helpless in the face of climatic changes, but the shift in habitat availability would certainly have affected demographic patterns and ultimately the genetic exchange that underpins human evolution.”