Behavioural genetics was pushed by a newly discovered African ‘climate seesaw’

Although it is commonly accepted that climate change aided our species’ evolution in Africa, the specific nature of that climatic change and its consequences are unknown. Glacial-interglacial cycles are thought to have regulated climatic changes in Africa during the important period of human evolution over the previous one million years, and they have a substantial impact on climate change patterns in many regions of the world. Early humans are assumed to have evolved and dispersed as a result of environmental changes caused by glacial cycles.

This perspective is challenged by a research released this week in the Proceedings of the National Academy of Sciences of the United States of America (PNAS). Ancient El Nio-like weather patterns were identified as the cause of major climate shifts in Africa by Dr. Kaboth-Bahr and an international group of diverse partners. The team was able to re-evaluate the prevailing climatic context of human evolution as a result of this.

Walking with the Rain

Dr. Kaboth-Bahr and her colleagues combined eleven climate records from across Africa, spanning 620 thousand of years, to provide a comprehensive spatial picture of when and where wet or dry conditions prevailed over the continent. “We were startled to see a definite climatic east-west ‘seesaw’, similar to the pattern caused by the weather phenomenon El Nio, which now substantially influences the precipitation distribution in Africa”, says lead researcher Dr. Kaboth-Bahr.

The impacts of the tropical Pacific Ocean on the so-called “Walker Circulation” - a belt of convection cells that affects the tropical rain and weather patterns - were the primary driver of this climatic seesaw, according to the authors. The results actually reveal that the wet and dry portions of the African continent moved between east and west on the timelines of about 100,000 years, with each climate transition followed by massive flora and animal fauna rotations.

“The dispersion and evolution of flora as well as mammals in eastern and western Africa seem to be regulated by this alternating between dry and rainy times”, says Dr. Kaboth-Bahr. “The resulting environmental patchwork was most likely a key component of human evolution and early demography as well”. The researchers are quick to point out that, while climate change was not the only element driving early human evolution, the new study does offer a fresh viewpoint on the close relation between climatic variations and our ancestors’ origins.

Dr. Eleanor Scerri, one of the co-authors and an evolutionary archaeologist at the Max Planck Institute for the Science of Human History in Germany, keeps adding, “We see many species of pan-African mammals whose distributions match the patterns we identify, and whose evolutionary history seems to articulate with the wet-dry oscillations between eastern and western Africa”. “These animals preserve the signals of the environments that humans evolved in, and it seems likely that our human ancestors may have been similarly subdivided across Africa as they were subject to the same environmental pressures”.

Ecotones: the transitional regions between different ecological zones

According to the researchers’ findings, a seesaw-like pattern of rainfall alternating between eastern and western Africa likely created critically important ecotone regions - buffer zones between diverse natural zones such as grassland and woodland. Dr. Kaboth-Bahr continues, “Ecotones provided diversified, resource-rich, and stable environmental settings thought to have been crucial to early modern people”. “They appear to have played a major role in other faunal assemblages”.

According to the researchers, Africa’s interior areas may have played a vital role in ensuring long-term population stability. “We see early members of our species’ archaeological signatures all over Africa”, Dr. Scerri continues, “but innovations come and go and are frequently re-invented, suggesting that our deep population history saw a persistent sawtooth like pattern of local population increase and collapse”. Eco tonal regions may have offered zones for longer-term population stability, ensuring that the overall human population survived even if local communities perished frequently”.

“Using a new climate framework to re-evaluate these patterns of stasis, change, and extinction will give new insights into the deep human past”, says Dr. Kaboth Bahr. “This isn’t to say that people were powerless in the face of climate change, but fluctuating habitat availability would have influenced demography patterns and, eventually, the genetic exchanges that support human evolution”.