Ancient El Nino-like weather patterns were the primary drivers of environmental change in sub-Saharan Africa when humans were evolving, according to research involving UCL, London's university.
Scientists have shown that over the last 620,000 years these alternately wet and dry patterns had more profound impacts in sub-Saharan Africa than colder, dryer glacial-interglacial cycles, which until now have been more commonly linked to human evolution.
Professor Mark Maslin of UCL Geography, who contributed to the international study, published in Proceedings of the National Academy of Sciences of the United States of America (PNAS), said the findings challenge previous theories on the impact of changes in the climate on the earliest ancestors.
"The detailed analysis of the past climate of Africa, led by Stefanie Kaboth-Bahr (University of Potsdam), has revealed an oscillation between wet and dry conditions across the continent. This means for at least the last 600,000 years there has been continual environmental change as wet lush conditions moved from the east to the west and then back again.
"This drove significant changes in plant and animal migration patterns and contributed to the evolution of new hominin species. This study shows that the long-term El Nino and Southern Oscillation (ENSO)-like climate shifts across Africa had a profound effect on the evolution of early humans."
It is widely accepted that climate change drove the evolution of our species in Africa, but the exact character of that change and its impacts have not been fully understood.
Glacial-interglacial cycles strongly impact patterns of climate change in many parts of the world and were also assumed to regulate environmental changes in Africa during the critical period of human evolution.
In this latest research, this influence of glacial-interglacial cycles has been challenged as the scientists identified ancient El Nino-like weather patterns as the drivers of major climate changes in Africa, allowing them to re-evaluate the existing climatic framework of human evolution.
To arrive at this conclusion, they integrated 11 climate archives from across Africa covering the past 620,000 years to generate a comprehensive spatial picture of when and where wet or dry conditions prevailed over the continent.
Stefanie Kaboth-Bahr of the University of Potsdam, who led the study, said: "We were surprised to find a distinct climatic east-west 'seesaw' very akin to the pattern produced by the weather phenomena of El Nino that today profoundly influences precipitation distribution in Africa."
The authors infer that the effects of the tropical Pacific Ocean on the so-called 'Walker Circulation' -- a belt of convection cells along the equator that impact the rainfall and aridity of the tropics -- were the prime driver of this climate seesaw.
The data clearly shows that the wet and dry regions shifted between the east and west of the African continent on timescales of approximately 100,000 years, with each of the climatic shifts being accompanied by major turnovers in flora and mammal fauna.
Kaboth-Bahr added: "This alternation between dry and wet periods appeared to have governed the dispersion and evolution of vegetation as well as mammals in eastern and western Africa. The resultant environmental patchwork was likely to have been a critical component of human evolution and early demography as well."
The scientists are keen to point that although climate change was certainly not the sole factor driving early human evolution, the new study nevertheless provides a novel perspective on the tight link between environmental fluctuations and the origin of our early ancestors.
Kaboth-Bahr continued: "Re-evaluating these patterns of stasis, change and extinction through a new climatic framework will yield new insights into the deep human past. This does not mean that people were helpless in the face of climatic changes but shifting habitat availability would certainly have impacted patterns of demography, and ultimately the genetic exchanges that underpin human evolution."