Our species originated in Africa around 300,000 years ago, but the ecological and environmental contexts of our evolution are still little understood. In the search for answers, rainforests have often been overlooked, generally thought of as natural barriers to human occupations. A new study recently published in Nature challenges this view by documenting the oldest human presence in rainforests within the present-day Côte d'Ivoire about 150 thousand years ago, demonstrating that human evolution occurred across a variety of regions and habitats.

Dr. Eslem Ben Arous, currently researcher at the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH) with a European Marie Skłodowska-Curie Fellowship (101107408- WATIME-HORIZON-MSCA-2022-PF-01) leads this study, which was carried out when she was a researcher at the Max Planck Institute of Geoanthropology (MPI-GEA) from 2020 to 2023.

The story of this discovery begins in the 1980s, when the site was first investigated by co-author Professor Yodé Guédé of l'Université Félix Houphouët-Boigny on a joint Ivorian-Soviet mission. Results from this initial study revealed a deeply stratified site containing stone tools in an area of present-day rainforest. But the age of the tools and their ecological and environmental context could not be determined precisely at the time.

“Several recent climate models suggested the area could have been a rainforest refuge in the past as well, even during dry periods of forest fragmentation,” explains Professor Eleanor Scerri, leader of the Human Palaeosystems research group at the Max Planck Institute of Geoanthropology and senior author of the study. “We knew the site presented the best possible chance for us to find out how far back into the past rainforest habitation extended.”

Consequently, the Human Palaeosystems team organized a fieldwork campaign in march 2020 to re-investigate the site. “With Professor Guédé’s help, we relocated the original trench and were able to re-investigate it using state of the art methods that were not available thirty to forty years ago,” says Dr. James Blinkhorn, researcher at the University of Liverpool. The renewed study took place just in time, as the site has since been destroyed by mining activity.

“Before our study, the oldest secure evidence for habitation in African rainforests was dated to around 18 thousand years ago, while the oldest known evidence of rainforest habitation was reported from southeast Asia at about 70 thousand years ago,” explains Dr. Eslem Ben Arous. “Through the combination of two dating techniques, the Optically Stimulated Luminescence (OSL) and Electron Spin Resonance (ESR), we demonstrate human presence in African rainforest about 150 thousand years ago, which significantly pushes back the oldest known evidence,” concludes Dr. Eslem Ben Arous.

At the same time, sediment samples were separately investigated for pollen, silicified plant remains called phytoliths, and leaf wax isotopes. Analyses indicated the region was heavily wooded, with pollen and leaf waxes typical for humid West African rainforests. Low levels of grass pollen showed that the site wasn’t in a narrow strip of forest, but in a dense woodland.

“This exciting discovery is the first of a long list as there are other Ivorian sites waiting to be investigated to study the human presence associated with rainforest,” says Professor Guédé.

“Convergent evidence shows beyond doubt that ecological diversity sits at the heart of our species,” completes Professor Scerri. “This reflects a complex history of population subdivision, in which different populations lived in different regions and habitat types. We now need to ask how these early human niche expansions impacted the plants and animals that shared the same niche-space with humans. In other words, how far back does human alteration of pristine natural habitats go?”.

Dating at CENIEH
The ESR dating analyses were carried out at CENIEH, illustrating the increasing role played by the Geochronology and Geology Scientific program and the Geochronology labs in refining the chronology of early human migrations across the African continent and beyond.

‘This study showcases the expertise and facilities currently available at CENIEH for geochronological studies, which enables to produce high-quality data in accordance to the highest standards, eventually publishable in international scientific journals’ concludes Dr Mathieu Duval, senior researcher at CENIEH and co-author of the work.