Earliest evidence for humans in rainforests

Rainforests are a major world biome which humans are not thought to have inhabited until relatively recently. New evidence now shows that humans lived in rainforests by at least 150 thousand years ago in Africa, the home of our species.

Our species originated in Africa around 300 thousand 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 habitation. 

Now, in a new study published in Nature, an international team of researchers challenge this view with the discovery that humans were living in rainforests within the present-day Côte d'Ivoire much earlier than previously thought. The article reveals that human groups were living in rainforests by 150 thousand years ago and argues that human evolution occurred across a variety of regions and habitats. 

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 the ecology of the site when they were deposited there – could not be determined.

“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.”

The Human Palaeosystems team therefore mounted a mission 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 and the Max Planck Institute of Geoanthropology. 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 around 18 thousand years ago and the oldest evidence of rainforest habitation anywhere came from southeast Asia at about 70 thousand years ago,” explains Dr. Eslem Ben Arous, researcher at the National Centre for Human Evolution Research (CENIEH), the Max Planck Institute of Geoanthropology and lead author of the study. “This pushes back the oldest known evidence of humans in rainforests by more than double the previously known estimate.”

The researchers used several dating techniques, including Optically Stimulated Luminescence and Electron-Spin Resonance, to arrive at a date roughly 150 thousand years ago. 

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 joyfully. 

“Convergent evidence shows beyond doubt that ecological diversity sits at the heart of our species,” says 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?”

The research was funded by the Max Planck Society and the Leakey Foundation.