Evrim Yazgin
Cosmos science journalist
The earliest evidence that humans inhabited rainforests has been found in Africa, a surprising find which pushes human settlement in these habitats much further back than previously thought.
Modern humans, Homo sapiens, evolved about 300,000 years ago in Africa. The ecological and environmental circumstances in which our species evolved are still not well understood.
It is likely that our ape-like ancestors millions of years ago did live in dense rainforests. But the retreat of Africa’s forests and the spread of savannah and grasslands as Earth’s climate dried is usually linked to the evolution of bipedalism in early human ancestors as far back as 7 million years ago.
As a result, rainforests have often been overlooked as important habitats in the evolution of early modern humans.
New research published in Nature has put a dent in this assumption.
The evidence comes from a site which dates to 150,000 years ago in present-day Côte d’Ivoire on the southern coast of West Africa.
“Before our study, the oldest secure evidence for habitation in African rainforests was around 18,000 years ago and the oldest evidence of rainforest habitation anywhere came from southeast Asia at about 70,000 years ago,” says lead author Eslem Ben Arous, from Spain’s National Centre for Human Evolution Research (CENIEH) and the Max Planck Institute of Geoanthropology in Germany.
The site was first investigated in the 1980s when ancient stone tools were discovered. But the age of the tools and the ancient ecology couldn’t be determined with the technology of the day.
Today, Côte d’Ivoire has roughly 9% forest cover which has dropped from nearly 50% in the 1960s due to agriculture from nearly 50% in the 1960s.
“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,” says senior author Eleanor Scerri, from the Max Planck Institute of Geoanthropology. “We knew the site presented the best possible chance for us to find out how far back rainforest habitation extended.”
The anthropologists used several dating techniques including optically stimulated luminescence and electron-spin resonance to determine the stone tools were 150,000 years old.
Sediment samples also showed the region was heavily wooded, with pollen and leaf waxes typically found in humid West African rainforests. Low levels of grass pollen show it wasn’t a narrow strip of forest either, but in a dense woodland.
This evidence suggests that some early modern humans lived in rainforests while others stuck to their grassland and savannah homes.
“Convergent evidence shows beyond doubt that ecological diversity sits at the heart of our species,” says 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?”
“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 Guédé.
The site which yielded these stone tools has since been destroyed by mining.
