Credit: CC0 Public Domain
Our human species emerged in Africa around 300,000 years ago but scientists don't yet have a clear picture of what kind of natural environment we evolved in. Until recently, the dominant idea was that grasslands and savannahs were the ecological "cradle" of human beings. Environments like the rainforest were considered to be barriers to human expansion, and inhabited only much later in human history.
This view is out of step with research in Asia, however. There, more and more evidence has been found of sophisticated behaviors and advanced cognition in ancient rainforest contexts.
Humans lived in a rainforest environment on Sumatra in Indonesia as far back as 70,000 years ago. They also coped well with the challenges of rainforests. At Niah Cave in Borneo, toxic plants obtained from nearby rainforest habitats were processed as far back as 45,000 years ago. This was soon after people were first documented in this region, around 46,000 years ago. Similarly, in Sri Lanka, there is evidence for direct reliance on rainforest resources from at least 36,000 years ago.
These discoveries suggest that humans were able to live in the rainforest before they left Africa, the home of our species. Until now, though, the oldest firm evidence for people living in African rainforests dated to around 18,000 years ago.
Our newly published study pushes that date way back. Our international team of researchers, working in Côte d'Ivoire, showed that human groups were already living in Africa's wet tropical forest 150,000 years ago.
Our research
The story of this discovery began in the 1980s, when the Bété I site in Côte d'Ivoire was first investigated by Professor François Yiodé Guédé of the Université Félix Houphouët-Boigny on a joint Ivorian-Soviet mission. Results from this initial study published in 2000 revealed a long sedimentary sequence, containing stone tools in an area of present-day rainforest.
This site is one of the few in Africa to feature a long history of layers of sediment being deposited. There is a sedimentary sequence of about 14 meters, with several levels of preserved tools. The stone tool assemblages, composed of more than 1,500 pieces, were found during the excavations of the 1980s and 1990s, but at that time the age of the tools—and the ecology of the site when they were deposited there—could not be determined.
We went back to the site 36 years later and found the exact location of the Bété I sequence. We took samples of sediment and studied them using a variety of analytical methods. This is a way to get the most reliable picture of how old the sample is and what kind of environment it was originally in.
To date the sediment in which the stone tools were found, we used two dating techniques: optically stimulated luminescence and electron spin resonance. These told us how old the quartz grains were at various points in the layers of sediment. We also examined the sediment for pollen, phytoliths (silica concretions produced by plants), and leaf wax.
These analyses together indicated that by 150,000 years ago, the site was heavily wooded, with pollen and leaf waxes typical of a humid West African forest. Low levels of grass pollen showed that the site wasn't in a narrow strip of forest, but in a dense woodland featuring plants that are important in that kind of ecosystem, like oil palms. This ecological information came from samples along the sequence and also in the same level where the deepest tools were found. This clue allowed us to say human groups were present at the site at most 150,000 years ago.
The birth of modern Man
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