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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 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 behaviours 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 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 C 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 C d'Ivoire was first investigated by Professor Fran Yiodé Guédé of the Université Félix Houphou 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 metres, 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 for 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 in the site at most 150,000 years ago.
Our findings
The results indicate that ecological diversity lies at the heart of our species, and humans have been able to live in different habitats and ecologies from an early stage in our history. In other words, humans are unique in their ability to both adapt to a huge variety of different environments, and become specialists within any of those environments.
The fact that this discovery was made in west Africa also highlights the importance of investigating different African regions for a more comprehensive picture of the earliest periods of human prehistory.
If we zoom into west Africa, archaeologists have also detected many distinctive cultural behaviours at this early stage in the region. For example, in 2021, Ravin Blanc I site in Senegal's Falémé valley shows associated occupations dating to around 125,000 years ago with stone tools that look very different from those found in the coastal mangrove site of Bargny 1 (Senegal). They are also different from those in Bargny 3 (Senegal) which, at 140,000 years ago, are only a little older than Ravin Blanc, and a little younger than Bété 1. And the material culture from these sites is different from what's found at the more distant and ecologically different Bété 1 site.
Looking forward
Although the story of human evolution in west Africa is only just beginning to be sketched out, it already looks like it has its own unique regional story to tell.
New discoveries here may upend what we know about human evolution. That remains to be seen, but what is already clear is that the deepest prehistory of humans was both geographically and ecologically expansive in Africa, before we even took our first steps beyond our "home" continent.
Eslem Ben Arous, MSCA Postdoctoral Fellow, Centro Nacional de Investigation sobre la Evoluc Humana (CENIEH) and Eleanor Scerri, Independent Group Leader, Max Planck Institute of Geoanthropology
This article is republished from The Conversation under a Creative Commons license. Read the original article.
(Authors: Eslem Ben Arous MSCA Postdoctoral Fellow, Centro Nacional de Investigación sobre la Evolución Human(CENIEH) Eleanor Scerri Independent Group Leader, Max Planck Institute of Geoanthropology)
(Disclosure statement: Eleanor Scerri receives funding from The Max Planck Society. Eslem Ben Arous does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.)
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