A groundbreaking study has shown that humans lived in rainforests approximately 150,000 years ago, much earlier than previously thought, suggesting that rainforests may have played a significant role in early human evolution.

This new timeline challenges earlier assumptions and shifts our understanding of how early humans adapted to various environments.

The study, conducted by an international team of researchers, revisited the archaeological site of Bété I in the Republic of Côte d'Ivoire in West Africa, where stone tools had been previously uncovered.

These tools, which include picks and smaller retouched implements, were originally discovered in the 1980s but could not be dated accurately at the time due to limitations in technology and the eventual loss of the tools during the Second Ivorian Civil War in 2011.

Eslem Ben Arous, lead author and archaeologist at Spain’s National Centre for Human Evolution Research, explained, "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. This pushes back the oldest known evidence of humans in rainforests by more than double the previously known estimate."

The site’s significance lies in the fact that rainforests were long considered obstacles to early human habitation. The dense vegetation and humid conditions were thought to be too challenging for human survival. However, the study challenges this view by showing that humans were living in rainforests much earlier than expected, potentially as animportant location for human evolution.

The Bété I site had been excavated by a joint Ivorian-Soviet expedition led by Professor Yodé Guédé from the Université Félix Houphouët-Boigny, with further research conducted by Ben Arous and her colleagues. The area where the tools were found was later analysed using modern methods unavailable during the original excavations.

James Blinkhorn, an archaeologist at the Max Planck Institute of Geoanthropology (MPI-GEA), noted: "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."

The excavation, however, came at a critical moment, as the site was later destroyed by mining activity during the COVID-19 pandemic, despite being designated a protected area. Professor Eleanor Scerri of MPI-GEA explained, "The site was given protected status. Unfortunately, the loss happened regardless of this."

Upon analysing the site’s quartz grains, the research team determined that the tools were deposited around 150,000 years ago. Additional analysis of pollen, leaf wax, and plant remains found in soil samples revealed that, at the time of the tools' use, the region was densely forested and consistent with modern-day West African rainforests. Low levels of grass pollen further indicated that the area was not a narrow strip of forest but a robust woodland.

This discovery raises important questions about the environmental contexts in which early humans lived and adapted. Professor Scerri remarked: "Convergent evolution shows beyond doubt that ecological diversity sits at the heart of our species. This reflects a complex history of population subdivision, in which different populations lived in different regions and habitat types."

Traditionally, it has been believed that early humans evolved in Africa’s grasslands and gradually adapted to other environments, including savannahs and semi-arid regions. However, the evidence from this study suggests that humans were living in diverse ecosystems much earlier than previously thought, even within tropical rainforests.

According to Prof Scerri: "What we have discovered in our results is that this view cannot be true, and for a very early stage, unexpectedly early, people were living in radically different ecosystems in Africa, the home of our species, and adapting to them."

The team also speculates that early human groups living in rainforests may not have frequently encountered those adapted to other ecosystems, such as savannahs, leading to distinct evolutionary paths. These adaptations to diverse environments likely influenced human interactions and gene flow, further shaping human evolution. As Prof Scerri pointed out: "Ecosystem diversity and our adaptations to radically different ecosystems also likely moderated how people interacted, and how genes were exchanged—the processes that underlie evolution."

The study opens new avenues of research into how early humans impacted their environments and how these habitats shaped the development of species. Prof Scerri raised the important question: "We now need to ask how these early human niche expansions impacted the plants and animals that shared the same niche-space with humans."

Professor Guédé highlighted the broader significance of the discovery: "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."