Pre-agricultural intensification of plant use in Pleistocene Sri Lankan rainforests

nature ecology & evolution Article https://doi.org/10.1038/s41559-026-03082-6 Pre-agricultural intensification of plant use in Pleistocene Sri Lankan rainforests Received: 5 November 2025 Accepted: 17 April 2026 Published online: xx xx xxxx Check for updates Nicolas Bourgon 1,2 , Marcus Oelze2, Noel Amano3, Oshan Wedage4, Nimal Perera5 & Patrick Roberts 1 Tropical rainforests have often been considered marginal environments for Pleistocene hunter-gatherers, yet archaeological research in Sri Lanka demonstrates long-term occupation of these habitats from ~48,000 years ago (ka). Material evidence indicates specialized hunting of arboreal mammals, as well as the use of plant resources, but plant consumption is often difficult to detect because organic remains preserve poorly in rainforest settings. Here we present zinc isotope data (δ66Zn) from Late Pleistocene to Late Holocene human (n = 24) and faunal tooth (n = 57) enamel, spanning ~20–3 ka of rainforest occupation in Sri Lanka. Our results show that humans consistently occupied an intermediate trophic position, indicating mixed diets of animal and plant foods. Over time, human δ66Zn values shift towards those typical of herbivores, suggesting an increasing reliance on plant resources. This pattern predates the regional introduction of crop agriculture and indicates that rainforest foragers were intensifying plant use long before farming emerged. In recent years, the notion that tropical rainforests were inhospitable1,2 and ecological barriers to Pleistocene human dispersals3–5 has been challenged by the increasing evidence of occupation and adaptation to these environments by our species6–12. Although rainforest ecosystems vary substantially across regions and should not be treated as direct analogues to each other, multidisciplinary investigations of four cave sites in Sri Lanka (Balangoda Kuragala, Batadomba-lena, Fa-Hien Lena and Kitulgala Beli-lena; Fig. 1) have provided an exceptionally rich and early archaeological record, ideal for understanding the earliest human adaptations to tropical environments and human tropical forest interaction over a long period10,13–15. These archaeological sites in the island’s lowland Wet and Intermediate Zones (Fig. 1) comprise a discontinuous record of human occupation reaching back to ~48,000 years ago (ka), providing some of the earliest direct evidence of human utilization of rainforest resources worldwide and several of the earliest Homo sapiens fossils found anywhere in South Asia10,16,17. Although chronological gaps exist at Fa-Hien Lena, probably reflecting local depositional or taphonomic processes rather than true absences of occupation18, the overall well-stratified cultural sequence of these cave and rockshelter sites, with consistent dating and no age reversals, points to sustained human presence in the region. Already in the oldest occupation layers, evidence suggestive of specialized hunting of arboreal and semi-arboreal small-bodied mammals can be found10, made possible through the use of microliths13,15,19,20, bone tools11,21 and what is currently recognized as the earliest bow-and-arrow technology documented outside of Africa11. Archaeological evidence further suggests that these populations gathered and processed freshwater and terrestrial molluscs, as well as fruits and nuts such as breadfruit and kekuna nut (also known as candlenut)10. This evidence contrasts with assumptions that coastal settings were preferred by humans migrating around the Indian Ocean rim22–24 and highlights our species’ ability to specialize, at the community level, in the use of more extreme terrestrial environments25–28. Although Sri Lanka’s lowland Wet Zone and Intermediate Zone rainforest archaeological record is striking for its abundance of archaeological material in a tropical rainforest context, it has been 1 Department of Coevolution of Land Use and Urbanisation, Max Planck Institute of Geoanthropology, Jena, Germany. 2Bundesanstalt für Materialforschung und-prüfung, Berlin, Germany. 3Max Planck Institute of Geoanthropology, Jena, Germany. 4Department of History and Archaeology, University of Sri Jayewardenepura, Nugegoda, Sri Lanka. 5Excavation Branch, Department of Archaeology of the Government of Sri Lanka, Colombo, Sri Lanka. e-mail: Nature Ecology & Evolution Article https://doi.org/10.1038/s41559-026-03082-6 9° N 8° N 7° N Colombo Batadomba-Iena Fa-Hien Lena Balangoda Kuragala 80° E 81° E 82° E Rainforest and grassland >1,500 m Rainforest and grassland 900–1,500 m Rainforest and grassland <900 m Intermediate rainforest Monsoon scrub jungle Semi-evergreen forest, grasslands and shrubs Fig. 1 | Vegetation zones and location of studied archaeological sites in Sri Lanka (Balangoda Kuragala, Batadomba-lena and Fa-Hien Lena). The Wet Zone (rainforest and grasslands) lacks a significant dry period and supports elevation-structured tropical rainforests; the Intermediate Zone (intermediate rainforest) has a short, muted dry season and transitional forest structure; and the Dry Zone (monsoon scrub jungle and semi-evergreen forest, grasslands and shrubs) has a distinct dry season dominated by monsoon forest and scrub jungle. The island’s vegetation zones were redrawn on the basis of published data from Gunatilleke et al. (2005)85 and Erdelen (1988)86. challenging to glean reliable insights into the overall dietary reliance on specific types of resources, as zooarchaeological and archaeobotanical remains may represent isolated events (such as season-specific activities, hunting, butchering or food processing) or year-long occupation, constitute a time-averaged assemblage spanning an unknown period or offer skewed interpretations on the basis of the absence and presence of material within the archaeological record29–31. The application of stable carbon and oxygen isotope analysis to human fossils from these rainforest sites has revealed their relative reliance on closed forest or open habitat resources9,14,18. However, what these resources were Nature Ecology & Evolution (for example, plants versus animals versus freshwater fauna), and exactly how these populations utilized their ecosystem (that is, land use) over time, has remained somewhat obscure. This is particularly significant in Sri Lanka, where there is evidence for early connections between forest communities and communities living along the coast11,13,32, and palynological and phytolith evidence has been used to suggest early, intense relationships with plants in parts of the island that may even constitute cultivation akin to that seen in other tropical regions by the Early Holocene33–35. Human fossils from the lowland Wet Zone and Intermediate cave sites provide an unparalleled opportunity to study long-term dietary reliance on plants through time in Sri Lanka and to test hypotheses relating to plant and land use. To date, however, such work has been lacking owing to methodological limitations. One (...truncated)