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

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.