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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.
Developments in multi-isotope applications now enable the production of direct and precise life history data of studied archeological and paleoanthropological specimens, with analysis of tooth enamel proving particularly powerful in deep-time preservation contexts. However, the increasing amount of sample material necessary too-often compromises the specimen's structural integrity. Finding ways of minimizing semi-invasive sampling while maximizing produced data thus becomes increasingly crucial. In this study, we examine ways of optimizing sample use for state-of-the-art isotope measurements of enamel-bound zinc (δ66Zn), an increasingly used method in multi-isotope archeological research. We show that certain desolvating nebulizer introduction systems can be used to measure δ66Zn with high efficiency and accuracy using less than half the usual sample size. Moreover, we demonstrate that remaining samples subjected to typical pretreatments used for tooth enamel carbonate (δ13C and δ18O) studies can be safely used without alterations to δ66Zn values. Similarly, we highlight that the often-discarded matrix fraction from zinc chromatography can be utilized for further analyses, such as measurements of 87Sr/86Sr ratios. Our results underscore the possibility of extracting the maximum information from specimens while altogether avoiding or significantly reducing semi-invasive sampling.
