Refining human palaeodietary reconstruction using amino acid δ15N values of plants, animals and humans

An established method of estimating the trophic level of an organism is through stable isotope analysis of its tissues and those of its diet. This method has been used in archaeology to reconstruct past human diet from the stable nitrogen isotope (δ15N) values of human and herbivore bone collagen. However, this approach, using the 15N-enrichment of human bone collagen δ15N values over associated herbivore bone collagen δ15N values to predict the relative importance of animal protein, relies on the assumptions that: (i) the δ15N values of plants consumed by humans and herbivores are identical, and (ii) the 15N-enrichment between diet and consumer is consistent. Bone collagen amino acid δ15N values have the potential to tackle these uncertainties, as they constrain the factors influencing bone collagen δ15N values. In this study, the δ15N values of glutamic acid and phenylalanine in human and herbivore bone collagen isolates from Neolithic sites in Germany, Greece and Turkey were determined by gas chromatography-combustion-isotope ratio mass spectrometry. The fraction of animal protein in total dietary protein consumed by the humans was estimated by: (i) comparing bulk human and herbivore collagen δ15N values, (ii) comparing bulk human and herbivore collagen and ancient charred cereal grain δ15N values, (iii) comparing human bone collagen δ15NGlutamic acid and δ15NPhenylalanine values, and (iv) comparing δ15NGlutamic acid values of human and herbivore bone collagen and estimated δ15NGlutamic acid values of ancient charred cereal grains. Where determined cereal grain δ15N values are higher than estimated herbivore forage values, estimates of animal protein consumption are significantly lower, emphasising the importance of the plant nitrogen contribution to human bone collagen. This study also highlights the need for further investigation into: (i) the Δ15NConsumer-Diet values of glutamic acid and phenylalanine in terrestrial ecosystems, and (ii) Δ15NGlutamic acid-Phenylalanine values of common plant foods in order to improve the accuracy and more widespread applicability of amino acid-based methods for palaeodietary reconstruction.