Coastal complexity: Ancient human diets inferred from Bayesian stable isotope mixing models and a primate analogue

An extensive ecological literature applies stable isotope mixing models to derive quantitative dietary reconstructions from isotope ratios of consumer tissues. While this approach works well for some organisms, it is challenging for consumers with complex, varied diets, including humans; indeed, many archaeologists have avoided the use of mixing models because uncertainties in model outputs are sufficiently large that the findings are not helpful in understanding ancient lifeways. Here, we exploit an unparalleled opportunity to evaluate the feasibility of dietary quantification in a nutritionally and isotopically complex context on the Cape Peninsula, South Africa. Delta values (δ13C and δ15N) of 213 indigenous food samples enable us to characterise four food groups: terrestrial plants, terrestrial vertebrates, marine invertebrates and marine vertebrates. A recent study of baboons that consumed marine and terrestrial foods provides insight into the relationship between such foods and consumer tissue isotopes. We use this information to refine our interpretation of δ15N and especially δ13C in bone collagen from 35 archaeological hunter-gatherers, achieving better estimates of the relative importance of marine and terrestrial foods in the diet than has hitherto been possible. Based on Bayesian stable isotope mixing model (SIMM) outputs, we infer that the trophic enrichment factor (TEF) for δ13Cbone collagen in these coastal humans is closer to +3 than +5‰. In the most 13C- and 15N-rich individuals, 65–98% of bone collagen (95% credible intervals) derived from marine foods. Conversely, in 13C and 15N-poor individuals, 7–44% of bone collagen derived from marine foods. The uncertainties discussed here highlight the need for caution when implementing SIMMs in studies of consumers with complex diets. To our knowledge, this work constitutes the most detailed and most tightly constrained study of this problem to date.

诸多生态学研究均采用稳定同位素混合模型（stable isotope mixing models），基于消费者组织的同位素比值推导定量膳食重建结果。尽管该方法在部分生物类群中应用效果良好，但对于膳食结构复杂多样的消费者（包括人类）而言却颇具挑战；事实上，诸多考古学者此前一直回避使用混合模型，这是因为模型输出的不确定性过高，所得结果难以助力我们理解古代人类的生计模式。本研究依托南非开普半岛这一兼具营养与同位素复杂性的独特研究场景，把握住了前所未有的机遇，对膳食定量分析的可行性展开评估。我们对213份本土食物样本的δ值（δ¹³C与δ¹⁵N）进行分析，由此界定出四类食物类群：陆生植物、陆生脊椎动物、海洋无脊椎动物以及海洋脊椎动物。近期一项针对食用海洋与陆生食物的狒狒的研究，为阐明这类食物与消费者组织同位素之间的关联提供了参考依据。我们依托该研究的结论，对35位考古学狩猎采集者骨骼胶原中的δ¹⁵N，尤其是δ¹³C展开重新解读，由此得到了比此前更为精准的膳食中海洋与陆生食物相对占比的估算结果。基于贝叶斯稳定同位素混合模型（SIMM）的输出结果，我们推断：这批沿海居民骨骼胶原中δ¹³C的营养富集因子（trophic enrichment factor, TEF）更接近+3‰，而非+5‰。在δ¹³C与δ¹⁵N含量最高的个体中，其骨骼胶原中有65%~98%（95%置信区间）源自海洋食物。与之相反，在δ¹³C与δ¹⁵N含量较低的个体中，其骨骼胶原中有7%~44%源自海洋食物。本次研究探讨的不确定性问题也提醒我们：在针对膳食结构复杂的消费者开展研究时，应用SIMM需格外谨慎。据我们所知，本研究是迄今为止针对该问题开展的最为详尽、约束条件最为严谨的相关研究。