A diet for all seasons: the role of intra-annual variability in the evolution of hominin diet in East Africa

饮食变化是哺乳动物进化的主要驱动力，包括早期原始人(人类谱系的成员)。稳定同位素分析的最新进展使我们能够量化单个化石人的饮食方面，从而彻底改变了我们对早期人的生物学和行为的理解。人类牙齿化石的碳同位素分析显示，在400万年前(Ma)之后，饮食发生了重大转变，从集中在C3植物(树木、灌木和草本植物的叶子、水果和坚果)到包括热带C4植物(草和莎草)和/或动物的饮食。这一创新的生物学意义，以及它与季节性草本植物为主环境的扩大的关系，目前尚不清楚。依赖基于C4的资源的个体季节性地改变他们的饮食，以便在可获得时包括更多的C3食物，或者全年专门从事基于C4的食物，这对与资源可获得性和与其他哺乳动物的竞争性互动相关的选择压力具有深远的影响。我们知道，在大约3.9 Ma以后，C4资源的比例在个体之间有很大的差异，这表明个体具有不寻常的灵活性。来自活着的人类和非人类灵长类动物的间接证据表明，原始人的饮食也可能随着季节的变化而变化，但没有直接证据表明这种变化的规模和模式。这项研究的目的是评估东非化石人类从4 Ma到1 Ma的饮食变异性的演变。以前对古人类化石的同位素分析表明，某些物种的古人个体之间的饮食差异很大。然而，他们依赖于每颗牙齿产生一个单一的整体同位素值，因此无法解决个体寿命内的可变性。哺乳动物的牙齿是逐渐生长的，牙齿内稳定的同位素图谱代表了个体人类或动物在牙齿形成期间的生活史档案。因此，通过使用激光消融显微采样，每颗牙齿产生多个同位素值，就有可能测量化石人类和其他灵长类动物的季节性饮食变化。这种方法已经成功地应用于广泛的哺乳动物食草动物，以及来自南非的人类和古人类化石。10多年前进行的后一项研究意义重大，因为它证明了这种方法在人类中的有效性，但由于南部非洲保存的化石的时间跨度有限，无法评估饮食变异性的长期变化。由于化石标本的同位素采样受到限制，这一方法尚未推广到东非化石人类。在肯尼亚国家博物馆的合作和全力支持下，现在可以使用横跨过去4 Ma的古人类化石来评估东非饮食多样性的长期变化。为了进行比较，我们还将包括灭绝的狒狒，它们的饮食转型显示出越来越多地依赖C4食物，部分反映了原始人，以了解在平行的陆地大型灵长类动物谱系中，C4摄食与年内饮食变化之间的关系。为了为化石记录中的其他生态推断提供现代基线，我们将对居住在热带大草原上的现代狒狒进行同位素研究，它们经常被用作早期人种的生态模型。我们使用粪便来提供高时间分辨率的饮食同位素记录，能够轻松地记录季节性变化，以更好地了解碳同位素如何跟踪已知的草和莎草消费的季节性变化，以及是什么影响了这些变化。粪便中的同位素变化将与从同一人群中收集的牙齿的釉质进行比较。

Dietary change is a major driver of evolution amongst mammals, including early hominins (members of the human lineage). Recent developments in stable isotope analysis have revolutionized our understanding of early hominin biology and behaviour by allowing quantification of aspects of the diets of individual fossil hominins. Carbon isotope analyses of fossil hominin teeth have revealed a major dietary transition after 4 million years ago (Ma), from a concentration on C3 plants (leaves, fruit and nuts from trees, shrubs, and herbs) to diets extended to include tropical C4 plants (grasses and sedges) and/or animals. The biological significance of this innovation, and its relationship to the expansion of seasonal grass-dominated environments, remains unclear. The degree to which individuals relying on C4-based resources shifted their diet seasonally to include more C3 foods when available, or specialized in C4-based foods year-round, has profound implications for the selective pressures associated with resource availability and competitive interactions with other mammals. We know that the proportions of C4 resources varied strongly between individuals after ca. 3.9 Ma, suggesting unusual individual flexibility. Indirect evidence from living human and non-human primates suggests that hominin diets may also have varied seasonally, yet there is no direct evidence on the scale and pattern of such variability. The aim of this study is to evaluate the evolution of dietary variability in fossil hominins in eastern Africa from 4 to 1 Ma. Previous isotopic analyses of hominin fossils demonstrate substantial dietary variability between hominin individuals among some species. However, they relied on generating a single bulk isotope value per tooth, and therefore could not address variability within the lifetimes of individuals. Mammal teeth grow incrementally, and intra-tooth stable isotope profiles represent life-history archives of individual humans or animals during the period of tooth formation. Thus it is possible to measure seasonal-scale diet changes in fossil hominins and other primates by generating multiple isotope values per tooth using laser ablation microsampling. This approach has been successfully applied to a wide range of mammalian herbivores, as well as humans, and fossil hominins from South Africa. The latter study, conducted over 10 years ago, is significant because it demonstrates the validity of this approach in hominins, but it was not possible to evaluate long-term changes in dietary variability due to the restricted time span of fossils preserved in southern Africa. This approach has not yet been extended to eastern African fossil hominins, due to restrictions on isotopic sampling of fossil specimens. With the collaboration and full support of the National Museums of Kenya, it is now possible to evaluate long-term changes in dietary variability in eastern Africa using fossil hominins spanning the last 4 Ma. For comparison, we will also include extinct baboons, which show a dietary transition towards increasing reliance on C4 foods that partly mirrors hominins, to understand the relationship between C4-feeding and intra-annual dietary variability in a parallel lineage of terrestrial, large-bodied primates. To provide a modern baseline to ground other ecological inferences in the fossil record, we will conduct an isotopic study on modern savanna-dwelling baboons, which are often used as ecological models for early hominins. We use feces to provide an isotopic record of diet with high time resolution, easily capable of recording seasonal-scale changes, to better understand how carbon isotopes track known seasonal variation in the consumption of grasses and sedges, and what influences these shifts. Isotopic variation in feces will be compared to enamel from teeth collected from the same population.