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，包括早期古人类（人类血统的成员）。稳定同位素分析的最新发展使我们对早期古人类生物学和行为的理解发生了革命性的变化，它允许对个体古人类化石饮食的各个方面进行量化。古人类牙齿化石的碳同位素分析揭示了400万年前（Ma）之后的主要饮食转变，从集中于C3植物（树木、灌木和草本植物的叶子、水果和坚果）到饮食扩展到包括热带C4植物（草和莎草）和/或动物。这种创新的生物学意义，以及它与季节性草为主环境扩张的关系，目前尚不清楚。依赖c4资源的个体在多大程度上季节性地改变他们的饮食，在可能的情况下包括更多的C3食物，或者全年专门食用c4食物，这对与资源可用性和与其他哺乳动物的竞争相互作用相关的选择压力有着深远的影响。我们知道，在约3.9 Ma之后，C4资源的比例在个体之间变化强烈，表明不同寻常的个体灵活性。来自现存的人类和非人类灵长类动物的间接证据表明，古人类的饮食也可能有季节性变化，但没有关于这种变化的规模和模式的直接证据。本研究的目的是评估非洲东部4 - 1 Ma古人类饮食变化的演变。先前对古人类化石的同位素分析表明，在某些物种中，古人类个体之间存在着巨大的饮食差异。然而，他们依赖于生成每颗牙齿的单一体积同位素值，因此无法解决个体一生中的变异性。哺乳动物的牙齿是渐进式生长的，牙内稳定同位素谱代表了牙齿形成时期人类或动物个体的生活史档案。因此，通过激光消融微采样产生每颗牙齿的多个同位素值，可以测量古人类化石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，以及人类和来自南非的古人类化石。后一项研究是在10多年前进行的，意义重大，因为它证明了这种方法在古人类身上的有效性，但由于非洲南部保存的化石时间跨度有限，无法评估饮食变化的长期变化。由于化石标本同位素取样的限制，这种方法尚未推广到东非古人类化石。在肯尼亚国家博物馆的合作和全力支持下，现在有可能利用过去400万年的古人类化石来评估东非饮食变化的长期变化。为了进行比较，我们还将包括已灭绝的狒狒，它们显示出对C4食物日益依赖的饮食转变，这在一定程度上反映了人类，以了解在陆地大型灵长类动物的平行谱系中C4摄食与年度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，我们将对生活在大草原上的现代狒狒进行同位素研究，这些狒狒经常被用作早期人类的生态模型。我们利用粪便提供高时间分辨率的饮食同位素记录，很容易记录季节尺度的变化，以更好地了解碳同位素如何跟踪草和莎草消耗的已知季节变化，以及影响这些变化的因素。粪便中的同位素变化将与从同一人群中收集的牙齿牙釉质进行比较。

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.