Natural History Museum of Utah, University of Utah, Salt Lake City, UT 84108;
Department of Anthropology, University of Utah, Salt Lake City, UT 84112.
Proc Natl Acad Sci U S A. 2019 Oct 22;116(43):21478-21483. doi: 10.1073/pnas.1909284116. Epub 2019 Oct 7.
Present-day African ecosystems serve as referential models for conceptualizing the environmental context of early hominin evolution, but the degree to which modern ecosystems are representative of those in the past is unclear. A growing body of evidence from eastern Africa's rich and well-dated late Cenozoic fossil record documents communities of large-bodied mammalian herbivores with ecological structures differing dramatically from those of the present day, implying that modern communities may not be suitable analogs for the ancient ecosystems of hominin evolution. To determine when and why the ecological structure of eastern Africa's herbivore faunas came to resemble those of the present, here we analyze functional trait changes in a comprehensive dataset of 305 modern and fossil herbivore communities spanning the last ∼7 Myr. We show that nearly all communities prior to ∼700 ka were functionally non-analog, largely due to a greater richness of non-ruminants and megaherbivores (species >1,000 kg). The emergence of functionally modern communities precedes that of taxonomically modern communities by 100,000s of years, and can be attributed to the combined influence of Plio-Pleistocene C grassland expansion and pulses of aridity after ∼1 Ma. Given the disproportionate ecological impacts of large-bodied herbivores on factors such as vegetation structure, hydrology, and fire regimes, it follows that the vast majority of early hominin evolution transpired in the context of ecosystems that functioned unlike any today. Identifying how past ecosystems differed compositionally and functionally from those today is key to conceptualizing ancient African environments and testing ecological hypotheses of hominin evolution.
当今的非洲生态系统是概念化早期人类进化环境背景的参考模型，但现代生态系统在多大程度上代表过去的生态系统尚不清楚。越来越多的证据来自东非丰富而年代准确的晚新生代化石记录，记录了大体型哺乳动物食草动物的群落，其生态结构与现代的大不相同，这意味着现代群落可能不适合作为人类进化的古代生态系统的类比。为了确定东非食草动物群的生态结构何时以及为何变得与现在相似，我们在这里分析了跨越过去约 700 万年的 305 个现代和化石食草动物群落的综合功能特征变化数据集。我们表明，几乎所有在约 70 万年前的群落都是功能上不相似的，主要是由于非反刍动物和巨型食草动物（物种 >1000 公斤）的丰富度更高。功能上现代的群落的出现早于分类上现代的群落 10 万年，这可以归因于上新世-更新世 C 草原扩张和 100 万年前以来的干旱脉冲的综合影响。鉴于大型食草动物对植被结构、水文学和火灾等因素的生态影响不成比例，因此，绝大多数早期人类进化发生在与今天任何生态系统都不同的环境中。确定过去的生态系统在组成和功能上与今天的生态系统有何不同，是概念化古代非洲环境和检验人类进化生态假设的关键。
