Relating woody cover with Australopithecus presence and relative abundance at eastern African fossil sites

Abstract

Many hypotheses regarding Australopithecus adaptations revolve around the idea that these hominins were ecological generalists. The diversity of site-specific habitat reconstructions and the wide geographic range of the genus support this notion, indicating that it could tolerate variable ecological conditions. However, previous analyses of Australopithecus biogeography and paleoecology have relied on qualitative descriptions with poorly defined ecological terms that lack precision, such as “mosaic environment.” Moreover, an imperfectly sampled fossil record complicates our ability to understand the spatiotemporal distribution of Australopithecus and the ecological and environmental variables that enabled australopith populations to live and thrive at a given site. For example, the observed absence of Australopithecus at a site could be due to one of two factors: (1) Australopithecus never lived at the site (true absence), or (2) it did but has not been discovered yet (false absence). Here, this study analyzes 49 eastern African fossil sites ranging from 4.5 to 2 million years ago using two Bayesian hierarchical models, which model true and false absences as explicitly different states. This approach quantifies the probability of Australopithecus presence and relative abundance at a given site as a function of woody cover. Woody cover is quantified using the site-level mean stable carbon isotope values (?13C) of mammalian tooth enamel and the within-site spread of those values. Model results positively associate eastern African australopith relative abundance with more open environments and sites with considerably greater breadth of vegetation availability. Therefore, communities where australopiths thrive exhibit within-site variability but also tend towards less woody cover. Results may indicate a negative correlation between australopith presence and mean and interquartile range ?13C values, but effective conclusions are not possible given the model’s performance. As the effect of the spread of ?13C values is larger than that of the mean, australopith abundance is driven by breadth of vegetation available, supporting the notion that they were a generalist taxon. The analysis rigorously tests the idea that Australopithecus was ecologically flexible—while simultaneously accounting for an incomplete fossil record—which sheds light on the ecology and evolution of a genus that is central to the study of human origins.