RECONSTRUCTING VEGETATION AND CLIMATE FROM FOSSIL MAMMALS: LATE CENOZOIC ECOSYSTEM CHANGES ACROSS 7 MYR OF HOMININ EVOLUTION IN EASTERN AFRICA

Modern African habitats range from rain forests to vegetation-limited deserts. Habitats between these extremes have varying tree densities together with grasses as ground cover—collectively referred to as savannas. Each vegetation habitat supports distinctive communities of large African mammals, and each mammal species possesses functional traits (e.g., diet, body size) that, when grouped, indicate the trait structure of the community. The functional trait structure is highly correlated with climate data such as mean annual precipitation and temperature seasonality. Here, we use these modern community characteristics as the basis for reconstructing vegetation and climate for eastern African fossil sites spanning the last 7 Myr.

We compiled databases of functional traits for 269 extant mammal species from 168 modern communities in sub-Saharan Africa, and for > 650 extinct taxa from 94 fossil assemblages located in Ethiopia, Kenya, and Tanzania. Vegetation and climate data for each modern community were taken from WorldClim (Hijmans et al., 2005) and White’s (1983) vegetation maps of Africa. We compared each fossil site to CO₂ proxies from Levin (2015), comparing functional traits (e.g., grazing) with CO₂ through time. Multivariate analyses and regressions of the functional traits in modern communities were used to deduce the vegetation and climate characteristics of each fossil site.

Proxy paleoenvironmental data, such as δ¹⁸O records of temperature and δ¹³C paleosol records of C₃-C₄ terrestrial vegetation, are often used to explain changes in late Cenozoic fossil mammal communities. Our results are generally consistent with these records and show that mean annual precipitation has significantly declined in eastern Africa over the last 7 Myr, while temperature seasonality has significantly increased. The proportions of grazing species in fossil assemblages tracks CO₂ levels as well as the expansion of C₄ grasslands though time, albeit with a slight lag. The timing of mammal community changes varies across different regions of eastern Africa, raising the possibility that local climatic factors may have been more important than global and/or continent-wide climate changes for hominin evolution.