Selective Pressures at the Asian Tropical Margin: Considering the Role of the East Asian Tropical Margin in Pleistocene Hominin Evolution

Robert Kenyon

Palaeoenvironmental analyses in Pleistocene Southeast Asia has typically focused on changes between or gradations of open (savannah-like), closed (rainforest-like) or mosaic environments. The impact of changes at the interface between tropical and subtropical landscapes, however, remains unexplored. Until recently tropical-subtropical distinctions were coarsely resolved, climate based, and spatially arbitrary. A more ecologically meaningful boundary is provided by the occurrence of frost which limits the northern extent of frost-vulnerable tropical flora, forming a non-arbitrary ‘ecological’ tropical margin which shifts over time. To investigate how this dynamic margin impacted hominin populations, ecologically defined environments were compared to identify the opportunities and risks for inhabiting populations. A time-series ecological niche modelling approach then tested responses of frost-sensitive, exclusively tropical flora to climatic changes over 800ka and 21ka, thereby modelling changes to the margin over time. Key aims were to assess environmental stability across the northern tropical margin, and to identify what, if any, role such changes may have played in regional hominin evolution. Results suggest the northern tropical margin, whilst diffuse, was a significant entity throughout the Pleistocene; biomes either side of which are likely to have exerted unique selective pressures on hominin populations. Moreover, the conservative models produced suggest the frequency and magnitude of ecological changes were substantial, with ecology in marginal regions oscillating at a finer scale than archaeological resolution can currently detect. These findings imply that palaeoenvironmental analyses and faunal assemblages at many sites potentially aggregate multiple environments, obfuscating interpretation of hominin lifeways.