Middle-Late Miocene paleoenvironmental evolution and its implications for hominoid distribution in the southeastern Tibetan Plateau

The discovery of hominoids dated to the Middle-Late Miocene in the southeastern Tibetan Plateau (SE TP) has attracted widespread attention and highlighted the significance of the SE TP as an important refuge for homi-noids. However, research on the living environment of hominoids in the region has primarily focused on the Late Miocene, and there are solely few earlier records on earlier migrations. A recently investigated sedimentary sequence within the Xiaolongtan Basin in the SE TP, which contains two hominoid-bearing levels dated respectively to 12.5 Ma and 11.7 Ma, provides a unique window to characterize and understand the living environment that attracted hominoids to the region. The clay mineral, geochemical and sedimentary analyses of the sequence, characterized by the highest kaolinite content and chemical weathering intensity and the coeval occurrence of thick lignite layers during -12.6-11.6 Ma, suggest that the climate when hominoids occupied the area was hot with higher humidity compared to the preceding and succeeding intervals. A spatiotemporal analysis of the hominoid distribution in the SE TP and the adjacent Indo-China Peninsula indicates a north--south-north pattern of movement, with their ages of -13-12.5 Ma,-11 to 8-7 Ma, and after -8-7 Ma. These movements accord roughly with the timing of the advance and retreat of the South Asian monsoon (SAM). We therefore infer that, against the overall cooling climatic trend of the Middle-Late Miocene, the increased pre-cipitation and flourishing vegetation promoted by the SAM expansion, was the principal factor that attracted the hominoids to the region. Overall, our results provide new insights into the relationship between the paleo-environment and hominoid migration/settlement.