Onset of summer aridification and the decline of Homo floresiensis at Liang Bua 61,000 years ago

The cause of the disappearance of the primitive hominin Homo floresiensis from the Indonesian island of Flores about 50,000 years ago is a key question in palaeoanthropology. While the potential roles of climate change and human agency continue to be debated, the history of freshwater availability essential for survival at the type locality, Liang Bua, remains poorly understood. Although speleothem δ18O is widely used to reconstruct monsoon rainfall, variations in summer and winter rainfall, with distinct δ18O values, can complicate interpretations of mean annual rainfall. Here, we combine speleothem Mg/Ca, a proxy for local rainfall, with δ18O to determine annual, summer and winter rainfall amounts concurrent with H. floresiensis and Stegodon, one of its primary prey. Geochemical modelling of the Mg-18O system reveals a sustained decline in mean annual rainfall from ~1560 to 990 mm between 76,000 and 61,000 years ago. Critically, summer rainfall decreased to a record low of ~450 mm at 61,000–55,000 years ago, alongside a marked decline in both the abundance and relative proportion of Stegodon remains in the fossil record. These findings increase the likelihood that progressive landscape aridification, and intensified human-faunal competition for dwindling resources, culminated in abandonment of Liang Bua.