Stochastic population projections in Sahul refine the human-refugia hypothesis for early Last Glacial Maximum

Abstract

Pleistocene archaeology in Australia has focussed on the survival and behaviour of Indigenous populations across Sahul during the Last Glacial Maximum (28.6 ± 2.8 ka to 17.7 ± 2.2 ka). A long-standing conceptual model proposes people occupied ecological refugia while abandoning drier regions during extreme climatic conditions, with inferred patterns of subsequent recovery essential for describing the evolution of societies in the Holocene. Radiocarbon-derived population estimates partially support the conceptual model while genetic evidence does not, but how human populations were influenced by the Last Glacial Maximum remains untested. To test the refugia hypothesis, we developed a spatial-demographic model of human movement to project population patterns across Sahul from 40,000 years ago (ka) to the mid-Holocene (5 ka). The model predicts little population change in eastern Sahul or New Guinea during the Last Glacial Maximum. However, extensive movement and potential abandonment in the central-western deserts and the north-northwest coastal regions are predicted during the first half of the Last Glacial Maximum (~ 26–20 ka), with some recovery after 15 ka. The demographic implications to societies appear to have extended beyond the Last Glacial Maximum, with increasing populations not evident until the early Holocene in many regions. Our model describes a complex pattern where large areas of Sahul provided refugia that maintained populations throughout the Last Glacial Maximum, providing a possible explanation for the disparity between archaeological and genomic evidence. There was also a correlation between predicted rates of population change and those derived from radiocarbon dates, supporting the realism of applying dates to infer past demography.

pastclim 1.2: an R package to easily access and use paleoclimatic reconstructions

“…The availability of such data paved the way for a large number of research questions among many disciplines, i.e. ecology (Miller et al 2021b, VanBuren and Jarzyna 2022), paleoecology (Leonardi et al 2018, 2022, Somveille et al 2020, Chen et al 2021, Schap et al 2021, Thorup et al 2021, Medina‐Castañeda et al 2022, Reade et al 2022), conservation (Beyer and Manica 2021), population genetics (Maisano Delser et al 2021, Miller et al 2021a, Warmuth et al 2021), archaeology (Betti et al 2020, Racimo et al 2020, Beyer et al 2021, Krzyzanska et al 2021, Bradshaw et al 2022, Cerasoni et al 2022, Park and Marwick 2022, Timbrell et al 2022), macroevolution of different taxa, including the genus Homo (Saupe et al 2014, Will et al 2021, Fordham et al 2022, Timmermann et al 2022, Troyer et al 2022), anthropology (Leonardi et al 2017, Padilla‐Iglesias et al 2022) and linguistics (Beyer et al 2019).…”