Did Rainforest Expansion Influence the Dispersal of Homo Sapiens into Sundaland and Wallacea? Insights From Stable Isotope Analyses of Mammalian Fauna

Mika Rizki Puspaningrum1*, Gerrit D. van den Bergh2, and Allan Chivas2

Modern humans are thought to have dispersed into Sundaland and Wallacea between ~128,000 and 50,000 years ago. To evaluate the environmental contexts accompanying these dispersals, we analysed stable isotopes from mammalian faunal assemblages recovered from key localities associated with the earliest evidence of Homo sapiens: Lida Ajer, Sumatra (76-63 ka); Punung, Java (128-118 ka); Leang Bulu Bettue, Sulawesi (50-40 ka); and Liang Bua, Flores (~50 ka). Our results indicate that rainforest environments became increasingly dominant across Sundaland following Marine Isotope Stage (MIS) 5. In Sumatra, rainforest conditions likely prevailed even earlier, consistent with the predominance of cave sites in upland settings. In Sulawesi, the arrival of modern humans also coincides with predominantly rainforest-like environments. In contrast, Flores does not exhibit the same pattern of vegetation change. These findings suggest that early Homo sapiens dispersal into Island Southeast Asia did not occur under a single environmental regime, but rather across variable ecological contexts. The repeated association with rainforest environments in parts of Sundaland and Wallacea raises the possibility that dispersal was facilitated by pre-existing adaptive flexibility to tropical forest habitats, rather than requiring region-specific innovations. This case study contributes to broader debates on whether early human expansions were primarily driven by adaptive plasticity or by the development of novel behavioural strategies.