Did Denisovans Cross the Wallace’s Line?

The migration of early modern humans out of Africa saw their initial stop in India, where the first diversifications of mitochondrial and Y chromosome lineages occurred. From there, groups moved eastward to Southeast Asia, facilitating exploration of Sundaland and Sahul. Sundaland, an extended landmass connecting islands like Sumatra and Java during glacial periods, and Sahul, comprising Australia and New Guinea, were linked to human migration routes due to lowered sea levels. These regions underwent significant geographical transformations over millennia, shaping the migration patterns of early humans. The biogeographic region of Wallacea, located between Sunda and Sahul, served as a transitional zone bounded by Wallace’s and Lydekker’s Lines. Its unique flora and fauna include marsupials in the east and placental mammals in the west. The islands of Wallacea, such as Sulawesi and Flores, provided stepping stones for human migration. Evidence from Flores, including the discovery of Homo floresiensis and Stegodon elephants, highlights the region’s role in human evolution and ancient biodiversity. Reaching Sahul approximately 50,000–65,000 years ago required advanced maritime skills and intentional voyaging. This migration represents one of humanity’s earliest seafaring achievements. Two primary routes, northern and southern, are hypothesized, each with distinct challenges and environmental adaptations. The migration also necessitated adapting to new ecosystems and likely involved following ocean currents and wind patterns. The arrival of humans in Sahul marked a significant milestone in human settlement and cultural evolution. Denisovans, an archaic human group, coexisted with Neanderthals and early modern humans. Discovered through mitochondrial DNA analysis in Denisova Cave, Siberia, their fossils and genetic evidence suggest they were widespread across Asia and Southeast Asia. Denisovans contributed significantly to the genetic makeup of modern humans, particularly in populations in Oceania and Southeast Asia. The adaptation to high-altitude environments in Tibetans, through Denisovan genetic introgression, exemplifies their influence. Denisovans exhibited significant genetic diversity, with evidence of interbreeding with early Homo sapiens and possibly other archaic species. Their genetic traces are most prominent in populations east of Wallace’s Line, such as Papuans and Australian Aboriginals. This distribution suggests concentrated interactions in Wallacea, likely shaped by migration patterns and ecological factors. Interbreeding likely provided advantages such as disease resistance and environmental adaptation, enhancing survival in diverse habitats. Wallacea, rich in hominin history, also hosted Homo floresiensis and Homo luzonensis. These species highlight the adaptability of early humans to isolated environments. Denisovans’ widespread presence and genetic contributions underscore their importance in understanding human evolution. Continued genomic research is expected to unravel more about their interactions with other human groups and their role in shaping modern human diversity.