The biogeographic threshold of Wallacea in human evolution

The Wallacean archipelago between the Indian and Pacific Oceans is a critical biogeographic boundary for all kinds of animals, from butterflies to birds. Humans are no exception, and in this paper we offer a three stage model for how our genus overcame this boundary. We review how Lower Palaeolithic hominins were able to colonize the larger islands of western Wallacea through incidental seagoing, and subsistence on the megaherbivores that also made these crossings. However, Lower Palaeolithic hominins were not able to maintain geneflow between islands, nor cross into eastern Wallacea and beyond into Sahul. This biogeographic threshold persisted for hundreds of thousands of years until the arrival of our own species, whose cognitive capacities for planning and abstraction likely allowed for the construction of composite material rafts, interdependent paddles, and multiple-day seafaring. With the sea-level rise of the terminal Pleistocene and early Holocene a greater connectivity is seen in the form obsidian exchange networks, inter-island bead and fishhook traditions, and the colonization of very small islands entirely depauperate in terrestrial fauna. We suggest that the appearance of shell adzes at this time reflects the production of dugout canoes to facilitate regular inter-island voyaging. Wallacea presents a key case study both in the limits of Lower Palaeolithic hominin dispersal and the capacities that allowed Homo sapiens to go beyond these, as well as the intensification response of our species to rapid benign environmental change.