The evolution of humans in Australasia from an environmental perspective

Abstract

Attempts to understand the evolution within the genus Homo in Australasia have relied on two main theories. The Multiregional scenario suggests that cranial morphology reveals a long-standing continuity between fossil and recent hominids in the region. Out of Africa theorist have favoured the idea that Homo sapiens evolved in Africa, migrated outward, and replaced archaic hominids, such as Homo erectus in Australasia. Neither Multiregionalists nor Out of Africans have seriously factored in the unique geographic and ecological context in which variation in Australasian members of the genus Homo evolved. Nonetheless, the mammalian faunas of this region have been strongly shaped by the existence of zoogeographic barriers and different ecological zones. Taking an environmental perspective on the evolutionary history of the human lineage in this region leads to scenario in which both local evolution and replacement play a role. Wallacea, as a water-barrier, has been a tremendously important geographic barrier influencing the distribution and evolutionary course of many mammals, including that of the genus Homo. Regional continuity may be assumed for Homo erectus in Southeast Asia, from earlier Javanese forms like Trinil and Sangiran to later forms like Ngandong. There is no evidence that Homo erectus crossed Wallacea and reached the Sahul. As between 126 and 81 ka the tropical rainforest spread as far as East Java, a number of archaic species, including Homo erectus, were replaced by modern species better adapted to survive in a more humid environment. In contrast to Homo erectus, Homo sapiens was capable of crossing Wallacea. Once Homo sapiens reached the Sahul, Wallacea was an important factor for its further evolutionary course. Interestingly, regional continuity may be assumed for Homo sapiens on both sides of Wallacea. Looking at the cranial morphology of recent humans, it is obvious that there are two clearly distinguishable cranial patterns, an East Asian (Sunda) and an Australian (Sahul) one.