Dating methods: the role of geochronology in studies of human evolution and migration in southeast Asia and Australasia

The origin of our own species has been a subject of intense debate since the development of the theory of evolution and the publication of Darwin’s book The descent of man in 1871. Within the last decade or so, a number of issues have been addressed with new discoveries of hominid remains and occupation sites, but the record is still fragmentary. As a result of the spatial diversity of the sites, obtaining accurate chronological control has been vital in piecing together the records. This article attempts to review the contribution played by geochronological techniques in addressing two main issues in the peopling of Asia and Australasia. The first is the timing of the arrival of Homo erectus in southeast Asia and, subsequently, the arrival of Homo sapiens. This has an important bearing on the debate concerning whether Homo sapiens evolved from erectus within a single geographically restricted area, probably southern Africa, and then migrated into other parts of the Old World, or whether this evolutionary step occurred more than once, and in different parts of the world. These two views are known respectively as the ‘Out of Africa’ and ‘Regional Continuity’ hypotheses. The second issue is the timing of the arrival of hominids in the Australasian continental area (Sahul). Throughout the Quaternary, an ocean barrier has existed between the southeast Asian continental area (Sunda) and Australasia. Therefore the first appearance of early man in Australasia implies an ability to navigate significant water crossings. This article describes some of the geochronological work undertaken in these areas over the last 10 years and also discusses the general obstacles involved. A significant contribution has been made due to the development of various dating techniques that can be applied to single crystals. This has had important implications for identifying contaminated samples which, without these methods, would yield incorrect results