The 'Two Layer' Hypothesis, or immigration hypothesis, is an archaeological theory that suggests the human occupation of mainland Southeast Asia occurred over two distinct periods by two separate racial groups, hence the term 'layer'. According to the Two Layer Hypothesis, early indigenous Australo-Melanesian peoples comprised the first population of Southeast Asia before their genetic integration with a second wave of inhabitants from East Asia, including Southern China, during the agricultural expansion of the Neolithic.The majority of evidence for the Two Layer Hypothesis consists of dental and morphometric analyses from archaeological sites throughout Southeast Asia, most prominently Thailand and Vietnam. The credibility of the Two Layer Hypothesis has been criticized due mainly to sim