Two layer hypothesis

The 'Two Layer' Hypothesis, or immigration hypothesis, is an archaeological hypothese that suggests the human occupation of mainland Southeast Asia occurred over two distinct periods by two separate racial groups, hence the term 'layer'.^[1] 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.^[2][3] 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.

Recent genetic and archeologic evidence found that both Australo-Melanesian and East Asian-related populations migrated along a southern route, with the Australo-Melanesians using a coastal route along the coast of the Indian peninsula into Insular Southeast Asia and Oceania, while East Asian-related groups used a route south or through the Himalayan mountain range into Mainland Southeast Asia, from where Basal-East Asians than expanded northwards and southwards respectively at 50,000BC. East Asian-related ancestry was far more widespread in Insular Southeast Asia than previously suggested, long predating the Austroasiatic and Austronesian expansions.