Two layer hypothesis

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 similarities between Southeast Asian and Chinese cranial and dental characteristics, excluding Australo-Melanesians.

The first fossilized skeletal remains and indication of early 'Proto-Australian' Southeast Asian inhabitants surfaced in 1920 during an excavation by Dubois on the island of Java. Despite this, a formal connection to mainland Southeast Asia and the suggestion of an initial population of Australomelanesoids was not suggested until 1952 by Koenigswald in his response to Hooijer, who sharply criticized the attribution of 'big toothed' dental remains to early Australo-Melanesians. The immigration hypothesis proposed by Koenigswald was formally termed the 'Two Layer' model by Jacob Teuku. In 1967, Teuku analyzed the cranial and dental proportions of 152 adult skeletal samples recovered from prehistoric sites in Malaysia and Indonesia, the majority displaying robust jaws and teeth, prominent glabellae, and slender, elongated limbs. Teuku argued these characteristics correspond to the Australo-Melanesian population proposed by Koenigswald that predated the East Asian immigrants of the Neolithic; also suggesting the initial inhabitants were likely forced south of Southeast Asia's mainland by the second wave of migrants, due to resource competition or conflict.

Excavations at Moh Khiew Cave resulted in the discovery of a female human skeleton, an AMS radiocarbon date on a charcoal sample that was gathered from the burial grave gave an age of 25,800 +/- 600 BP. Measurements such as the bimaxillary breadth, upper facial height, bicondylar breadth, mandibular length, nasal breadth and height, palatal height, and mandibular angle were compared from the cranial and dental area to other female human specimens from East Asia and the Southwest Pacific regions. Twelve measurements were used in total for the statistical comparison in addition to 14 buccolingual crown diameters of the maxillary and mandibular teeth. After a comparison was completed the closest sample to the Moh Khiew Cave sample is the Late Pleistocene Coobool Creek from Australia. The next closest sample is the modern Australian Aborigines. However, those from Flores, Vietnam, and Thailand are distant samples in comparison. This discovery suggests that the Moh Khiew Cave skeleton may have shared a common ancestor with Australian Aborigines and Melanesians and also supports the ‘Two Layer’ hypothesis.

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