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 Late Pleistocene 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.
A study conducted in 2005, examined and compared permanent teeth from 4,002 individuals found in 42 prehistoric and historic samples throughout East Asia, Southeast Asia, Australia, and Melanesia. Metric dental traits were denoted by mesiodistal and buccolingual crown diameters. Measurements were taken as maximum diameters and male individuals were primarily measured because males generally have larger teeth and therefore show more differences. One culture that was prevalent during this time was the Đa Bút culture which occurred during 6000-5000 BP. The metric and nonmetric dental analyses of the Đa Bút resulted in one culture that are highly differentiated from the East Asians. A cluster analysis of the transformed Q-mode correlation coefficients of the dental crown measurements, showed a major cluster with Australo-Melanesians. Da But also has similarities with Australo-Melanesians in tooth size proportions and nonmetric traits. These close traits indicates a likeness with Australo-Melanesians.
Modern debates and controversies
The main controversy concerning the 'Two Layer' hypothesis is whether or not the evolutionary process truly involved the Australo-Melanesians. Archaeologists such as Matsumura suggest Southern Chinese people comprised the initial population of Southeast Asia, rather than Australo-Melanesians while researchers such as Turner argue that prehistoric Southeast Asians did not mix with either racial group. Though the early prehistoric Vietnamese and Malaysians both resembled the Australo-Melanesian samples the most, the Màn Bạc people had a greater resemblance to the Đông Sơn samples dating back to the Iron Age. Analyzing cranial and dental remains, Matsumura concluded based on chronological differences that the Man Bac people were immigrants affiliated with peoples near the Yangtze River region in Southern China. Molecular anthropologists have used classical genetic markers and mtDNA to analyze the similarities between early Chinese and Southeast Asians. Such genetic markers suggest the genetic layout of Southern Chinese peoples is quite similar to that of Southeast Asians.
Other controversies completely reject the 'Two Layer' hypothesis. Using dental evidence, Turner’s Sundadont/Sinodont hypothesis suggests the “Sundadont” trait seen in present-day Southeast Asians is a result of long-standing continuity. Turner created a cluster analysis of MMD values in order to test existing hypotheses of origins, concluding that all Southeast Asians, Micronesians, Polynesians, and Jomonese form their own branch and descend from a common ancestor. The Australians and Melanesians, however, are scattered over the African and European branch along with a side branch of Tasmanians and Solomon Islanders. Howell analyzed crania of major racial branches worldwide, and linked Australian and Melanesian cranial morphology most closely with African cranials. Howell discovered, however, that the size and features of present-day Asian cranial morphology differed significantly from that of Australians, Melanesians, and Africans.
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