The following article seems to confirm, on the basis of dental traits, the hybrid origin of Southeast Asians, with the earliest inhabitants being Australoids, to which Mongoloids were later added, starting probably with the Neolithic.
It seems to me that in South and Southeast Asia, and perhaps in South Arabia to the west, and stretching into Oceania in the east, the earlier inhabitants were of an old, pigmented subequatorial "Paleo-Asian" type. Over this Paleolithic stratum, the main Eurasiatic races were added: Caucasoids in Arabia and India, and Mongoloids in Southeast Asia.
American Journal of Physical Anthropology (Early View)
Dental perspectives on the population history of Southeast Asia
Hirofumi Matsumura et al.
This article uses metric and nonmetric dental data to test the two-layer or immigration hypothesis whereby Southeast Asia was initially occupied by an Australo-Melanesian population that later underwent substantial genetic admixture with East Asian immigrants associated with the spread of agriculture from the Neolithic period onwards. We examined teeth from 4,002 individuals comprising 42 prehistoric and historic samples from East Asia, Southeast Asia, Australia, and Melanesia. For the odontometric analysis, dental size proportions were compared using factor analysis and Q-mode correlation coefficients, and overall tooth size was also compared between population samples. Nonmetric population affinities were estimated by Smith's distances, using the frequencies of 16 tooth traits. The results of both the metric and nonmetric analyses demonstrate close affinities between recent Australo-Melanesian samples and samples representing early Southeast Asia, such as the Early to Middle Holocene series from Vietnam, Malaysia, and Flores. In contrast, the dental characteristics of most modern Southeast Asians exhibit a mixture of traits associated with East Asians and Australo-Melanesians, suggesting that these populations were genetically influenced by immigrants from East Asia. East Asian metric and/or nonmetric traits are also found in some prehistoric samples from Southeast Asia such as Ban Kao (Thailand), implying that immigration probably began in the early Neolithic. Much clearer influence of East Asian immigration was found in Early Metal Age Vietnamese and Sulawesi samples. Although the results of this study are consistent with the immigration hypothesis, analysis of additional Neolithic samples is needed to determine the exact timing of population dispersals into Southeast Asia.