In the comments to the post below about Indian ancestry in Thailand, some observed that this should not be surprising due to reciprocal gene flow and proximity. Implicitly, I think what is being suggested here is that there is isolation by distance and continuous gene flow. Obviously some of this is true, but there details here which suggest that it is simply not just geography at work.
The reason I was curious about the Dusun people in coastal Borneo is that while Malays all seem to have Indian ancestry, many tribal Austronesian groups in maritime Southeast Asia do not. The Indian admixture into the Malays is not just recent. Some of it seems quite a bit older than the colonial period.
In the context of Southeast Asia, it seems that some of the more ancient Austro-Asiatic people, in particular, the Mon and Khmer, have Indian ancestry, and groups which mixed with Austro-Asiatic substrates, such as Burmans and Thai, also have this.
Additionally, some groups in the northeastern states of India have less “Indian” admixture than the Thai and Khmer. To show this, see this PCA:
The Garo and Naga live in India (some Garo are in Bangladesh). The “East Indian” samples seem to be mostly Mizo. Of course, some of these groups are intrusive to the northeast. But still. Here are admixture and TreeMix:
The issue in Southeast Asia is that ethnolinguistic groups are the product of several syntheses and migrations. Most of what is today “Thailand” was the domain of Mon-Khmer people in 500 AD. Most of the ancestry seems to date to that period, though there was an overlay from Tai people to the north. In Burma the population is a synthesis of Burman elements with connections to northern East Asians, and Austro-Asiatic people such as the Mon in the south. Additionally, a later movement of Tai people also occurred in Burma (the upland Shan). In Vietnam, the Kinh moved south and seem to have replaced the indigenous Chams and Khmer (there is very little Indian-like ancestry in any Vietnamese samples).
When looking at the map the plausible route of gene flow is clearly from the northeastern part of the subcontinent overland. But several people have pointed out to me that this is very difficult terrain. Recently, I have been convinced that a maritime intrusion of Munda languages into Odisha is plausible. One of the potential points of departure for the proto-Munda is the Tanintharyi region of today’s far southern Burma, adjacent to southern Thailand. I propose that the Tanintharyi region served as a cultural and demographic valve, initially mediating overseas expansion by a group of Southeast Asian rice farmers, who eventually established connections across the Bay of Bengal between South and Southeast Asia.
The absorption of lowland Munda domains in Odish by Indo-Aryan speakers did not entail the disruption of the flow of goods and people in the preestablished trade network. Rather, these routes which were preexistent were co-opted by the Kalinga state, and later on by various southern Indian polities facing east on the Bay of Bengal. Inspection of the Y and mtDNA haplogroup profile suggests these were elite Indian males, with few females. This is very different from a folk expansion through Arakan, which would involve both and women.