|Black, Michael L., et al. “Genetic ancestries in northwest Cambodia.”|
This Austronesian group was once dominant in what was termed Annam by the French, the central regions of coastal Vietnam between the deltas flanking the northern and south (dominated by the Vietnamese and Khmer respectively). The Cham were a seafaring population and had extensive contacts with maritime Southeast Asian and the network of Austronesian peoples.
As such, the Cham were influenced by the currents of cultural change to their south, and as by the early modern era many had become Muslims. But a minority resident in Vietnam retained their Hindu religious identity, and this reflects a deep current of Indianization which took root among them in the centuries before 1000 AD. The boundary between ancient Champa and Đại Việt was also a civilizational boundary, between the elite culture of India and China.
The commenter states:
As far as I can see, this sample of Chams from Binh Thuan Province, Vietnam does not exhibit any clear South Asian influence in its mtDNA. This contrasts starkly with the significant (18.6% to 32.2%) South Asian influence that is apparent in the Y-DNA of the male subset of the same sample
This seems right. As you can see above I’ve found plenty of evident that R1a1a is found in Southeast Asia where it shouldn’t be. Notice that among northern groups in China R1a1a is pretty frequent too. Obviously from a different source, but the same general pattern. And in that case we have plenty of historical evidence of interaction with Indo-Europeans on the steppe.
I’m not very conversant in mtDNA. This paper argues that the Mon people of Thailand have some mtDNA affinities with India. I created this pivot table for readers to double-check (the “MO” populations are Mon).
The history of Southeast Asia, or perhaps more accurately the quasi-history of Southeast Asia since so many of the records are from China and elsewhere, indicates strong Indian influence in the period before 1000 AD. The standard model is that this is cultural diffusion. And by and large Southeast Asian peoples are are mostly indigenous. But, a non-trivial minority of their ancestry is recent, but pre-colonial, gene flow from the Indian subcontinent. Additionally, the imprint is easier to see in the Y chromosome than the mtDNA. The legends of marriages between Indian Brahmins and native princesses in places like Cambodia probably do reflect something real in the dynamics of the early Indianization.