The Indian admixture into Southeast Asia is not just a function of distance

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.

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4 thoughts on “The Indian admixture into Southeast Asia is not just a function of distance

  1. Do we have a timeline by geography of rice domestication & cultivation in India

    I find it interesting that IVC was mainly wheat, while Dravidian Tamil is rice based

    At some point a mixture of IVC & Rice cultivation must have happened before Dravidian was formed. And what was the climate during that period? Droughts/floods !?

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  2. @Marees: My theory is that IVC was Dravidian, but as you can see IVC transitioned to rice farming in the more tropical parts which they colonised, for which the Near Eastern crops are not suitable.

    The pressure to move South fast increased with the Aryan invasion. The Indo-Aryans adopted the Dravidian developed rice cultivation pretty fast themselves.

    This allowed the higher culture and Indo-Aryan as well as Dravidian people and culture to move deeper into the subcontinent on a high level. Before that Caucasoids had more of a problem with that ecological border, they didnt do welk it seems. Rice cultivation, adaptation and admixture made the expansion possible.
    The difference between Indo-Aryan and Dravidian is not the main one, but between those two and tropical foragers (= AASI).

    That Dravidian is no longer wheat farming in the North is just because Indo-Aryans came out on top there and while some of their elites decided to integrate, others preferred to stay independent and flee South, found new colonies.

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  3. Always cool to see how the Nagas seem to be polar on a pole seemingly distinguishing Sino-Tibetan/NE Asian from Austroasiatic/Austronesian, which is somewhat different from the Siberian->”NE Asian” vector. Shame they aren’t on Davidski’s PCA, but it always seems like he didn’t want to sign a particular waiver to get access to the samples.

    Of course it’s not just distance but distance+cultural distance. The sort of inland thick dense jungle/hill territory was not really good territory for the S Asian migration stream, whatever it was, compared to integrating in some manner with whatever societies (kingdoms / states?) existed relatively closer to open coastal littoral.

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  4. Read this interesting paper on rice cultivation today:

    We have a case of multiple origins of the various Asian rice subspecies/subpopulations (in China 18k yrs ago and India 12k yrs ago) but only one original de novo domestication (in China).

    Domestication occurred first with O. Sativa japonica (O. rufipogon) in the Yangtze Valley in China, associated with fixation of alleles for non-shattering, color, and other domestication traits.

    Proto-indica may have been cultivated in India without being domesticated for millennia until introduction of domestication alleles from japonica

    Modeling of archeological data suggests that japonica rice moved westward out of China and into South Asia via Central Asia along a precursor of the historical Silk Route (the so-called Hexi corridor) perhaps shortly after ∼4 ka

    Archeobotanically, there is no strong evidence for proto-indica having any domestication traits until the 2nd millennium BC, which is substantially later then the domestication time for japonica

    O. sativa may be considered a complex of between 2–5 genetically distinct and (to greater or lesser degrees) reproductively isolated subspecies, bound together as a single crop species by shared presence of domestication alleles.

    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5400379/

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