A reader pointing me to a paper whose hypothesis is novel to me. But, I have to say that reading the paper, I am now convinced this is highly likely. The paper is The Munda Maritime Hypothesis:
On the basis of historical linguistic and language geographic evidence, the authors advance the novel hypothesis that the Munda languages originated on the east coast of India after their Austroasiatic precursor arrived via a maritime route from Southeast Asia, 3,500 to 4,000 years ago. Based on the linguistic evidence, we argue that pre-Proto-Munda arose in Mainland Southeast Asia after the spread of rice agriculture in the late Neolithic period, sometime after 4,500 years ago. A small Austroasiatic population then brought pre-Proto-Munda by means of a maritime route across the Bay of Bengal to the Mahanadi Delta region – an important hub location for maritime trade in historic and pre-historic times. The interaction with a local South Asian population gave rise to proto-Munda and the Munda branch of Austroasiatic. The Maritime Hypothesis accounts for the linguistic evidence better than other scenarios such as an Indian origin of Austroasiatic or a migration from Southeast Asia through the Brahmaputra basin. The available evidence from archaeology and genetics further supports the hypothesis of a small founder population of Austroasiatic speakers arriving in Odisha from Southeast Asia before the Aryan conquest in the Iron-Age.
For me, the Brahmaputra migration always implied that Bangladeshis should have lots of Munda ancestry. And yet that is not clear from genetics (though a few individuals are shifted in that direction). In contrast, they do have a strong affinity to the Khasi. This paper proposes that the Khasi are quite distinct from the Munda.
Rather, the Munda are placed further south, and their arrival in South Asia was through maritime means. One of the possibilities suggested is a relation to the Aslian subgroup of Austro-Asiatic languages in central Malaysia. This could actually help explain the enrichment for AASI in the Munda: the indigenous Negritos of Malaysia are similar to the people of the Andaman islands!
Remember, the arrival of Austro-Asiatic farmers in northern Vietnam dates to ~4,000 years ago. The Munda could be relative latecomers to South Asia…