Razib Khan notes a key observation of Peter Bellwood's new book "First Migrants" in which Bellwood notes based upon unpublished materials that ancient DNA and human remains from a Northern Vietnamese frontier of the rice farming phase of the East Asian Neolithic around 4,000 years ago (i.e. 2000 BCE) show the presence of a minority population with “Austro-Melanesian” physical attributes and genetics.
Modern Southeast Asian Negritos are not a good fit for this profile. This hints at an entire layer of SE Asian population history that would otherwise have been completely missed. I'm not aware of any prior study observing these kinds of populations outside Australia, Papua New Guinea and islands connected to them by Austronesian sea faring.