Here, I will propose that Austroasiatic started out as a creole during the Middle Neolithic in Lingnan (Guangxi and western Guangdong). Archaeological evidence is also used in my argument.
Austroasiatic is well known for its creoloid-type syntax typical of the MSEA linguistic area, with isolating features and SVO word order. I believe this is because Austroasiatic, just like Kra-Dai and Hmong-Mien later on in the Bronze Age, was a creole mixing an autochthonous layer with early Sino-Tibetan, pre-Austronesian, and likely also Old Middle Yangtze, although the evidence would be difficult to sort out due to the back-migration of a Northern Austroasiatic branch back into the original Old Middle Yangtze homeland in Hunan when it started mixing with it to form Hmong-Mien during the Bronze Age. The creole formation of Proto-Austroasiatic would have occurred around 4,000-6,000 BP, before the migration of Austroasiatic into Indochina but also after Neolithic farmers had adopted major crops and started to expand demographically.