Paul Sidwell (2018) considers the Austroasiatic language family to have rapidly diversified around 4,000 years B.P. during the arrival of rice agriculture in Indochina, but notes that the origin of Proto-Austroasiatic itself is older than that date.
pˈɔːl sˈɪdwɛl tˈuː θˈaʊzənd ˈeɪtiːn kənsˈɪdɚz ðɪ ˌɔːstɹoʊsɪˈæɾɪk lˈæŋɡwɪdʒ fˈæmɪli tə hæv ɹˈæpɪdli daɪvˈɜːsᵻfˌaɪd ɚɹˈaʊnd fˈoːɹ,zˈiəɹoʊzˈiəɹoʊ zˈiəɹoʊ jˈɪɹz bˈiː.pˈiː. dˈʊɹɹɪŋ ðɪ ɚɹˈaɪvəl ʌv ɹˈaɪs ˈæɡɹɪkˌʌltʃɚɹ ɪn ˌɪndoʊtʃˈaɪnə, bˌʌt nˈoʊts ðætðɪ ˈɔːɹɪdʒˌɪn ʌv pɹˈoʊɾoʊˌɔːstɹoʊsɪˈæɾɪk ɪtsˈɛlf ɪz ˈoʊldɚ ðɐn ðæt dˈeɪt.