Proto-Austroasiatic is the reconstructed ancestor of the Austroasiatic languages. Proto-Mon–Khmer (i.e., all Austroasiatic branches except for Munda) has been reconstructed in Harry L. Shorto's Mon–Khmer Comparative Dictionary, while a new Proto-Austroasiatic reconstruction is currently being undertaken by Paul Sidwell.
Scholars generally date the ancestral language to with a homeland in southern China or the Mekong River valley. Sidwell (2022) proposes that the locus of Proto-Austroasiatic was in the Red River Delta area around .
Phonology
Shorto (2006) The Proto-Mon–Khmer language is the reconstructed ancestor of the Mon–Khmer languages, a purported primary branch of the Austroasiatic language family. However, Mon–Khmer as a taxon has been abandoned in recent classifications, making Proto-Mon–Khmer synonymous with Proto-Austroasiatic; the Munda languages, which are not well documented, and have been restructured through external language contact, have not been included in the reconstructions.
Proto-Mon–Khmer as reconstructed by Harry L. Shorto (2006) has a total of 21 consonants, 7 distinct vowels, which can be lengthened and glottalized, and 3 diphthongs.
Proto-Mon–Khmer is rich in vowels. The vowels are: *a, *aa *e, *ee *ə, *əə *i, *-iʔ, *ii, *-iiʔ *o, *oo *ɔ, *ɔɔ *u, *uu, *-uuʔ
The diphthongs are: *iə, *uə, *ai
Sidwell & Rau (2015) Paul Sidwell and Felix Rau (2015) propose the following syllable structure for Proto-Austroasiatic. *Ci(Cm)VCf
Also possible are more complex forms with prefixes and infixes, as well as presyllable "coda-copying" from main syllables. *(Cp(n/r/l))CiVCf
Sidwell & Rau (2015) reconstruct 21-22 Proto-Austroasiatic consonants (the reconstruction of *ʄ is uncertain).
All of the Proto-Austroasiatic consonants except for implosives and voiced stops can occur as syllable finals (Cf).
All of the Proto-Austroasiatic unvoiced stops and voiced stops, as well as *m-, *N-, *r-, *l-, and *s-, can occur as presyllables or sesquisyllables (Cp).
Medial consonants (Cm) are *-w -, *-r -, *-l -, *-j -, and *-h-.
Sidwell & Rau (2015) reconstructs 8 Proto-Austroasiatic vowels, for which there is vowel length contrast. A long vowel will be appended with triangular colon (ː) instead of doubling.
Proto-Austroasiatic diphthongs are *iə and *uə, and possibly *ie and *uo.
Morphology
Common structures include *CV(C) and *CCV(C) roots. *CVC roots can also be affixed either via prefixes or infixes, as in *C-CVC or *CVC (Shorto 2006). Sidwell (2008) gives the following phonological shapes for two types of stems.
Monosyllabic - C(R)V(V)C
Sesquisyllabic - CCV(V)C
Note: R is one of the optional medial consonants /r, l, j, w, h/.
Sidwell (2008) considers the two most morphologically conservative Mon–Khmer branches to be Khmuic and Aslian. On the other hand, Vietnamese morphology is far more similar to that of Chinese and the Tai languages and has lost many morphological features found in Proto-Mon–Khmer.
The following Proto-Mon–Khmer affixes, which are still tentative, have been reconstructed by Paul Sidwell (Sidwell 2008:257-263).
Nominalizing *-n- (instrumental in Kammu, resultative in Khmu) Nominalizing agentive *-m- Nominalizing iterative (expressive of repetitiveness/numerousness) *-l-/*-r- Nominalizing instrumental *-p- Causative *p(V)- (allomorphs: p-, pn-, -m-) Reciprocal *tr-/*t(N)- Stative *h-/*hN- (?)
Roger Blench (2012) notes that Austroasiatic and Sino-Tibetan share many similarities regarding word structure, particularly nominal affixes (otherwise known as sesquisyllables or minor syllable prefixes). Blench (2012) does not make any definitive conclusions about how these similarities could have arisen, but suggests that this typological diffusion might have come about as a result of intensive contact in an area between northern Vietnam, Laos, and northeast Myanmar.
Syntax
Like the Tai languages, Proto-Mon–Khmer has an SVO, or verb-medial, order. Proto-Mon–Khmer also makes use of noun classifiers and serial verb constructions (Shorto 2006).
However, Paul Sidwell (2018) suggests that Proto-Austroasiatic may have in fact been verb-initial, with SVO order occurring in Indochina due to convergence in the Mainland Southeast Asia linguistic area. Various modern-day Austroasiatic languages display verb-initial word order, including Pnar and Wa (Jenny 2015). Nicobarese also displays verb-initial word order.
Lexicon
Pronouns
Proto-Austroasiatic personal pronouns as follows, with reconstructions from Sidwell & Rau (2015) and Shorto (2006).