>This mystery, of the Northeast Asian Sprachbund (language area) has puzzled linguists and historians for decades.
Most likely a bronze age expansion of pastoralists with typological influences on Koreanic/Japonic. Though the agropastoralists that roamed the region were originally quite similar to northern Chinese cultures(millet,pig farming etc.)
Japonic may even have a continental origin in the far past.
academia.edu/7869241/Out_of_Southern_China
>Ainu
Ainu is probably a remnant of the Jomonic that was displaced by Japonic.
>Siberian
Paleo-Siberians were replaced by Tungusics.
>Chinese developed thousands of miles away around northern Burma/southeast China.
There's no linguistic consensus where Sinitic originated,when and how it displaced languages in northern China and its relationship as whole to the Sino-Tibetan(see Sinitic's relationship with Bai and Tujia).
The earliest usage of Sinitic is attested by Shang oracle bones where it served as a liturgical/administrative language of the elites.
>This culture, at the time probably called "Huaxia" by its inhabitants
"Hua" and "Xia" are Zhou era anachronisms,they do not apply to the Shang.
>For most of their history, the Huaxia people only inhabited a small territory.
Huaxia is a misnomer,rooted in Western Zhou concepts of Hua(originally a toponym or an elite ethnonym) and Xia(political descent from the mythical Xia dynasty).
There isn't enough to data to say how widespread Sinitic was prior to Shang/Zhou hegemony as well as the possibility of neighboring polities such as the Rong-Di having Sino-Tibetan origins(Pulleyblank).
>Languages related to Thai, Vietnamese, Laotian, and Hmong were and are spoken throughout the region.
Only Tai-Kadai and Hmong Mien have merit(see Chu/Wu/Yue lexicon). Austroasiatic has been debunked by Sagart while pre-Austronesian is completely speculative.
Save for Min,most Sinitic topolects are derived from Middle Chinese.