Certain subbranches of Trans-Himalayan (Sino-Tibeto-Burman) stand out as islands of complexity in a Eurasian sea of simplicity Others show a radically simpler verbal system more consistent with their South and Southeast Asian neighbors. The complex systems include elaborate systems of argument indexation; most of these reflect a hierarchical indexation paradigm, which can be traced to Proto-Trans-Himalayan. This morphology has been lost in many languages, including the most familiar branches of the family such as Sinitic, Boro-Garo, Tibetic, and Lolo-Burmese, as a result of creolization under intense language contact. The archaic system is preserved fairly intact in rGyalrongic and Kiranti and with various structural reorganization in several other branches. The Kuki-Chin branch has innovated an entirely new indexation paradigm, which in some subbranches has completely replaced the original system, while in others the two paradigms coexist.
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Scott DeLancey (2015) studied this question.