Why cannot linguists confidently reconstruct one shared Proto-Tibeto-Burman tone system?

Linguists cannot confidently reconstruct a single shared Proto-Tibeto-Burman (PTB) tone system because the tonal systems across various Tibeto-Burman subgroups do not exhibit regular correspondences. While many Tibeto-Burman languages are tonal, these systems appear to have evolved independently and cyclically in response to universal phonetic tendencies in monosyllabic languages, such as the loss of voicing contrasts in initial consonants or the loss of syllable-final laryngeal consonants.12

Because these tonal developments are tied to unstable diachronic processes—such as the interaction between prefixes and root-initial consonants—any tonal similarities observed across different subgroups are likely the result of convergent evolution rather than descent from a common ancestral system. Even if a phase of tonal uniformity existed in the distant past, it would not have been a stable equilibrium, and it has left no unambiguous traces. Consequently, while researchers can identify the conditioning factors that led to the development of tones, the specific tones themselves cannot be reconstructed for the proto-language.132