How did tones develop in many Tibeto-Burman languages?

In many Tibeto-Burman languages, the development of phonemic tone is primarily attributed to the loss of specific phonetic contrasts in the syllable structure.12

As established by linguist André-Georges Haudricourt, tonal systems evolved to compensate for two main factors:
* Loss of voicing contrast: The disappearance of the distinction between voiced and voiceless initial consonants (e.g., when p- and b- ceased to be differentiated) often triggered tonal development.12
* Loss of syllable-final laryngeal consonants: The loss of final consonants such as -h or the glottal stop -ʔ frequently resulted in the emergence of tones.13

These conditioning factors, which were often linked to morphological processes like affixation and consonant alternation, caused the pitch or voice quality of the syllable to change. Consequently, tone is often deeply connected to phonation types—such as clear, breathy, or creaky voice—determined by the configuration of the glottis. While these conditions encouraged the development of tones, linguists generally agree that a single ancestral tonal system cannot be reconstructed for Proto-Tibeto-Burman; rather, tones have likely emerged and fluctuated cyclically across different subgroups due to universal phonetic tendencies in monosyllabic languages.231