Tonogenesis

Tonogenesis is the appearance of contrasting tone in a previously non-tonal language, generally as a result of regular phonological changes.

(copy from Tone (linguistics))

Generally tone in a language is an areal, not a genetic, feature: that is, a language tends to, but does not always automatically, acquire tones if many neighboring languages also are tonal. For example it is accepted that tones in the East-Asian language area spread from the Chinese family (Sinitic) or from Tai-Kadai, more probably from Sinitic.

An interesting question is how tones arise in a language, i.e. tonogenesis. In the Chinese languages it is known that they arose as a reinterpretation of initial or final consonant clusters as a pitch inflection of the vocalic nucleus of syllables. It is also known, in all languages, that surrounding consonants influence the pitch of the adjacent vowel. The same thing happened for Vietnamese, probably under the influence of Chinese; note that Khmer, which is genetically related to Vietnamese, is not a tonal language. Another example is Korean and Japanese. Although they were under heavy influence from Chinese cultures and a vast amount of Chinese loan words exist, both are not tonal languages. This mechanism seems to account for the appearance of contour tones.

In the Algonquian language Cheyenne, tone arose via vowel contraction; the long vowels of Proto-Algonquian contracted into high-pitched vowels in Cheyenne, while the short vowels became low-pitched.