One characteristic feature of Yakut is vowel harmony. For example, if the first vowel of a Yakut word is a front vowel, the second and other vowels of the same word are usually the same vowel or another front vowel: кэлин (kelin) "back": э (e) is open unrounded front, и (i) is close unrounded front. Yakut initial s- corresponds to initial h- in Dolgan and played an important operative rule in the development of proto-Yakut, ultimately resulting in initial Ø- < *h- < *s-. Ubrjatova (1985) suggests that the rule was either reintroduced through Evenki contact, or may be a substratal feature of the Yakut language itself. An example is shown in the word meaning "not": Dolgan huoq and Yakut suox. This phonetic observation gave rise to two acknowledgements:[1]
that the original phenomenon in proto-Yakut may have likewise been the result of substratal influence.
that the change of *s > h (debuccalization) is, however, well known and is far from unusual, being characteristic of such languages as Greek and Indo-Iranian in their development from Proto-Indo-European, as well as such Turkic languages as Bashkir, e.g. höt 'milk' < *süt.