This article shows that Yukaghir underwent a regular sound change whereby all word-internal and word-final w phonemes became j, probably in Early Proto-Yukaghir. After degemination had occurred, possibly in Middle Proto-Yukaghir, any j in an intervocalic position of disyllabic roots was followed by an epenthetic l, as it still is in the modern Yukaghir languages. Palatalization, labialization, uvularization, and assimilative effects finally formed the Late Proto-Yukaghir forms from which the modern languages have arisen. Word-class prosody controls epenthesis, vowel lengthening, and any further word-final vowel changes. Identifying these historical processes also strengthens the evidence that Yukaghir is genealogically related to Uralic. The Uralic and Yukaghiric correspondences are carefully analyzed as to phonology and semantics, resulting in over fifty new or revised cognate suggestions. Further, Yukaghiric shows a trend towards a reduction of the number of root syllables in the comparison. The semi-vowel w remained unchanged word-initially in Tundra Yukaghir and has thus been a continuous part of the Yukaghir phonemic register. Lexemes containing the semi-vowel w found in modern Yukaghir in word-internal and final positions arose from other sources only after the semi-vowel alternation sound change rule.