It is a law of linguistics, that languages change over time, and Nahuatl is no exception. By comparing Nahuatl to its closest relatives, the other Southern Uto-Aztecan languages, this paper shows aspects of the language's early development. The process that created the earliest form of Nahuatl included sinificant changes to Phonology, morphosyntax and vocabulary, and some significant changes in each of these sections are briefly summarized. 1. Phonology – three processes of sound change were particularly significant in creating the phonological profile of Nahuatl, loss of unstressed syllables, leveling of diphthongs, and development of the tl sound. 2. Morphosyntax – several processes transformed the Uto-Aztecan morphosyntactic profile into what the Nahuatl grammar we see today - the introduction of headmarking as the main syntactic principle for marking grammatical relations, the development of verb-initial word order, the introduction of relative nouns instead of locative postpositions, and the grammaticalization of honorification. 3. Vocabulary – Nahuatl vocabulary was restructured with the loss or semantic change of many inherited Uto-Aztecan lexemes and their replacement with new coinages or calques or loans from Mesoamerican languages. The paper shows that Nahuatl conserves many traits that are shared with other Uto-Aztecan languages, but also how other traits are innovations some of which appear to be adopted through diffusion from the languages of Mesoamerica.