How does the Altaic hypothesis connect Mongolian, Turkic, and Manchu-Tungus languages?

The Altaic hypothesis proposes that Mongolian, Turkic, and Manchu-Tungus languages constitute a single language family sharing a common genetic origin.123

Proponents of this hypothesis point to several shared characteristics as evidence of this relationship:

• Agglutinative Morphology: These languages are characterized by an agglutinative structure, where words are formed by adding a sequence of suffixes to a root, with each suffix typically serving a distinct grammatical function.4
• Structural and Lexical Similarities: The groups exhibit noteworthy similarities in their morphological and syntactic structures, as well as commonalities in vocabulary.123
• Phonological Features: Some scholars argue that systematic sound correspondences exist across these languages, supporting the theory of a shared ancestry.1

Despite these observations, the Altaic hypothesis remains a subject of significant controversy among linguists. Many scholars now reject the idea of a genetic relationship, suggesting instead that the similarities observed are the result of prolonged geographic proximity, leading to areal convergence and mutual borrowing. Consequently, while some continue to classify them as a language family, others view "Altaic" merely as a useful category for describing an areal grouping of languages.123