Why cannot linguists precisely date the divergence of Indo-European language branches from linguistic evidence alone?

Linguists cannot precisely date the divergence of Indo-European language branches using linguistic evidence alone because there is no reliable method to determine the exact "time depth" of language families. While computational approaches have been proposed, they have not been proven to yield consistent or reliable results.1

Several factors contribute to this difficulty:

• Limitations of the Comparative Method: Traditional models, such as the "family tree," often rely on a misleading genealogical metaphor that assumes languages diverge through sudden, sharp cleavages and never converge again. In reality, languages frequently undergo both divergence and convergence due to ongoing contact between speech communities.2
• Flaws in Glottochronology: This method, which attempts to calculate time depth based on the rate of vocabulary replacement, is rejected by many linguists. Critics argue that it is impossible to compile a culturally unbiased basic vocabulary list and that the rate of linguistic change is neither constant for a single language nor uniform across all languages.3
• Lack of Direct Records: As Proto-Indo-European was spoken by populations without a writing system, its reconstruction is hypothetical, making it impossible to observe the divergence process directly.4

Consequently, linguists must rely on "dead reckoning"—comparing estimated degrees of difference against language families with known divergence dates—and correlate these findings with archaeological evidence to achieve a more accurate historical picture.1