How did retroflex consonants become important in Indo-Aryan languages?

The proliferation of retroflex consonants in Indo-Aryan languages is largely attributed to the influence of Dravidian languages due to their long-standing proximity on the Indian subcontinent.12

While some retroflex sounds developed internally within Indo-Aryan through regular phonetic evolution—such as from inherited Indo-European clusters—the influence of Dravidian is considered a major factor in the extension and proliferation of these sounds beyond their limited original occurrence. This process occurred through several mechanisms:2

• Substrate Influence: As Indo-Aryan speakers interacted with Dravidian speakers, the latter may have transferred features from their native speech into the Indo-Aryan languages they adopted.2
• Borrowing: Indo-Aryan languages borrowed numerous terms from Dravidian that contained retroflex consonants. Because Indo-Aryan had already developed some contrastive retroflex sounds, these borrowed terms could be integrated without shifting the retroflex consonants to dentals.2
• Middle Indo-Aryan Developments: Sound changes within Middle Indo-Aryan vernaculars also generated new retroflex consonants, which were subsequently maintained or adopted into literary forms of the language.2

This development sets Indo-Aryan apart from other Indo-European languages, which generally lack this specific phonological feature.12