How did Vedic Sanskrit become the oldest major record of an Indo-Aryan language?

Vedic Sanskrit is considered the oldest major record of an Indo-Aryan language primarily because it is preserved in the Rigveda, which dates to approximately 1500 BCE and serves as the earliest literary document of South Asia.12

This status as the oldest record is supported by several factors:

• Oral Preservation: The entire corpus of Vedic literature, including the Samhitas, Brahmanas, Aranyakas, and Upanishads, was maintained through a rigorous oral tradition known as Shruti ("What Is Heard"). This allowed the language to be handed down with precise intonation and rhythm from the early days of Vedic religion.3
• Linguistic Archaism: The Sanskrit found in the Vedas represents the most archaic stage of Old Indo-Aryan. Philological analysis shows that early Vedic dialects exhibit specific phonological features, such as the replacement of the Indo-European l sound with r, which aligns with other ancient Indo-Iranian languages like Avestan.45
• Historical Context: While the precise timing of Indo-Aryan migration into the Indian subcontinent remains a subject of study, the geographical references within the Rigveda confirm that the earliest Indo-Aryan settlements were established in the northwest of the subcontinent by the 2nd millennium BCE.1