Why is the exact origin of Mahayana Buddhism difficult for historians to determine?

The origins of Mahayana Buddhism are difficult for historians to determine because the movement likely emerged gradually across multiple locations rather than as a single, sudden event. The exact date and place of its inception remain unknown.1

Several factors complicate historical reconstruction:

• Lack of Early Records: The earliest available sources are sutras compiled approximately four centuries after the death of the Buddha. These texts attribute their teachings to the Buddha himself, obscuring the actual historical development of the movement.1
• Sectarian Bias: Modern historical understanding has been heavily influenced by the agendas of later sectarian movements. Furthermore, the scriptures most valued by later groups do not necessarily represent the movement during its formative period.1
• Mutual Influence: Mahayana did not simply replace an earlier tradition. It developed alongside non-Mahayana schools, and both sets of literature reflect mutual influences, making it difficult to isolate a clear "pre-Mahayana" starting point.1
• Terminology Issues: Terms like "Hinayana" were often used polemically within Mahayana texts to criticize perceived deviations rather than to describe a specific, unified historical sect.1