priviously thought 인 이유를 알고 싶어요

[..]

This suggest that the origin of the poems may lie much earlier than previously thought.

than 뒤에 previous thought 가 아닌 previously thought 인 이유를 알고싶어 질문드려요

감사합니다^^

[ㅇㅇ]

(What was) previously thought

[...]

This suggests that the origin of the poems may lie much earlier than (it was) previously thought.
"it was"가 생략된 거고 thought는 명사가 아니라 과거분사이기 때문에 형용사가 아닌 부사 previously
가 쓰인 겁니다.

[..]

정말 감사드립니다^^