수능특강 3강 8번 지문에서 having이 쓰인 이유가 궁금해요

수능특강 3강 8번 지문 문장입니다.
“Instead, it is the result of the chemical having been either transported into the lake from elsewhere or produced by a chemical or biological reaction from other compounds which have already existed in the lake.”
여기서 왜 to have가 아니라 having이 쓰였나요?