Sentence:  The sequence of events leading to his important discovery still remains ambiguous but it seems that one of the advanced students at the university related that the <WSD>first</WSD> direct event that led to the publication of Oersted 's discovery occurred during a private lecture made before a group of other advanced students in the spring of 1820 . 
Meaning of the word marked with <WSD> tags:  preceding all others in time or space or degree