The phrase "main precursor" refers to the most important or significant thing that comes before another event or development. It is the primary factor or cause that leads to something else happening. Full definition
The two main precursors for the screenplay category are the Writers Guild award and the Scripter. (awardsdaily.com)
We believe the importance of singing together as a family is the main precursor to actually singing together as a congregation. (premierchristianity.com)
Particularly in PAUSD, where we have such an obvious need for understanding the two main precursors to suicide: anxiety and depression. (paloaltoonline.com)