The phrase "direct precursor" refers to something that came before another thing and directly influenced or led to its existence or development. Full definition
The correlation between binding strength in clusters and the kind of amorphous phase and polymorph finally formed furthermore suggests that the clusters are direct precursor species of ACC, in which the particular structure may be also preformed. (science.sciencemag.org)
One wire screened door glass that's been wiped with a dirty rag gives a gauze to the stairwell beyond and reads like a pretty direct precursor to the later paintings in this context. (artcritical.com)
In 1992, the Ross Perot campaign, a direct precursor to the Reform Party campaigned against the North American Free Trade Agreement (NAFTA). (reformparty.org)