A precursor is something that came before, and that helped the later thing become what it is. An old steam engine can be a precursor of the modern train; a mild tremor can be a precursor of a bigger earthquake. In science, a precursor is a substance the body or a factory uses to make another substance. The word carries a feeling of “this opened the door.” Though actually, it is not just “the previous one.” A predecessor is often the person who had your job before you. A precursor is more about influence and development, and it is used as much for things, ideas, and chemicals as for people. It is a formal, slightly academic word. In everyday talk, people say “that came first” or “that led to…”