What term signifies a forerunner or predecessor that indicates something to come?

The term "precursor" specifically denotes something that comes before another event or entity, indicating its presence or likelihood in the future. In various contexts, such as science or literature, a precursor serves as an early example or sign that foreshadows and sets the stage for subsequent developments.

While "harbinger," "indicator," and "sign" can also denote elements that predict or suggest future occurrences, they carry slightly different connotations. For instance, a harbinger often implies a more immediate or dramatic signal of change, while an indicator typically points to data that supports a specific conclusion or trend over time. The term "sign" is broader and can refer to any mark or symbol that conveys meaning but doesn’t specifically emphasize a predecessor relationship as "precursor" does.

In summary, "precursor" is the precise term that emphasizes the idea of a forerunner or something that precedes and suggests what is to come, thus making it the most suitable choice in this context.