precursor
noun [ C ] formal uk/ˌpriːˈkɜː.sər/ us/ˌpriːˈkɝː.sɚ/
先驱，先锋;先兆，前兆
something that happened or existed before another thing, especially if it either developed into it or had an influence on it
Sulphur dioxide is the main precursor of acid rain. 二氧化硫是形成酸雨最重要的先决条件。
Biological research has often been a precursor to medical breakthroughs. 生物学研究往往是医学突破的先驱。