precursor

Concise Oxford English Dictionary © 2008 Oxford University Press:
precursor/priːˈkɜːsə(r)/

▶noun

• 1 a person or thing that comes before another of the same kind.

• 2 a substance from which another is formed, especially by metabolic reaction.

– derivatives
precursory adjective.

– origin ME: from L. praecursor, from praecurs- ‘preceded’, from praecurrere, from prae ‘beforehand’ + currere ‘to run’.