ancestor vs posterity

ancestor

noun
  • One from whom a person is descended, whether on the father's or mother's side, at any distance of time; a progenitor; a forefather. 

  • One who had the same role or function in former times. 

  • One from whom an estate has descended;—the correlative of heir. 

  • An earlier type; a progenitor 

  • A word or phrase which serves as the origin of a term in another language. 

verb
  • To be an ancestor of. 

posterity

noun
  • All the future generations, especially the descendants of a specific person. 

How often have the words ancestor and posterity occurred in a corpus of books? (source: Google Ngram Viewer )