Why are there Sibyls and Prophetesses in the Sistine Chapel?

For what purpose did Michelangelo add each Sibyls or Prophetesses to the ceiling of the Sistine chapel? This is an explanation to why they're there in the first place. > The latter, although belonging to the pagan world, are represented here because of their prophetic gifts, in this way extending the wait for Redemption from the chosen people to all mankind. I'd imagine they each have some sort of story - or at least represent some different part of the pre-redemption world and pointed in some vague way to Christ. But what is the vague way? Here are the Sibyls: - Erythrean Sibyl - Persian Sibyl - [Libyan Sibyl](http://www.museivaticani.va/content/museivaticani/en/collezioni/musei/cappella-sistina/volta/sibille-e-profeti/sibilla-libica.html) - Cumaean Sibyl - [Delphic Sibyl](http://www.museivaticani.va/content/museivaticani/en/collezioni/musei/cappella-sistina/volta/sibille-e-profeti/sibilla-delfica.html) Source