What is the origin of the first pope being Saint Peter and the last pope being Saint Peter?

The origin of Peter being thought of as the first Pope isn't Biblical. There may be some pagan influence particularly from the Roman god, Janus, who was the guardian of the gates and the doors to heaven and also used the emblem of the crossed keys which are now associated with the papacy. This pagan influence was then probably linked in with Matthew 16:19.