In hindsight, this makes Jonathan's sacrifice in that final fight with Dio all the more impressive, because if he hadn't convinced Erina to escape with the baby, the Earth and Humanity would be fucked right now.
I don't get it. Wasn't she Arina and Jonathan's daughter?
Nope. Back when Dio's head got snuck aboard the ship, his zombie slaves killed all the passengers leaving an orphaned baby. Jonathan convinced Erina to escape with that baby, but she was also pregnant with Joseph's father at the time.