When is it an actual human being? The view of abortion as homicide is actually somewhat new. The initial reason for the Church's opposition to abortion was to cover up a sexual sin. Thomas Aquinas didn't believe that the abortion was homicide unless ensoulment had taken place. So the basic question is "When do the cells become a human [or ensouled]?"
Because it has a heartbeat. An individual cell e.g a sperm doesn't have a heartbeat. Yes, it is living, but it isn't a human being, it only has the potential to be one.