How did bull fighting start?

The so-called sport was known to have existed among the Pagan Greeks. It is not known if they used professional or amateur athletes, or recruits from Prisoners of War or slaves, etc ( Slaves and Prisoners were used in the Roman Gladiatorial combats). As the Greeks and Romans were frequently at war- it makes sense Greek POW"s were enslaved and used as gladiators- such as Spartacus- who was a Greek, a war-like Spartan as the name suggests.