The tonsure itself is not an ordination properly so called, nor a true order. It is rather a simple ascription of a person to the Divine service in such things as are common to all clerics. Historically the tonsure was not in use in the primitive Church during the age of persecution. Even later, St. Jerome (in Ezech., xliv) disapproves of clerics shaving their heads. Indeed, among the Greeks and Romans such a custom was a badge of slavery. On this very account, the shaving of the head was adopted by the monks. Towards the end of the fifth, or beginning of the sixth, century, the custom passed over to the secular clergy. (New Advent Article)
Priests playing Handball (notice tonsures).