Grambo
To earn money for flying aeroplanes, you have to - by law - hold a commercial pilot's licence. A basic commercial pilot's licence (CPL) entitles you to fly as pilot in command (PIC) of small aircraft. An Airline Transport Pilot's Licence (ATPL) entitles you to fly in command of large aircraft.
It doesn't matter what your aircraft carries, human passengers, animals or boxes, the licencing requirements for the pilots are the same. A pilot flying cargo today is licenced to fly passengers tomorrow with no extra training or qualifications