Except for the fact that, as the Oxford English Dictionary notes, there's a 50-year gap between the battle for Port Mahon and mayonnaise's appearance in the textual record. But nature abhors a vacuum (sous-vide aside), and plenty of theorists took advantage of mayonnaise's unclear origins to come up with their own.
Depending on the structure of the program, you may or may not need to complete your bachelor's degree before entering the program...especially if you apply to the doctorate level program. So, yes, one does need to go to grad school to become a PT. It would be worth your while to go to the apta website at www.apta.org and click under education to get a list of accredited PT programs. Then you can visit each school's website to get a better idea of their entrance requirements.
Before you begin formal training in graduate school to become a therapist, you will have to finish a bachelor's degree at a college or university. A major in a field such as psychology or social work can give you the strong background you will need to excel in graduate school. Studying psychology as an undergraduate will give you exposure to the field, and will allow you to fulfill basic prerequisite courses for a graduate school program in psychology.