The physician assistant model started in the military in Canada. It's been in existence for over 40 years. It was a requirement that came strictly out of necessity. You could never send doctors everywhere. Ships have a small number of people. We couldn't send doctors there, and of course many of the army units are very far forward. You have to be a soldier also. The term “physician assistant”, and for that matter paramedics, came out of the military in the historical sense during the First World War, with stretcher bearers and so on. The physician assistants came out of the military necessity to provide acute care in a setting where there were no physicians and there were no other health care providers available. These were trained by the military using a medical model to be able to have the skills to look after acute requirements to buy time for a patient to then be evacuated back to a facility manned by physicians or to a surgical facility.
That scheme then expanded over time to beyond just simple acute care to doing primary care. Why would we in the military want to have physicians looking after colds and simple sprains and so on when we could spend more time on more complex cases? We simply evolved the physician assistants in the military. Basically you are leveraging physician services so that instead of a physician looking after 100 patients, let's say, with a PA he or she may be able to look after 180 patients with physician oversight.
Then there are nurse practitioners in our clinics who use the nurse practitioner model, but they come from a nursing model, so it's slightly different. They do have a separate origin, but they are both in the same care delivery unit. They provide similar services in different contexts with different backgrounds, but there is enough room in our system for both of them.