Many people think that reward training is the modern method of dog training.
It's my belief that reward training predates leash training. It is reasonable to believe that reward training was fairly well developed by early humans. In my library I have a book from the 1930s that describes many modern reward training principles. This is before much of the scientific research about operant conditioning was published. When considering the historic significance of reward styles of training, it's important to remember that the scientific study and application of these systems is only about half as old as leash training.