The Humble Beginnings
The earliest ancestors of the modern American Pit Bull Terrier (APBT, or more casually, "Pit Bull") can be traced back to the British Isles, circa early 1800s. These were not a pure breed of dog, rather, mixes of bulldogs and terriers, commonly known as "Bull and Terriers". The tenacity and drive of terriers, combined with the athletic build and strength of the bulldog, created the ultimate champion in the world of "bull baiting". Bull baiting was an ancient bloodsport, dating back to the Romans, where a bull was tethered down and allowed to be attacked by dogs - the ultimate goal was the immobilization of the bull. It is also said that bull baiting had a practical value as well; butchers commonly used Bull and Terriers to subdue the bulls before slaughter. And when they weren't being used to take down bulls, Bull and Terriers were used in "rat pits" as well. These were popular "sports" amongst all walks of life - a form of "entertainment" for the upper- and middle-class alike.
It is important to note that these dogs were bred for performance and working ability, rather than towards a physical standard.
When bull-baiting became illegal in 1835, the focus began pitting the dogs against each other. Despite the bloody nature of the sport, the commoners inadvertently began breeding in the traits that Pit Bull owners appreciate today. Though tenacious and determined in battle, the dogs developed an undying love and devotion towards humans, along with a very high bite inhibition. This was done through selective breeding and culling practices. Handlers had to be sure that while in the midst of fighting another dog, that they could be able to enter a pit and pull the dogs apart if need be, without being bitten themselves. If a dog redirected on a handler during a match, that dog was usually culled.