All machines, if they are around long enough, undergo some kind of evolution. Today’s Ford Mustang is not the same car it was decades ago. So it goes with the AR-style rifle. The AR-15 that Gene Stoner introduced in the 1960s has undergone a serious evolution. I believe most shooters would agree that the current crop of “black rifles” is more reliable and robust than those shipped to Southeast Asia all those years ago.
Naturally, all these changes did not occur overnight. Enterprising engineers and gunsmiths have considered the AR and offered improvements in the controls, the internal components, and the outward appearance. Of course, not every “improvement” or modification has stuck, and various manufacturers have components that are unique to their own brand.