My version:
In the beginning (late 1980s) there was acid house. (This is all in the UK BTW.) Acid house was like house (off-shoot of disco) crossed with electro (came out of hip hop) but more druggy. Then came rave, which was like acid house in a field, with more drugs and a more epic sound. Then drugs got speedier, and the music sped up and changed in several different directions. One of those directions was hardcore (which was originally quite different to Rotterdam-style gabber techno). Hardcore maximised use of sampled breakbeats (so stuff from this time or in this style can be called breakbeat hardcore), at the same time introducing elements from rap and particularly from dub (drugged-out reggae, often with MCing or "toasting" over the top). The most important element taken from dub was the massive bass line.
All these elements - breakbeats, basslines, techno and rap samples - were refined until they became an identifiable style on their own - at this point generally known as Jungle, after a kind of reggae identified with Jamaican tough guys known as junglists. At this point, as the technology advanced, breakbeats became much more highly edited and complex (check Goldie's 'Inner city life' for example). This complexity didn't last, but it helped clear the way for a complete break with 'naturalness'. Jungle was now more techno-influenced, and as the breakbeats began to be stepped back, we saw the rise of "two-step", so called because of its simplistic one-two beat. At this point the expression "Drum and Bass" became more popular than Jungle.
Most D'n'B is a variation on two-step, and to be honest I stopped listening at that point, so I can't really say if the style has ossified or continued to develop. I have noticed, strangely, that you can often hear D'n'B in TV advertisements, but it's still considered too weird for mainstream radio play.
