First, some numbers: colloquially, most hip-hop runs about 90 beats per minute, or BPM. Most techno runs at about 120, and most jungle, around 170. These are rough averages, and real tempos may vary by about 10 BPM in either direction.
Now, consider the Technics SL 1200 MK2 turntable, the most popular piece of DJ hardware on the planet. Over 3 million units of the 1200 series have been sold since their introduction in 1972. The SL 1200 MK2 can play records at 33 1/3 or 45 RPM (revolutions per minute, not quite the same as BPM), and the pitch control allows these speeds to be modified by as much as +/- 8% for beatmatching.
Historically, hip hop came first, call it a stable form by about 1985, techno had stabilized by the late 80s/early 90s, and jungle was largely formed by around 1995 or 1996. So during that decade, we've got underground dance music, largely played and produced by DJs on Technics 1200 turntables, speeding up. And it's not speeding up gradually, there are these large jumps in tempo, with gaps in between that define the difference between one genre and the next.
What means criticality? Criticality is used here to mean the constant change in systems of expression: speech, art, architecture, fashion, music. Changes in underground dance music during the eighties and nineties were largely determined by the hardware that was used to play and produce it.
DJs are omnivores. If a tune's got a good beat and you can dance to it, a DJ'll play it. If a record sounds good with the record played before it, then it'll get played next. That's why the major genres: hip hop, techno/house, and jungle, are largely separated by tempo and little else. Hip hop became techno when enough records were released that could only be played with each other. They were too fast to get beatmatched with everything else - Critical Mass.
Picture those pitch sliders: phono 1 is a hip hop record pitched all the way up to +8%, we've got Rakim coming out the speaker sounding like Alvin the Chipmunk, phono 2 is pitched down to -8%, the bassline sounds like a flat tire, and the records still aren't matched - WTF? Speciation: if two entities can't be joined to produce viable offspring - that is, a mix, a blend, a beatmatch - then they must be different species, different genres, different things.
So the new songs that are coming out can't be blended smoothly with the older songs, the tempos have risen, and they're starting to accrete around a new steady state, a new invisible point in phase space: an attractor. Beats in the new tracks are too fast to go with the older tracks that hover around 90 BPM, the newer tracks are headed towards a new tempo to stabilize around: 120 BPM.
At this point the records we're talking about are long playing singles: one track per 12 inch side. A lot of these tracks are at 33 1/3, in order to sqeeze the most music onto the grooves. DJs play a lot of records, and it's easy for a DJ or performer to get tired of a song before their audience does. This is one reason that DJs are natural experimenters, if you're bored with a song, it's fun to fuck around with it: play two copies at once, bounce them back and forth, flip it, scratch it, play it at the wrong speed. A 90 BPM track that's meant to be played at 33 1/3 becomes, when played at 45, a track with a tempo of 121.6 BPM: a jump to a new genre. The same thing happens when a techno track is played at the wrong speed: 120 BPM becomes 162, techno becomes jungle. Now layer the beginning back in: hip hop records, at 90 BPM, can be pitched down low enough to match the faster end of the jungle spectrum, since 85 is half of 170, and it's all in 4/4 time, slow hip hop goes perfectly with fast jungle.
The +/- 8% on the pitch control binds genre music together locally, but the 33 / 45 switch allows larger scale jumps, into new undiscovered localities, which are then themselves bounded at the top and bottom by the limits of the pitch control.
The person who plays records all the time is like William Gibson's "... bored researcher who kept one thumb permanently on the fast-forward button," hyper-accelerating cultural change just to keep themselves interested. And in the process of avoiding that disinterest, they'll find out everything that their equipment can do, and this will open up new territories and new means of expression. The technical limitations impose boundaries on the space of possibility, and the desire to avoid cliche and repetition ensures that this space will be fully explored: limits times boredom equals innovation.