dubstep doesnt have much to do w/ dub reggae, at least not inherently. the 'dub' most likely is a reference to the dub side of vocal records, as the genre evolved out of dark instrumental 2step garage like el-b, wookie, oris jay, etc. really, the point at which it becomes dubstep is indistinguishable assuming you don't require dubstep be half time (since the term was in use before everything became half time anyway). my personal theory is it probably evolved as a way of distinguishing club nights that focused exclusively on that dark instrumental kind of 2step from those that might incorporate darker stuff into sets composed largely of more commercial material. the first dubstep allstars mix cd for instance is pretty much a dark garage set.
that said, horsepower productions were definitely doing the reggae thing early, eg:
...but that's just a continuation of a thread that runs through jungle and garage, not really an innovation.