They are a reletivly new species, crossed between humans and the aratis bug, either by the ancients or vanir. They then are quickly able to grow and essentially eliminate a spacefairing race that has existed for millions of years. They lose most of their battles but with persaveriance they are able to win the war. Plus, they lack some of the greatest technology, shields, intergalactic hyperdrives, high level power generation (their main weakness). They take over the galaxy and pretty much become the reapers. I would say that is largly succesful from humble orgins.
They really do share a lot in common with the reapers, organtic based tech, periods of destruction on a galactic scale, telpathy, massive (on universe scale) ships, huge numbers (More ME3), using the flesh of their enemies, destroing their races when their tech advances too much (less I guess in ME, but that is largly their reasoning)... Any others