There is evidence in one or two of the cards that the Fallen were once like the Guardians, touched and raised up by the Traveler. The Fallen lost their homeworld to the Darkness and it makes me wonder if we are merely another in a long line. Perhaps the reason the Darkness came to their world was because of the Traveler, like it did to ours, and when it did it nearly destroyed them completely. As their world burned, it fled to find another host species to use as a proxy army to defend itself. As it ran, it made victims of all it passed, perhaps not intentionally, but certainly without any visible remorse. The Cabal flee the Darkness as well, so one may infer that the Traveler at least passed near their system and brought a storm in its wake. I'm not sure what this means in the long run, but the pieces fit. There's also a good possibility that the Hive were once a human-like species (note the similarities in their soldiers' garb to flight suits, optic helmets and reinforced armor; perhaps this calls back to what they once were) and that the Vex and the Traveler may be related somehow, though whether they share a creator or one made the other is uncertain.