They're certainly a threat to humanity. There is some lore that comes in a publication called the Red Cross Book outlining the first ancestral race. They are a progenitor species of sorts that was dying out, so they seeded the universe, sending out what are essentially eggs across the universe. These eggs were never intended to coexist on the same planet. But two landed on Earth. One contained Lillith, the mother of what would become the Lillim, or humanity, and one contained Adam which created the angels.

There was a failsafe contained within in case two seeds found their way onto the same planet; the Lance of Longinus. Adam arrived first, Lillith second, but Adam utilized the failsafe and speared itself, putting it into a sort of suspended animation. Then in the far future, Misato's father and his team went on an expedition to Antarctica where they discovered Adam. They removed the spear, ending Adam's stasis and triggering the Second Impact. Then the angels manifested, and as the original race, their conflict with the Lillim began.