If the ring had been cast into the pit of fire earlier, Sauron would have fallen after the last alliance. He was defeated firstly by the ring being cut from his hand, and then he would have been utterly cast down with it's destruction shortly after if Isildur had cast it into the fire of Mt. Doom. Thus the quest would have never come to pass. Perhaps Bilbo would have been killed in the orc mountain where he found it, although it wouldn't have been by Gollum's hand, but by something else most likely. Remember, Gollum would have never came to be. Middle Earth would have drastically been changed, seeing as how the elven rings would have been affected byt he destruction at that time.
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