its probably cause you don't have the national dex in emerald.
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You must first get it from Emerald version and then trade it over. But if you have just Sapphire, you can't get it. you can"t get him but you can get him on gts You can't. You need to get it in another game pak. Fire Red for instance. Capture the Mew in Fire Red or Leaf Green and trade it to sapphire.
you don't. You can't find either Mew or Mewtwo in Emerald; you have to trade from FireRed or LeafGreen to get Mewtwo. You can't get Mew without an event or cheating.
It is impossible to obtain a Mew in Pokemon Emerald outside of Nintendo events without cheating or trading from another game.
You cannot get Mew legit in Pokemon FireRed, for the event for Mew has ended. You would need to use a GameShark or trade for it from Pokemon Emerald.
Mew doesn't evolve at all.