I believe that extrateresstrial life originated on earth because all life that God created that we know of was created on Earth, which is also stated in the Bible. If God created life that existed on other planets from the beginning, he'd have told us, because God does not keep secrets from humans. Also, God created everything that has existed, exists, or will exist, so they cannot be created through means not involving God, though it may seem as if they are.

But there is also the possibility that God created aliens after the events that have already happened in the bible took place, so my theory does have its faults. Also, most others on this thread base their theories on science rather than religion, and that may be why mine is somewhat unrealistic.

Or perhaps aliens do not exist. That is also a possibilty.