Joseph Fielding Smith Jr. (10th Mormon prophet) wrote:

I tell you, life did not commence upon this earth spontaneously. Its origin was not here. Life existed long before our solar system was called into being. The fact is, there never was a time when man -- made in the image of God, male and female -- did not exist. The Lord revealed to Joseph Smith the truth that man was also in the beginning with God....

The Lord has given us the information regarding his creations, and how he has made many earths, for there never was a beginning, never was a time when man did not exist somewhere in the universe, and when the time came for this earth to be peopled, the Lord, our God, transplanted upon it from some other earth, the life which is found here. man he created in his own image. If it were our privilege to go out and visit some of the other creations, other worlds in space, we should discover that they are peopled with beings who look like us, for they, too, are the offspring of God, and of the same race from whence we came. Perhaps they would be more exalted, but, nevertheless, they would be in the image of God, and so are we.

Source: McConkie, Bruce R., ed. Doctrines of Salvation: Sermons and Writings of Joseph Fielding Smith, Vol. 2. 139 - 140. Salt Lake City: Bookcraft, 1954-1955.