Joseph Fielding Smith, The Progress of Man, p.66

...this earth was prepared for the habitation of the sons and daughters of God, so that they could all pass through mortality and here gain the experience so essential to advancement on to perfection, after the resurrection from the dead. Adam and Eve were placed at the head, and given the responsibility of introducing the mortal life, and making it possible for all the spirits who were assigned to this earth to come in their proper times and seasons, to obtain the experiences here awaiting them.  (Italics added)