Everybody starts off as a simple life form--single cell, algae, or whatever. Then they come back is something more complex--grass, weed, etc. Then they move up to something more complex--insect, aracnid, etc. Then they move up to animal. All of this happens automatically. When they finally reach the human stage, then their reincarnation is based on karma. Their karma can cause them to move back down the scale.
