Evolution Source

Evolution Source is a term used for the creatures that are stacked underneath an evolution creature or cross gear when they enter the battle zone.

Details
The conditions of evolving are defined by the text of the evolution card. Most refer to a race or Civilization as the common source.

The evolution source can also refer a place known as "under evolution creature". It is however not treated as an actual game zone.

When an evolution creature is put into the battle zone, an evolution source is also required. For Vortex evolution, 2 are required. For Galaxy Vortex evolution, 3 are required (except Super Terradragon Variant Vades). For an Evolution Cross Gear, the single cross gear is needed.

Rulings

 * While under an evolution creature, any abilities or characteristics of the source are ignored.
 * When the evolution source is put into another zone along with an evolution creature an ability that triggers as "When ~ leaves the battle zone" doesn't trigger. If the evolution creature is destroyed, the evolution creature isn't also treated as being destroyed.
 * If only the top creature of the card is removed and the evolution source remains, they become separate creates. As they are treated as being originally in the battle zone, Summoning Sickness doesn't occur and Come Into Play abilities don't trigger. However, the tap state is inherited. This is important to know for cards such as Soul Phoenix, Avatar of Unity and Royal Durian.
 * Evolution Creatures will be put into the battle zone tapped if the evolution source was tapped.
 * By the ruling change of January 17th, 2014 "when you choose a card in the battle zone", the "evolution source" was also changed to be counted as a card. Therefore you can choose to remove the bottom card underneath.
 * Evolving from creatures that had "Summoning sickness", due to a ruling on February 21, 2014, when the top card of an evolution creature leaves the battle zone during that turn, the creature will still have summoning sickness. Conversly, if you evolve from a creature that wasnt summoned that turn and the top card of the evolution creature was removed, the summoning sickness is ignored.
 * According to the announcement on the official website, only the 2 cases were listed. But according to other official responses, evolution creatures with abilities such as Graveyard evolution and the top card is removed during the same turn, the evolution source has summoning sickness.
 * These ruling changes are seen as an adjustment that large finisher creatures could be summoned into the battle zone by turn 4 with graveyard evolution and card removal such as Senju Splash.
 * If you bounce an evolution creature with Persistent Prison of Gaia, the evolution creature returns to your hand, as well as any evolution source. If that same evolution creature that was on the top wasn't the card randomly discarded, then the shield burn ability isn't triggered.
 * Psychic creatures are also able to evolve and stay in the battle zone while under an evolution creature. If they would leave the battle zone by removal or a Meteorburn ability, they are returned to they are returned to the hyperspatial zone.
 * When you evolve over a linked God, separate them into separate cards under the evolution creatures. If the top card is removed by a card such as Royal Durian, if they were originally God Linked, they can be left in the battle zone without link.
 * When an Psychic Super Creature is evolved over, the are put under the creature as a psychic cell. As a psychic cell isn't treated as a creature, they can be include by effects that refer to "underlying card", but not the "underlying creature".
 * When counting the number of cards included in a creature, it is the total sum of the top card and evolution sources underneath.
 * There are rare cases where the evolution source isn't a card, such as Pack evolution and Ore evolution.