The Planting

 was an organic virus.

It arrived on planets in a sapling form, and started to take roots. Then, it started to grow and spread, until the whole planet was covered by it. Thus, all native life was replaced by it. After the planet was covered by it, the Planting formed a new sapling and went on to another planet.

Once, the Scream took one sapling from a planet which had already been taken over by the Planting, and transferred the Eleventh Doctor's and Alice Obiefune's memories into it, to form a "new creation". (COMIC: The Scream)