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Ancestor cells were living things from which all life in the universe descended. This didn't include the more exotic lifeforms which came into being soon after the creation of the universe and died out before developing sentience.

The ancestor cells were exposed to energy leaked from the bottle universe, allowing them to grow and develop in a universe damaged with chronon decay and temporal paradoxes created by the Time Lords' time travelling. Greyjan the Sane feared that they had evolved into the Enemy. (PROSE: The Ancestor Cell)

In the pre-universe, (PROSE: Christmas on a Rational Planet) the Great Houses were very concerned that other species might develop and impose understandings and histories onto the continuum that were inhospitable to the Houses' civilisation. This lead them to develop the earliest time technology and eventually pursue the anchoring of the thread. (PROSE: The Book of the War)