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)