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. (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)
After the universe was damaged with chronon decay and temporal paradoxes caused by the Time Lords' time travelling before and during the War in Heaven, the remaining ancestor cells used energy leaked from the bottle universe to grow and develop. Greyjan the Sane feared that they had evolved into the Enemy. (PROSE: The Ancestor Cell)