An ancestor was any lifeform from whom another was descendant. (TV: The Daleks, The Web Planet et al.) As such, ancestors were parents, followed by grandparents.

Traveling back to Earth in the year 2008, the Toclafane slaughtered their human ancestors. However, they were protected by the temporal reprecussions of this act by the Paradox machine created by the Master, beginning the Year That Never Was. (TV: The Sound of Drums/Last of the Time Lords)