ANCESTRAL means ---> relating to, or inherited from ancestors, or serving as a forerunner, prototype or inspiration.
While SAVANNAHS ---> are grasslands ecosystem characterized by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of grasses.
So therefore I conclude that, ANCESTRAL SAVANNAHS means ---> inherited grasslands. :)