Mountain gorillas are mostly herbivorous, and their diet is primarily composed of plants such as roots, leaves, stems, vines, bark, bamboo shoots, and entire small plants. They will also eat fruit if they can, however, mountain gorillas live at extremely high elevations, and very little fruit can grow at these altitudes and so only about 1-2% of their diet is fruits. For their cousins, the eastern lowland gorilla (Gorilla beringei graueri), fruits can make up as much as 25% of their diets.

Chororapithecus abyssinicus is the oldest ancestor of gorilla ever found, and dates back to 10.5Ma, and was found in the Chorora Formation in the Ethiopian Rift. The teeth of Chororapithecus appeared specialized for eating stems and leaves, and resembled those of modern gorillas, which suggests that Chororapithecus probably represents an ancestral branch of the gorilla lineage.

What could have caused Chororapithecus to adapt to eat stems and leaves, is a loss of access to fruits. The mountain gorilla inhabits mountains ranging in altitude from 2,200–4,300 metres and little fruit can grow at those altitudes, and so if the region where Chororapithecus ancestors lived found itself elevated, from tectonic forces, then that would over time have decreased access to fruits. Those apes that found themselves with less and less fruit, would gradually adapt to instead eat shoots and stems, as well as bark and roots, and they would spend more and more time eating, a trait that is preserved in their descendant the mountain gorilla who spends 30% or more of a typical day searching for and eating food.

How big was the mountain gorilla population before humans took over the world?

Due to human activity, such as poaching and habitat destruction, the mountain gorilla has become the most endangered type of gorilla, with between 600-880 individuals remaining. That the mountain gorilla is near extinction, could mean that there are many populations of mountain gorillas which have become extinct as a result of human activity.

Tectonic history of the Ethiopian Plateau from an expansion tectonics perspective

Expansion tectonics is a new model for how the continents and oceans have formed, and is based on the idea that the earth's radius has increased.

Within the hypothesis of a mountain-origin for Chororapithecus abyssinicus, the mountains that make up the Ethiopian Plateau would by 10.5Ma have reached high-enough elevations that little fruit could grow.