Most likely, they were small and rodent-like. We are not sure what the early relative of the gorilla was, but it can be traced back to an early ape known as Proconsul africanus. The chart on the last page details the development of primates. Minor differences between the lowland and mountain forms suggest that the divergence between the two occurred in the past. There seems to be two primary factors that limit the growth of gorillas as a species.
Rivers are barriers to gorillas.
In the past gorillas may have been a single population, but due to climatic change, their population has become changed. Maybe at one time, central and western Africa were an uninterrupted stretch of rain forest. Due to climatic change where there was a much drier period caused the rain forest to be in pockets Volcanic activity also suggests the population may have splintered into different directions. In the Pleistocene period there seems to have been considerable tectonic activity which caused the split. When the environment changed, the gorillas adapted to whatever their new habitat's conditions supplied, such as food.