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. From the development of teeth, we can note that the lowland gorillas have a varied diet of leaves and fruits, while the mountain version is adapted to eating mainly leaves and shoots of the higher elevation. The habitat disjunction is related to the landscape change due to climate, which has allowed the three subspecies to evolve into different types of gorilla. Most of the evidence has not contributed to an agreed theory.
Many different scientists have a different view of how the gorillas evolved.
Some say it is due to climate change and some say it is due to the availability of food. It may even be a combination of both. For sure it has to do with the its diet and habitat. This is a question that has soon to be resolved and answered. According to data from various studies, the lowland gorilla far outnumbers the eastern and mountain races and is more widespread.