There are no mammal fossils of any kind where African great apes currently live. But using DNA to estimate the time of the last common ancestors, we can imagine a plausible sequence of speciation events for Gorilla, Pan, and Homo.

About 15.8 million years ago, some orangutans migrated from SE Asia to Africa where they evolved to become the Eastern gorilla. Their bodies were large because their diet consisted of large quantities of low quality foods. 
About 9.1 million years ago, some Eastern gorillas migrated to the west, where they ate higher quality foods and evolved a smaller body. They became the Western chimpanzee.
About 6.7 million years ago, some chimpanzees rafted to the barren volcanic island of Proto-Biobo. Their diet was marine, and they evolved to become Humans. 

About 3.1 million years ago, some Eastern gorillas migrated to the west, where they lived in the same areas as the chimpanzee. There was no competition, because the gorillas ate larger quantities of lower quality foods.  
About 2.8 million years ago, some chimpanzees became isolated on the south side of the Congo River, where they evolved to become the Bonobo.
About 1.0 million years ago, some chimpanzees migrated to the east, where they lived in the same areas as the gorillas. But there was no competition, because the chimpanzees searched more for higher quality foods.



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BiokoApes.com