DNA evidence shows us that the last common ancestor of gorillas and chimpanzees lived about 10 Ma ago. The two branches that diverged − gorillas (Gorilla) and chimpanzees (Pan) − look rather similar today (ape-like) because the habitats they evolved in were about the same. They did not change much from their common ancestor.
The gorilla branch evolved in 4 different places into 4 different taxa: Gorilla gorilla gorilla, Gorilla gorilla diehli, Gorilla beringei beringei, Gorilla beringei graueri (Map of the 4 gorilla places)
About 6 Ma ago, the chimpanzee branch diverged into the chimpanzee (Pan) and human (Homo) branches. Then the chimpanzee branch evolved in 5 different places into 5 different taxa: Pan troglodytes verus, Pan troglodytes vellerosus, Pan troglodytes troglodytes, Pan troglodytes schweinfurthii, Pan paniscus (Map of the 4 chimpanzee places.) (Map of the bonobo place.)
There is no fossil record of the chimpanzee or gorilla evolution, because the places where they evolved were not suitable for fossils. The only knowledge that we have about their evolution comes from living animals and their DNA.
Humans and chimpanzees are remarkably different, even though their DNA is quite similar. It is assumed that humans evolved to be different, because they evolved in a different habitat. Paleoanthropologists thought that this habitat was drier (e.g. savannah) than the chimpanzee habitat because fossils are found in much drier areas (East Africa, South Africa). Elaine Morgan thought that humans evolved in a wetter habitat because human features can easily be explained as aquatic adaptations (hairless body, sweat-cooling, large brain, bipedal running, babies with subcutaneous fat or blubber, protruding nose, etc.)
There is not a single fossil that helps us understand gorilla evolution or chimpanzee evolution. Are there any fossils that help us understand human evolution? I don't think so.
There are lots of fragments of pre-human hominin taxa that are younger than about 6 Ma old. The finders want these pre-human taxa to be related to humans, but the taxa do not show progression in brain size or other human-like features. Suddenly about 300,000 years ago, we get real human fossils (Homo sapiens, Homo neanderthalensis). The pre-human taxa are not human-like, and some younger taxa seem to be throwbacks -- more ape-like than some of the older taxa. No one knows how or if they are related to each other or to modern humans.
How can we explain the origin of so many taxa, that are not showing progression toward human features? And how can we explain throwbacks to more ape-like hominins that lived during younger geologic times?
I imagine that there was a progressive increase in brain size and other aquatic human-like features on Bioko, all the way from 6 million years ago to 300,000 years ago. Homo sapiens with large brains may have existed on Bioko as early as two million years ago -- we don't know.
During the six-million-year aquatic evolution on Bioko, many individuals were escaping to mainland Africa. When they arrived in Africa, they found fruits and other forest foods that were not available on Bioko. Their aquatic adaptations ceased, and they did not become more human-like. Their descendants later left fossils in dry areas. These are unsuccessful offshoots, whose progeny did not survive to the present.
Some of these human-like apes may have mated with forest apes, resulting in new and strange hybrid taxa with smaller brains than the apes living on Bioko at that time. Other hybrid features would also have been more ape-like than the Bioko apes. Their descendants may have evolved in various terrestrial habitats for a few million years on mainland Africa before their fossils appear in the dry areas. There are many possibilities.
The bipedal apes with small brains and other ape-like features that are found as fossil fragments of various ages in the dry areas of Africa may all represent unsuccessful offshoots. Only the apes who remained on Bioko developed the human features, and we find their fossils on Africa and then in Eurasia, beginning around 300,000 years ago.