A proposed evolutionary tree of Gorilla-Pan-Homo speciation
Here is a proposed evolutionary tree, showing how humans may have evolved directly from chimpanzees that became isolated on Bioko about 4 million years ago. It is modified from the image on Wikipedia.
Homo erectus essentially had the same skeleton as the modern human, except that the brain size is less. In the Bioko model, H. erectus evolved its human features on Bioko, and then escaped to other places (Europe, East Africa, Java, China) where bones could be preserved as fossils. The brain was big enough for it to invent tools, shoes, and clothes, necessary for survival in places other than Bioko.
There are no fossils of Gorilla or Pan where they evolved in central western Africa, because fossils are not possible there. However, three teeth of a chimpanzee and many H. erectus bones have been found in E. Africa, where climate was suitable for fossil preservation. E. Africa was not the place where H. erectus or chimpanzee evolved, but some managed to wander there and leave fossils.
Chimps, gorillas, and all other primates are not very mobile, and keep to a single range and habitat. H. erectus and H. sapiens, with large brains, wandered far and wide, over the continents of Africa and Eurasia.