What is the genetic mechanism that may have been responsible for the base sequence changes between humans and gorillas?

Many genetic changes happened. One involved the joining of two "ape" chromosomes (they have 24 pairs of chromosomes) end to end to make human chromosome #1 (we have 23 pairs of chromosomes, but still have about the same amount of genetic material). This would have made it impossible for the "new" humans to successfully breed with their most recent prehuman ancestors, even though both peoples probably had identical appearance and lived together for several generations of both.