Chimpanzees and bonobos look very similar to the untrained eye. Indeed, in the past it was not uncommon for them to be housed together, at least until the keeper saw the bonobo getting beaten over and over again.
Indeed they have been separate species for between 1.5 million to 2 million years.
What’s more, it is thought that this split occurred as a result of the formation of the Congo river. As such, it has been a big surprise to researchers looking at their DNA to find that they have exchanged genetic material far more recently.
Central and eastern chimpanzees share significantly more genetic material with bonobos than do other sup species, leading scientists to suppose that there was some interbreeding roughly 200,000 years ago and 500,000 years ago.
Given the difference in behaviour, and the fact that they generally cannot be housed together (apart from anything this stems from the chimpanzee way of dealing with stress through violence where as bonobos deal with it through sex) this has stunned scientists. It also begs the question as to how this mixing occurred given the Congo river being in the way.