Other HPV types, such as HPV-13, vary relatively little in different human populations. In fact, the sequence of HPV-13 closely resembles a papillomavirus of bonobos (also known as pygmy chimpanzees).[26] It is not clear whether this similarity is due to recent transmission between species or because HPV-13 has simply changed very little in the six or so million years since humans and bonobos diverged.[2]