I found an explanation here, which is basically saying that sometimes the centromeres are duplicated, breaking a chromosome into two halves. But how could this propagate? I don't think inbreeding is efficient enough for this mutation to become dominant, and it should take a lot of pregnancies for the mutants and the regular ones to produce an healthy offspring. Is the explanation in the link backed by any evidence? Thank you!
By the way, do we know how many chromosomes did the latest common ancestor of human and chimpanzee have, and when did chimpanzee gain the extra pair of chromosomes (or when did we lose it)?