I believe that human cells contain 23 chromosomes, while those of our nearest ape relatives have 24, so that either our ancestors or theirs changed number.Tossing a mental coin, assume it was our ancestors who changed, then initially there would have been an individual with 23 chromosomes in a population where everyone else had 24. A number of questions arise.1 - How would that individual differ? There must have been substantially advantageous differences for it to have become the new standard.2 - Could 23 and 24 chromosome individuals interbreed? If not, there would be an Adam-and-Eve situation when two 23-chromosome individuals occurred. If such cross-breeding could occur, I wonder if there are populations of any animal with differing numbers of chromosomes?3 - How long would it have taken for the species to change from all having 23 chromosomes to all having 24?