As I understand, for sexually reproducing creatures, it’s very important for both parents to have the same number of chromosomes, or the offspring is not viable. So how did we get to the point where there are sexually reproducing organisms with different numbers of chromosomes?
Chromosomes readily split and fuse, and closely related species often may differ in chromosome number. While differing chromosome numbers can cause mispairing during meiosis and hence sterility, this is not necessarily a problem early in the process. If a chromosome splits, as long as the two parts can still align with the original chromosome during meiosis - which is often the case - the hybrid may still be viable and fertile; likewise for the reverse case.
It’s also a matter of mutation, many of the earlier organisms had actually larger numbers of chromosome sets which decreased with complexity.
Well in a way people are just a bacterium with a really bad case of down syndrome.
ducks
Here’s a good page with diagrams showing how an chromosome can be gained or lost in humans. More stuff is linked-to therein. Almost all result in miscarriage but some survive, even until adulthood. (E.g., downs syndrome.) Small chromosomes and the sex chromosomes are typically less problematic.
Especially in plants, there can be multiplication of all chromosomes. Linky. (In plants it actually seems to an advantage surprisingly often.)
Apparently in some species at certain times, the number of chromosomes is slightly more “flexible”, so that males and females within the same extra chromosomes could exist in the same area and fertilize/mate and there you go.