Chromosomes decide your basic features sex and mind usage, and DNA decides many physical features including color, shape, and immunity (has to do with chromosomes too) to some diseases and not to others.
Evolution happens over millions of years... like 1,000's of millions. Over time, the proteins have combined in a way that creates an asexual being that split into separate sexes if those are the creatures that survive. Everything that exists depends on if it will survive. Everything exists the way it is, all species, because it can survive how it is. That is simply how it works. And throw in millions and millions of years, you get quite a combination of evolutions.
I don't think something such as number of chromosomes matter, since mentally hadicapped are down syndromed.... and they die out sometimes, that is why not many are handicapped.
The worlds plants and animals branch out with chromosome and DNA types because of their environment. Only the right chromosome type and number will survive in their environment.