[RocketMoose]

Originally Posted by Alawen Everywhere [You must be logged in to view images. Log in or Register.]

I'd like to thank you for this question. No, seriously. I had no idea what the answer was, and this is actually one of my fairly serious hobbies, so I started digging.

The answer to your questions is that chromosomes are not completely fixed. There are several ways for a particular gamete to change chromosome count. It's fairly technical but it basically boils down to that sometimes the two parts of a gamete don't split successfully (non-disjunction) and sometimes two chromosomes that are not part of a pair get stuck together (translocation).

The great apes all have 24 chromosome pairs. We, of course, have 23. There is no way to know for sure whether the great apes added a chromosome or humans dropped one, but the general thinking is that if the majority of species within a group (we group with great apes at the family level, hominidae) have the same number of chromosomes but another varies by one, it is likely that species is the one that varied. Ergo, human DNA was probably subject to translocation which reduced our chromosome count from 24 to 23. It seems likely that there was some pattern highly susceptible to translocation in one of our predecessors.

Interestingly enough, humans are a very inbred species compared to the other great apes. A small population of chimpanzees will apparently reflect as great a genetic diversity as all of mankind. You have, interestingly enough, hit upon the answer to why there are no 80% human great apes--the mutation that led to homo sapiens could no longer breed successfully with other closely related species.

It may go without saying, but gene mutations of this sort are fatal and almost always sterile. However, it's not extremely rare. In the last 17,000 years or so since humans began domesticating animals, we have created domesticated species whose chromosome counts vary from their feral ancestors.

I like it when I learn something new. I like it a lot.

Interesting, just another supporting fact for creationism and the fact that God created Adam and Eve...