Evolution, Chromosome Fusion, Man & Chimp Question?

Question

I heard something interesting about human evolution in relation to other modern primates. 
We all know that Humans have 46 chromosomes and our near relatives such as chimpanzees have 48 chromosomes. Now what I heard was that two chromosomes from our earlier ancestral species had 48 chromosomes and that a pair of chromosomes fused into one large chromosome.

I can sorta buy into that, it's testable, not only that, it's confirmed, so I hear, but I only heard that ">a pair< of chromosomes fused into one," which would have to mean that we should have 47 chromosomes, not 46, SO I AM CLEARLY MISSING SOMETHING!

This proposal would require, it seems to me, TWO SETS (4) of chromosomes fusing into TWO chromosomes, eliminating the ODD NUMBER issue, giving us 46 chromosomes.

The only chromosomes I learned about are the 9th and the 14th ape chromosomes which combined into the human 12th chromosome. What are other two that fused and into what # chromosome?

Formerly Peter S · Accepted Answer

Think of it this way.
Chimps  - 24 pairs
Humans - 23 pairs.

One pair of the 9th fused with one pair of 14th - one pair lost  = now 23 pairs.

Confused Dave · Answer

You are missing something.

The autosomes come in pairs, one from the maternal line and one from the paternal line.  Humans don't have 46 distinct chromosomes, we have 23 pairs.  There isn't a "chromosome 39" or "chromosome 41" in humans - the chromosome numbers only go up to 23.

Since this is a pretty rare event, it's pretty much a given that it only happened once.  That means that there was probably a while where you had individuals with one "conjoined" chromsome and one pair of separate chromsomes.  This kind of mismatch can cause problems during meiosis, but if the chromsomes themselves are similar enough, it probably won't make as much difference as you might think.  It's likely (although not definite - neutral changes can become fixed in populations) that the fused chromsomes conferred some kind of slight selective advantage over the separate chromsomes, because eventually the fused form displaced the separate ones.  But yeah, between chimps and humans, this only happened once.

Evolution, Chromosome Fusion, Man & Chimp Question?

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