The reason why we think the chromosomes fused in human ancestors is the fact that one of our chromosomes has two centromere locations (one no longer functions, and both centromere locations match those found in two chromosomes in other apes) instead of the single one the rest of our chromosomes have. Furthermore, when you stick the two ape ones together, you pretty much can't tell between that and the actual human chromosome, which is chromosome 2 (except the X and Y chromosomes, they are numbered by size). Chromosome 2 is also unique in that it has telomeres (repeating segments found at the ends of chromosomes) in the middle of it. Does that not seem exactly like what two chromosomes merging together would result in?
In the most basic sense, the chromosomes found in chimps and other nonhuman apes alive today are like the two on the left, while the human one is like that on the right
https://upload.wikimedia.org/wikipedia/commons/7/71/Chromosome2_merge.png
It would be weirder if this was caused by something other than the fusion of two chromosomes.
Look at the bottom two rows of this cancer cell's chromosomes. These are ones comprised so much of a mix of other chromosomes that they can't be placed into the numbering system for human chromosomes.
http://cdn.phys.org/newman/gfx/news/hires/2011/2-arecancersne.jpg .
-_- the translocation of a chromosome is literally when part or all of a chromosome merges with another one.
https://upload.wikimedia.org/wikipedia/commons/c/cb/Translocation-4-20.png this is what happens to make some people more likely to have kids with Down Syndrome.
In people that inherit the form of Down Syndrome that runs in families, it does change the chromosome number. Carriers themselves have no detrimental health affects despite having a huge portion of one chromosome get stuck to another and the rest on its own, and not all of their children will have Down Syndrome.
The human Y chromosome has degraded a lot in a short amount of time. Even in ancient human DNA taken from mummies, the chromosome is way bigger and has more genes on it than in modern humans, and that's within a few thousand years. There are some ideas as to why that happened, but I think that explains the huge differences in that particular chromosome pretty well. In any case, most of the "missing" genes are present on the X chromosome anyways, so the genetic impact is not as big as you would think. It does make men more prone to being born with certain genetic problems and conditions, though, such as color blindness. To basically explain further, besides the genes directly tied to making biological structures associated with males, genes on the sex chromosomes are completely redundant. In the cells of women, only 1 X chromosome is active while the other is shut off completely (which one it is, the one that came from dad or mom, is completely random and varies from cell to cell). This is also why cases where people have extra sex chromosomes (examples being XXY, XXX, and others) don't have all that many phenotypic consequences compared to having extras of any other chromosome. Some can even reproduce. But I am getting off track. The same likely happened to chimpanzees and their Y chromosomes; it is detrimental to have energy expended on unnecessary active gene sites, but neither the whole X or Y chromosome in men can be turned off and produce a fertile and functioning male individual.
Actually, pretty much all of the few genes on the human Y chromosome are for the development of the male reproductive system. In fact, during my many genetics classes (and classes that happened to talk about genetics), whenever the Y chromosome was brought up, instructors always commented on how the only genes on it that we know of are associated with generating biological sex. More genes does not equate to significantly different structure, especially considering how redundant they tend to be.
The translocation associated with people more prone to having children with Down Syndrome doesn't physically harm the carrier. Physical location of a gene is practically irrelevant to it's function so long as the start codon and end codon go along for the ride. It isn't as if certain chromosomes behave differently than the others (aside from the X chromosomes in women that are rendered inactive). A gene will have the same function regardless as to where it is in a chromosome, or what chromosome it is on unless it is a sex chromosome.
There is literally no other reason a chromosome would have the remnants of a second centromere and telomeres in its center unless it was the result of two chromosomes getting stuck together. The limited function telomeres have only works if they are at the very ends; they exist as disposable segments that shorted every time our linear DNA replicates thanks to imperfection in replicating the lagging strand.
That's a fair enough outlook, but I ask that you read more about chromosome 2 and why people think it is the result of two chromosomes merging. It's like looking at two different colored Lego blocks stuck together.
Just something to note as far as the genetic differences between, say, chimpanzees and humans, we don't determine which had the mutation just by comparing the two, but by comparing all modern apes. If there is a gene or genetic quirk found in only one ape species, it makes far more sense for a mutation to have occurred somewhere down the line in the ancestors exclusive to that species than it does for similar or identical mutations to have occurred in all the others. As a person that finds some validity in evolution, you must agree that much makes sense. Because only humans and no other modern apes have that strange chromosome that physically is two smaller ones stuck together, it makes a whole lot more sense for that to have been a fusion that occurred in human ancestors and not in the ancestors of other living apes than it does for there to have been some sort of split in all of their ancestors. Especially considering the fact that a lot of those apes are not very close genetically compared to, say, between humans and chimpanzees.
Also, if ancestry is shared, how else to explain the genetic differences than mutation?
Chromosome 2 has structures that ONLY make sense if it was once two separate chromosomes.
Sure, but if you have doubts, be sure to read up on the material in question as best as you can.
