OK, I browsed the Chromosome 2 on the wikipedia. So, the difference between ape and human is on that chromosome.
Not really. It is only
one difference and not very important for all I know about genetics.
And we also know how much differences are there between ape and human.
Technically, we
are apes but taxonomy isn't the point of this discussion, so...
So does that say Chromosome 2 is responsible for all the differences?
No.
If not, than what is the point of the argument?
The point of the argument is that chromosome 2 shows our common ancestry with chimps. Now how does it do that?
(1) It has the same genes in the same order as two chimp (or gorilla, or orang-utan...) chromosomes. One half of it corresponds to one ape chromosome, the other to another ape chromosome.
(2) It has telomeres in the middle. Telomeres are normally found at the
end of a chromosome.
Because of certain features of our DNA replication (could go into details if you want me to), some of the end of a chromosome gets lost every time it's copied. You obviously don't want to lose genes and other important bits of DNA... so every chromosome has long repetitive sequences that can get lost instead of the more important stuff. You don't normally find telomeric sequences anywhere other than the ends of chromosomes.
(3) Chromosome 2 has two centromeres (one non-functional). Centromeres are where a chromosome gets grabbed and dragged apart during cell division. A normal chromosome has only one of these - and only one of the centromeres in human chromosome 2
functions as a centromere, but the other is still recognisably similar to a working centromere.
In short, human chromosome 2 looks exactly like two chimp chromosomes stuck together. And...
Even all the Chromosomes are the same, ape and human are still very different. Two things are "very similar" does not mean one is derived from the other.
No it doesn't. And we are not saying that. We are saying that these particular similarities are best explained by common ancestry (and
not explained otherwise).
Why do you say this piece of evidence is VERY strong for evolution?
You could call it an argument from lack of function. If two
functional things are similar you could say they are similar because this design worked well for whichever function, so it was reused*. However, the telomeres in the middle of human chromosome 2 and the second, non-functional centromere
don't really do anything. There is no reason why they should be there - unless they are leftovers from our evolutionary history.
A (rather famous) presentation of the chromosome evidence:
http://www.youtube.com/watch?v=BXdQRvSdLAs
*which, it turns out, isn't a good argument once you start looking at DNA or protein sequences. Cytochrome c is the classic example - the protein's sequence changes by massive amounts (50%-ish IIRC) between distantly related creatures, and it has the same functions anywhere from a yeast to us (as far as I'm aware).