I recently came across a page giving evidence for fusion in human chromosome 2 -- it was the first time I had heard about this. The conclusion of the article is:
The evidence that human chromosome 2 is a fusion of two of the common ancestor's chromosomes is overwhelming.
I searched the web in vain for any creationist response to this--I only found some creationist skepticism about whether there really were telomene structures in the middle of the human chromosome (which seems unjustified), and large numbers of evolutionists doing victory dances. So I was forced to actually think about it myself (oww, painful!), taking it on trust for now that the facts presented on the site linked above are basically correct. The process was quite helpful to me, and perhaps might be useful to other people, so here goes. I'm mainly looking at this from an explicitly creationist perspective, but some of this may apply to some intelligent design positions.
The problem with the conclusion, and with much of the discussion, is that it mingles the evidence with the explanation--so the conclusion, as stated, assumes that there was a common ancestor between apes and humans, which makes it difficult to use it as evidence for a common ancestor. To untangle this, let's rewrite the conclusion so that it no longer makes this assumption:
The evidence that human chromosome 2 is the fusion of two chromosomes is overwhelming. The two chromosomes it appears to derive from are directly analgous to chromosomes that are found in apes.
However, once you make this change, an explanation that agrees with creationist accounts of human ancestry just falls out. It would go something like this:
Humans were created in a separate creative event from the other primates, but with 24 chromosome pairs just like them. At some point early in human history, fusion of the chromosomes occurred to give us the 23 pairs we have now. All living humans are descended from those in which the fusion occurred.
The point is simply this: the evidence points to the fusion of human chromosomes, but gives no indication when this happened, except that it must have occurred to a creature that was the ancestor of all living humans. Since none of the apes share this fused chromosome, there is no reason at all to date this fusion any further back than warranted, so it becomes unnecessary to even posit the existence of a common ancestor between apes and humans based on this evidence alone.
What is interesting is this: at first glance, looking from an evolutionary perspective, this seems to have dealt a death blow to creationist accounts of human origins. (The main problem for the creationist is why God would have chosen to create humans with chromosomes that have useless bits in them i.e. the telomene sequences in the middle, and in a way that mimics fusion of 2 chromosomes).
But actually, the evolutionary interpretation has overstepped the mark -- it doesn't demonstrate common ancestry, it assumes it. All the way through it talks about the chromosomes of a common ancester etc. But by putting the evidence into the evolutionary framework ahead of time it has confused what the evidence actually points to. The evidence does indicate (assuming that the God-created-it-to-look-like-fusion explanation is invalid) that human chromosomes were once more similar to those of apes than they are currently, but that is all.
The article also states:
Not only is this strong evidence for a fusion event, but it is also strong evidence for common ancestry; in fact, it is hard to explain by any other mechanism.
The main basis for saying this appears to be the statement: "At the place where we would expect it on the human chromosome we find the remnants of the chimp 2q centromere". But of course, as the author explains later, we don't actually find the chimp 2q centromere -- there is no label on it saying "I'm a chimp 2q centromere". What we find is the remnants of centromere that is in the same place, relative to certain sequences, as the 2q centromere is in chimps. All this shows is that the ancestor chromosomes (i.e. before the fusion event) of current human chromosomes were closely analagous to chromosomes of apes, including chimps.
So the author claims we can rule out other accounts of human origins, but is unjustified in doing so. The reason for this logical mistake is that he/she has intertwined the evidence and the evolutionary explanation so tightly, and seen that they fit so well, that the author cannot imagine another explanation would fit. In reality, the evidence fits a creationist explanation equally well--there is nothing contrived or tricky about what I have presented, and if evolutionary theory did not exist, there is nothing in this evidence that would make creationists think of common descent. The idea that humans originally had a complete set of chromosomes that correspond to ape chromosomes, rather than only 23 out of 24, does not shake creationist ideas one little bit. We already knew there was a vast amount of similarity between humans and primates both in terms of physical characteristics and genetic material and structure. It is a mistake of the evolutionary mindest to assume that observing similarities necessarily brings you to the conclusion of common descent. Taxonomy based on physical characteristics was already a very well established science when the idea of common descent came on the scene, and people from both the intelligent design and the creationists camps have no problem in understanding physical and genetic similarities that are not rooted in common descent -- that is, it is easy to think of perfectly adequate reasons why a designer (either an unknown intelligence in the case of ID, or God in the case of creationism) might have reused designs.
A lesson for both sides to take away is the importance of detaching yourself from your current position in order to see where the evidence actually points. It is easy to imagine that because evidence fits with your position, and fits very well, it therefore not only confirms your position but disconfirms the rival theories.
[Note, I realise that I'm likely to get flamed for this entry, as I have been in the past for the smallest references to things of this nature. Since this is my blog, and I'm interested in intelligent debate, and don't want my readers to have to trall through screens of rubbish, I will not hesitate to delete comments that are just stupid and contribute nothing. I promise not to censor you if you have something intelligent to say, and I am genuinely interested in any flaws in my logic or gaps in my knowledge of the evidence.]