And yet it looks exactly like what you would expect a fusion of two chromosomes to look like. They share synteny to the closest relatives without fused chromosomes, the fused chromosome has double centromeres and end-to-end telomeres. What should a chromosome fusions look like if not this?
You might be able to convince yourself with some elaborate rationalization that it’s not a chromosome fusion, but you can’t seriously argue it doesn’t conform to what a chromosome fusion should look like.
Yes. Since humans have fewer chromosomes than our closest primate relatives, logically speaking if we are to share common ancestry there must be some mechanism by which humans could end up with fewer chromosomes. One of which would be a fusion event. Assuming here we can rule out loss of an entire chromosome that has numerous essential genes.
The hypothesis of a fusion event would in turn also lead to numerous other predictions. Think about it this way. If two human chromosomes which are highly similar to chimp chromosomes, fused, we would expect that the fused human chromosome should look like it was derived from two chimp chromosomes fused together end-to-end. This would entail for example double centromeres, with one part showing synteny to one chimp chromosome, and another part to another chimp chromosome. And so on and so forth. All of these predictions bear out. As I said, it looks exactly like a chromosomal fusion should.