How do you know that the two genes have fused together?
We are not talking about genes here, we are talking about entire chromosomes.
And yes, we know that with an extreme amount of certainty.
Because chromosomes have markers, at their ends (telomeres) and at their centers (centromeres).
So, if two fuse you would expect there to be two centromeres in the new chromosome, and also telomeres in the middle.
And in our chromosome #2, this is exactly what we found.
This chromosome makes no sense... unless it is a fused one.
And given that our entire genome looks so similar to the other ape-genomes... except for this one chromosome that we have, which mirrors exactly the two chromosomes we DON'T have anymore, the number of coincidences to explain this without the model of a fusion of the chromosomes become unbelievably high.