Is this assumed to be a mutation that occurred in a single member of the species that propagated throughout the gene pool? If so does that say anything about this individual, as in, they were of superior genetic fitness (does not have to be related to fusion) or does that not factor as much within the size of the population it occurred in?

I know that this type of fusion isn't a completely unique thing (though it's not very common as far as I know). I guess where I'm stuck at is, does the fusion have any properties which are different than any other genetic change which finds itself in all humans? On that level, is there anything significant about it's spread or would it have passed on like any other natural selection. I'm also lacking in knowledge concerning how monumental the fusion was, what level of evidence do we have to suggest it's possible impact? Thanks.