The '46-fix' probably originated in one mutant chimpanzee, and punctuated
the speciation of humans.
Chromosome differences are associated with marked differences in physical traits. A typical example of a chromosome abnormality is Turner syndrome, where an X-chromosome is lacking. This mutation is associated with special traits, including a short and webbed neck, low-set ears, low hairline at the back of the neck, and short stature.
We can imagine a chimpanzee being born with 46 chromosomes and bizarre physical traits (perhaps a large head, weak neck, bald body, subcutaneous fat, hooded nose, ... ). If this chromosome fusion appeared in the African rainforest where chimpanzees are well adapted, such a baby chimp would probably have died. But if it appeared in a marine habitat, where some of the traits were advantageous, this chromosome abnormality might have been passed on to future generations. See my message 202.
Now geneticists have the technology to study chromosomes and try to understand how the 2A-2B fusion might be related to our unique human features. They will eventually be able to genetically engineer a chimpanzee with fused chromosomes 2A-2B, and see what physical features appear. ( Oof! )