Though the novel exon is because of the splicing change. But this draws attention to an important detail I neglected.
Because this extra 267 amino acids is required for the function of the new protein, it is another example of the complexity springing from non-coding DNA de novo, without any prior history as a protein sequence. This is also an important example of "exaptation," the adaptation of one structure (a non-coding DNA frame) to another (a coding DNA frame) with an important function.
If functional sequences are extremely rare in sequence space, this should be impossible. Yet here we see it happen, all do to a single point mutation change.
Beautiful story. It is great for teaching. Keep posting examples like this.