@SkyWhisperer - I hate to shoot down your theory, but it’s not borne out by the article. It’s clearly stated that there is no real use for the muscle.
So if there’s not much practical use, why would nature factor it in for the survival of a particular race? The impression I get from the article is that we can all live without this muscle and still go on our merry way.
If this is indeed the case, variations within the species should have no effect either way in my opinion. Although I would have to admit that it’s still curious why some people have it and some people don’t.