Protamines that, in the AndrewC fictional version, should be identical to any pruative protamine that may have served that function a billion years ago. Whatever. If you can't be bothered to address the argument as it actually is, rather than some ridiculous parody that exists solely in your head, there's precious little point in continuing - you do not discuss this subject in good faith.
It doesn't matter how long ago, a minor change is a minor change. We know that the histone that we are considering is practically "eternal". A minor change constrains the changee if the change-target is "fixed". Any more futile attempts to distance yourself from your fiction? homologs such as the protamines may well have served some entirely different function before doing what histones do now. A minor change to that homolog, and presto - histones .