What do you think feeling shame means?
You are scared that you might be ostracized from the group
But I feel shame about things that other people don’t even care about.
You are scared that you should be ostracized from the group.
That seems unrelated to the social realities. Why would evolution bother to equip me with a feeling for that?
Because you need to be able to justify yourself verbally. It is important that you think you should not be ostracized, so you can argue for it. Even if nobody shares your standards, if other people try to ostracize you for some stupid reason and you—for other reasons—believe that you really should be ostracized, then you are in trouble. You want to believe wholeheartedly that you are worthy of alliance. So if you don’t, an alarm bell goes off until you manage to justify yourself or sufficiently distance yourself from the activity that you couldn’t justify.
(From a discussion with John Salvatier)
Meteuphoric 
I have trouble even seeing why you would think this. First and most importantly, there would be no evolutionary sense to equipping us with highly visible emotions (for example, the hanging head of the ashamed), as this would serve also to inform others that we did something we can’t justify!
Second, people are ashamed of many things, such as congenital bodily traits, that have nothing to do with any lack of justification.
On second thought, my first point is wrong: there may be advantages to others having a way to check that you didn’t do something unjustifiable.
But I’ll stick by the second criticism.
Feelings of shame aren’t particular to perceived chance of ostracism by a group; they anticipate any social devaluation, for example from an individual mate or trade partner. If your voice in the conversation is represented by the non-italicized text, it would be nice to have some examples of thing you feel shame about “that other people don’t even care about”.
Social devaluation could occur after information about any number of things is revealed (ex. individual wealth, sexual orientation, illness, ancestry, …), not only after revelations about one’s activities.
Regarding the behavioral profile of shame, the big three I see are “withdrawing from public activities when participation might give others adverse information”, “displays of compliance or appeasement”, and “aggression to deter inquiries”. “Search for verbal justification of self-value to others” doesn’t match my mental picture of shame however.
If it’s related to anything, it’s guilt rather than shame.
The usual evo psych story about emotions emphasizes that they serve to guarantee your internal state through involuntary outward display. That you are equipped with shame but don’t appear ashamed during a particular encounter that helps prove you have nothing to be ashamed about.
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