Difference between "unto" and "to"

What are the differences between "unto" and "to"? It seems that in many contexts where the word "unto" is used, "to" could be substituted and would be perfectly correct. It reminds me of flammable/inflammable, where "flammable" came into use because the "in" in "inflammable" caused people to think that it meant not inflammable. Is this a similar situation?

Solution 1:

To did not come from unto (if anything, vice versa), so the situation is not the same as with flammable and inflammable. Though to is an older form, unto was never as prevalent, and is now either archaic, or used in limited contexts, such as shown here (Idiom: unto itself).

Solution 2:

The origin of unto, as reported from the Oxford English Dictionary is the following:

ORIGIN: from until preposition with to preposition replacing northern equivalent till preposition.

Therefore, the relation between to and unto is not the same relation there is, e.g., between touchable and untouchable.  

One of the meaning of unto reported from the same dictionary is until, till. The dictionary reports also that unto indicates "regular recurrence within specified units of time, as day unto day."

Doctor Manley…praises Kate unto this day.—I. Maclaren