Some background info. These two symbols don't have a reading on their own, but rather affect the sound that appears right before them. In modern Russian they are:
The distinction covers front-vowels and back-vowels (just like in most Fenno-Ugric languages). It also gave a consonant shift (just like in some Fenno-Ugric languages). Some languages spoken at the Slavic language area are Fenno-Ugric.
Can this be a result of Fenno-Ugric influence?