fingerprint, retina, or even DNA are stored in the blockchain
That's one way to do it, link physical bodies with digital identities.
The other way to do it is to make accounts user-owned, have value, and have an almost un-gamable reputation system that determines who can access what based on it. Or something like that.
That's interesting. I guess the transparency of blockchain can help with that un-gamable system. Real world applications of the blockchain technology are exciting.