[__MatrixMan__]

That sort of chicken/egg situation is typical for bootstrapping a currency. Coinage in ancient Rome was a solution to a military logistics problem: How to feed distant soldiers... make the locals do it. How to do that? Intimidate them with your well-fed military. The innovation was to require that collected taxes weren't just valuable metal, but that they had Caesar's face on them. This proved that the payer had been supporting the soldiers that so recently attacked their community.

An effective solution to the how-to-distribute-UBI problem could itself be the thing that backs the new currency in a similar circular fashion. I mean, it's not like the bar is very high. Currently our money is backed by games of chicken over whether the Strait of Hormuz is open.

But biometrics are just the wrong way. They require too much trust to be placed in a sensor, and an authoritative source of truth about the data. Any time such a source of truth exceeds a certain importance threshold, it becomes corrupt.

I believe the solution is out there, but it's in a different part of the landscape dictated by the CAP theorem: CRDTs not blockchains.