We derive a substrate-independent physical criterion for the distribution of biological complexity in the universe. The criterion, called the Causal Firewall, follows from a single physical requirement: that the environment maintain a universally agreed-upon causal ledger of actualization events over the timescales required for complex molecular assembly. Two conditions are necessary: (F1) the Quantum Darwinism redundancy threshold, requiring that actualized states are encoded in at least R_ ∼ O (10) independent environmental fragments; and (F2) the non-relativistic decoherence condition, requiring that the local Unruh temperature TU = ℏ a / (2π c kB) satisfy TU Tₑnv. Crucially, neither condition makes reference to molecular class, solvent chemistry, temperature range, or biological alphabet. We show that the Causal Firewall corresponds to the μ = 1 Erdős-Rényi percolation threshold of the local causal graph --- the same mathematical transition governing QCD confinement, galactic rotation curves, and quantum fault-tolerance thresholds. We further derive a quantitative constraint on origin-of-life scenarios: the coarse-grained assembly depth ARA of the first proto-metabolic network is bounded both above (by the Causal Firewall cycle time constraint) and below (by the Assembly Theory floor). The framework predicts that life is structurally impossible in highly relativistic environments regardless of energy availability, that non-carbon-based life is not prohibited but is subject to the same causal coherence conditions, and that SETI search programs should be weighted toward environments satisfying the Causal Firewall criteria rather than merely toward circumstellar habitable zones. Two falsifiable predictions are stated.
Joshua Sandeman (Thu,) studied this question.