Relational Actualism: Life, Complexity, and the Causal Depth of Biological Systems

The origin and persistence of biological complexity present one of the deepest explanatory challenges in science. This paper develops the biological applications of Relational Actualism (RA), a foundational physics framework in which irreversible on-shell actualization events are the primitive physical reality. Building on the five foundational theorems proved in the companion paper RAHC---the Assembly Index Correspondence, the DAG Markov Property, the Markov Blanket Isomorphism, the Landauer Cost of Actualization, and the Epistemic-Physical Equivalence bound---this paper establishes: (1) the Actualism Assembly Index as a physically grounded measure of biological causal depth, supported by a B3LYP/6-311+G* density functional survey across the five primary bond types of life; (2) the Causal Firewall as the physical phase transition that separates abiotic chemistry from stable biological complexity; (3) the thermodynamic grounding of natural selection as simultaneous optimization of causal assembly depth, edge-of-chaos operation, and Markov blanket fidelity; and (4) the categorical impossibility of Boltzmann Brains. Five empirical predictions follow, spanning astrobiology, origin-of-life chemistry, evolutionary biology, and the universal search for life. A new result: the Causal Firewall threshold σ = λτ_dℓ^3 = 1 is proved (not merely conjectured) via the directed Erdős-Rényi percolation theorem applied to the Poisson-CSG causal graph, identifying the Firewall with the onset of a giant strongly connected component and universal causal ledger agreement.