The Evolution of Large Organism Size: Disparate Physiologies Share a Foundation at the Smallest Physical Scales.

Life is defined by self-governing networks of molecules that change conformation cyclically, converting thermodynamic motion into directional work and structure. A spectrum of scale, from nanoscopic to macroscopic, involves a shift from intracellular thermodynamically driven processes (thermal agitation ultimately rooted in quantum phenomena) to intercellular bulk flows described by classical physics; from short-distance transport involving diffusion and cytoskeletal transport to long-distance pressure fluxes in hydraulic networks. A review of internal transport systems in macroscopic eukaryotes suggests that a key evolutionary step favoring large size and multicellularity involved exploiting molecular-scale stochasticity to generate organized bulk flows (e.g., motor proteins collectively generating mechanical pressures in metazoan tissues such as cardiac muscle; within tracheophytes, active and passive phloem loading/unloading inducing pressure gradients, and active regulation enabling passive xylem function and hydraulic reliability; sieve-like conduction in heterokonts; and peristaltic shuttle streaming in myxogastrian plasmodia). Macroscopic physiologies are underpinned by Brownian molecular thermodynamics and thus quantum mechanics; the apparently disparate physiologies of large organisms share a fundamental operating principle at small scales. However, the specific translocation mechanisms that extend this functioning to larger scales are embroiled in bauplans, representing phylogenetic constraints to body size.