Natural Philosophy of Protoplanetary and Planetary Discs: On the Origin and Evolution of Life

XIAOMING LI 

This paper examines the four pivotal and most contentious issues within the field of biogenesis by analyzing the life-originating processes centred on protoplanetary and planetary discs. These include: 1. Geological environment; 2. Source of nutrients/initial molecules; 3. Source of energy; 4. RNA world versus metabolic world: which came first? To date, scientists have been unable to reach a consensus on these questions. Regarding geological environment, the distinct conditions within the inner, middle, snow line, and outer regions of the protoplanetary disc—including solar, cosmic, and isotopic radiation—along with the exchange of dust and gases between these zones, provided an exceptional reaction platform for prebiotic chemistry. Concerning the origin of life's food/precursor molecules, the protoplanetary disc—the stellar gas and dust disc formed during star formation—served as a crucial transitional stage linking interstellar matter to life's chemical potential. Transition metals, short- and long-lived radioactive isotopes, polycyclic aromatic hydrocarbons (PAHs), fullerenes, and the metal-organic compounds formed from these substances play a crucial role in life's composition. Analysis of meteorites reinforces the credibility of the protoplanetary and planetary discs as potential sites for the origin of life. Quantum mechanics may significantly influence the possibility of life emerging within protoplanetary discs. Regarding energy sources, the energy required for the emergence of life is quantifiable. The energy driving life's birth and evolution within protoplanetary and planetary discs can be summarized by the equations F = ma and ΔE = Δm × c². These energy sources derive not only from the kinetic energy of the protoplanetary disc and interactions between dust particles, expressed by F=ma, but this kinetic energy can also be converted into chemical reactions, ultimately yielding prebiotic effects. Energy generated by radiation from the decay of radioactive isotopes within the protoplanetary and planetary discs, alongside energy produced by solar nuclear fusion, can be expressed by the formula ΔE = Δm × c². This energy not only generates heat but, crucially, produces radiolysis effects through solar radiation, cosmic rays, and isotope decay. These effects facilitate prebiotic chemical reactions and provide sustenance and nutrients for life within the planetary disc. Concerning the RNA-world versus metabolism-first debate, the high concentrations of PAHs and fullerenes in protoplanetary discs, coupled with their influence on the formation of aromatic amino acids and aromatic hydrophobic proteins affecting nucleic acid molecular chirality, underscore the metabolism-first theory. The discovery of Hemolithin in meteorites and its implications for RNA generation reinforce the "metabolism precedes genetics" theoretical framework. Starting from Hemolithin and primitive metabolic systems found in meteorites, lipid membrane vesicles encapsulated metabolic small molecules and informational polymers. Sustained by external energy sources, these gradually evolved into primitive systems possessing the tripartite functions of "replication–metabolism–boundary". It subsequently led to the integration of energy, protein metabolism, and membrane–genetic systems within protoplanetary and planetary discs. Ultimately, a self-maintaining and self-replicating cycle system was established within the planetary disc. The core emphasis of this paper lies in the continuity and consistency, spanning hundreds of millions of years, of the gas and molecular dust composition within the protoplanetary disc and the gases and molecules that could be generated within planetary disc planetesimals. It provided a stable energy source and an excellent sanctuary for the birth of life within the protoplanetary disc and the evolution of life within planetary disc planetesimals. The philosophical coherence that life should emerge wherever gases and molecules exist elucidates the availability of protoplanetary disc gases and molecules (such as H2, CO, and electrons) alongside the bacteria consuming these substances within the disc's planetesimals. These gases and molecules are generated within planetary disc planetesimals by radioactive isotopes (such as 60F, 238U, ⁴⁰K), simultaneously producing oxygen radicals and hydrogen peroxide (e.g., H₂O₂, •OH, O₂•⁻ , NO₃⁻ ). Thus, isotopes provided both the energy for bacterial survival and the nutrients required for bacterial metabolism, while simultaneously generating oxygen-free radicals and hydrogen peroxide that constituted a hazard to these bacteria. Consequently, bacteria acquired the capacity to resist oxygen-free radicals and hydrogen peroxide while obtaining the nutrients essential for their metabolism. The core of this process lies in the "radiation-metabolism coupling" and "radiation-metabolism co-evolution" hypotheses concerning the origin of life in protoplanetary and planetary disks. Within this theoretical framework, radiation-generated gases, molecules, and oxygen free radicals coexisted long-term with bacterial metabolism and antioxidant functions, forming the "radiation-metabolism co-evolution" model for life's origin. Only this "radiation environment-metabolism co-evolution" hypothesis can explain the survival characteristics exhibited by all bacteria and viruses we observe today. The fundamental reason why bacteria, archaea, and viruses possess exponentially higher radiation resistance than mammals lies in the direct consequence of these microorganisms' prolonged coexistence with radiation within planetary disks. Migratory birds and octopuses, which traverse Earth's magnetic field lines, share connections with bacterial antioxidant mechanisms and primordial genetic material. Finally, constructing a novel Tree of Life reveals that the evolutionary lineage of life differs entirely from our currently accepted model. The roots and trunk of this evolutionary tree do not originate on Earth, but rather on the planetary disc. Two branches diverge from the trunk: bacteria and archaea. The archaea branch further gives rise to a branch of eukaryotes, constituting the primary life forms on our planet today. Meanwhile, bacteria, archaea, and viruses continue to evolve both on the planetary disc and on Earth, exerting influence upon the life forms evolving on our planet.