AbstractWe consider life as a cyclic physicochemical process that makes heredity and Darwinian evolution observable through living cells. We elaborate four principles that constrain current speculations about life’s emergence to natural processes driven by diurnal physicochemical gradients, primarily of temperature, water activity and electromagnetic radiation. First, Earth’s prebiotic chemical evolution is historically continuous with Darwinian evolution; second, cycling energies of solar radiation are primary drivers of chemical evolution; third, environmental molecular complexity must be high at the origin of life; and fourth, non-covalent molecular forces determine molecular recognition and cellular organization. Under normal physiological conditions of high ionic strength and high macromolecular crowding, hydration interactions (hydrogen bonding), screened electrostatic forces and excluded volume repulsions act over acommensuratedistance of about one nanometer. This intermolecular distance governs chemical coevolution of proto-biomacromolecular surfaces (nucleic acids, proteins and membranes) toward Darwinian thresholds and living states. The above physicochemical principles of life’s emergence are consistent with the second law of thermodynamics, and with the current facts of molecular microbiology and planetary sciences. New kinds of experimentation with crowded molecular mixtures under oscillating temperature gradients - a PCR-like mechanism of life’s origins - can further illuminate how living states come about.Graphical abstractLife’s emergence follows from chemical and Darwinian evolution, a high degree of molecular complexity and a high crowdedness, and non-covalent molecular forces that determine molecular recognition and cellular organization. The macromolecules divide the cytoplasm into dynamically crowded macromolecular regions and topologically complementary electrolyte pools. Small ions and ionic metabolites are transported vectorially between the electrolyte pools and through the (semi-conducting) electrolyte pathways of the crowded macromolecular regions.
Complex molecular mixtures under cycling gradients as basis for life’s origins