Origins of Life: Transition from Geochemistry to Biogeochemistry

Life as we know it is completely dependent on metal ions. Gradients of metal ions drive metabolism, metal centers often form the active sites of enzymes, and metal-ion coordination is largely responsible for protein and RNA folding. This dependence on metal ions likely reflects the environment from which cellular life emerged. However, long chain biological polymers were not present on prebiotic Earth. Therefore, the chemical reactions leading to Earth’s first cells must have made use of alternative catalysts that were later superseded by RNA and protein. Here, we discuss the similarities between free metal ions, minerals, and biological enzyme catalysts, and how cellular life could have exploited prebiotic metallocomplexes.

Life as we know it is completely dependent on metal ions. Gradients of metal ions drive metabolism, metal centers often form the active sites of enzymes, and metal-ion coordination is largely responsible for protein and RNA folding. This dependence on metal ions likely reflects the environment from which cellular life emerged. However, long chain biological polymers were not present on prebiotic Earth. Therefore, the chemical reactions leading to Earth’s first cells must have made use of alternative catalysts that were later superseded by RNA and protein. Here, we discuss the similarities between free metal ions, minerals, and biological enzyme catalysts, and how cellular life could have exploited prebiotic metallocomplexes.

Systems consisting of mineral surfaces, water, salts and organic molecules are considered to be plausible models of early Earth’s prebiotic environments. The probable involvement of clays, highly soluble minerals, sulfides and other minerals at the beginning of life have spurred a number of experimental studies to investigate organic molecule adsorption, polymerization and catalytic reactions of relevance to prebiotic chemistry. This article reviews current ideas in how life originated, summarises experimental results and presents some of the existing challenges that still beset the field of the origins of life.

The onset of life on Earth was preceded by prebiotic chemistry in which complex organic molecules were formed from simpler ones in the presence of energy sources. These prebiotic organics were either synthesized on Earth itself (endogenously) or synthesized extraterrestrially (exogenously) and then delivered to Earth. Organics have been detected in space and have been successfully synthesized under experimental conditions simulating both extraterrestrial environments and early Earth environments. Homochirality and enantiomeric enrichment of organic molecules, which were once considered to be biosignatures, can, in fact, be achieved abiotically. It is important to determine conditions that allow the formation of prebiotic organics and those that preserve them against degradation.

The stage for the origin of life may have been set during a period that was as short as 20 million years within the first 100 million years after the formation of the Moon (at ~4.5 Ga). The atmosphere at that time contained more carbon dioxide than at any other period thereafter. Carbon dioxide sustained greenhouse conditions, accelerated the weathering of a primitive crust, and may have led to conditions conducive to forming the building blocks of life. The conversion of inorganic carbon and nitrogen to the essential building blocks of life may have been facilitated by clays, zeolites, sulfides, and metal alloys that had been formed as the crust reacted with a warm and carbonated (seltzer) ocean. Geochemical modeling constrains the conditions favorable for the formation of these potential mineral catalysts.

Paradigm-changing discoveries about stellar and planetary evolution, the survival of organic molecules and microorganisms under extreme conditions, and geochemical environments on early Earth and other planets are sparking a synergistic dialogue between geoscientists, chemists, and biologists to understand how life originated. To achieve this goal, we must (i) explain the non enzymatic synthesis of biologically relevant organic molecules under geologically plausible conditions; (ii) overcome the rigid conceptual dichotomy of the “RNA world” versus the “metabolism-first” hypotheses; and (iii) develop high-throughput analytical systems to sample the myriad possible combinations of environmental conditions to find those that could initiate life. This issue of Elements highlight the roles of minerals and geochemical environments in the emergence of protocells, the cell-like entities that might have preceded the Last Universal Common Ancestor.