Thematic Articles

I will argue that the driving force for the transition from geochemistry to biochemistry was natural selection operating, in its earliest stages, on inorganic materials. The most critical requirement for truly primitive evolvable systems is truly primitive genetic materials. These should have the kind of permutable structure that can hold information, and they should be able to replicate this information—very accurately for the most part. They should be like DNA in these respects. But, unlike DNA, they must do it all without any pre-evolved systems. Mixed-layer and polytypic materials will be featured in attempts to sketch what we should be looking for.

The early Earth was hot and chaotic, bombarded intensely from 4.5 to 3.8 billion years ago. In ponds near the flanks of volcanoes, feldspars and zeolites from volcanic flows and ash were alternately washed by fluids and dried, fostering adsorption and catalytic processes. Some silica-rich surfaces favored adsorption of organic molecules, including amino acids, which were produced by lightning in volcanic clouds. Catalysis then promot- ed polymerization to generate more complex molecules. Dissolution of alkali feldspars created a honeycomb of cavities, which may have acted as tempo- rary cell walls, while phosphorus released from the weathering feldspar framework was available for energy molecules. Following the emergence of the first cells, geochemical processes continued to influence biological evolution. Alkali-rich volcanoes introduced metallic elements, which served as nutrients in the food supply and may also have accelerated the rate of primate evolution prior to the appearance of hominids.

Montmorillonite, a clay mineral formed by the weathering of volcanic ash, may have played a central role in the evolution of life. Because of its structure, montmorillonite tends to adsorb organic com- pounds and this contributes to its ability to catalyze a variety of organic reactions critical to scenarios of life’s origins. We have shown experimentally that RNA molecules bind efficiently to clays and that montmorillonite can catalyze the formation of longer molecules (oligomers), thus lending support to the RNA world hypothesis. This theory proposes that life based on RNA preceded current life, which is based on DNA and protein.

Many microorganisms make extensive use of transition metal sulfide clusters in their metabolic chemistry. Similarly, transition metal sulfide minerals, e.g., pyrrhotite and pyrite, have the potential to provide the essential catalytic chemistry for Earth’s earliest life. Experiments reveal that transition metal sulfides have the capacity to both catalyze and, in some cases, participate in organosynthetic reactions that bear similarity to modern biosynthetic pathways. These experiments are buttressed by recognition of natural cases of extensive abiotic organosynthesis in the Earth’s crust—reactions that could have provided the first life with a large complement of functionally useful protobiological organic compounds.

Life arose on the young Earth as a natural chemical process. More than half a century of experimental research has underscored the dynamic interactions of atmosphere, oceans, and rocks that fostered this ancient transition from geochemistry to biochemistry. Researchers on the origin of life now conclude that rocks and minerals must have played key roles in virtually every phase of life’s emergence—they catalyzed the synthesis of key biomolecules; they selected, protected, and concentrated those molecules; they jump-started metabolism; and they may even have acted as life’s first genetic system.

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