The mystery remains difficult to solve because the full sequence of events that led to life is impossible to observe directly and extremely challenging to recreate. Over the past century, scientists have proposed numerous hypotheses, most of them centered on chemical evolution occurring either on Earth or in space. Yet each explanation has limitations, often relying on specific experimental findings and/or theoretical assumptions.

Several well known models have attempted to explain the (terrestrial) chemical OoL, including the Metabolism-first world (FeS world), Zinc world, Thioester world, RNA world, and Lipid world. While each provides valuable insights, none offers a complete explanation of how life emerged from nonliving matter. No single theory has successfully integrated all aspects of the process into a unified and convincing scenario.

A New Framework Built Around Nanozymes

To address this challenge, Prof. Yongdong Jin of the School of Biomedical Engineering at Shenzhen University in China has proposed the "nanozymes hypothesis" for the OoL on Earth.

The hypothesis suggests that primitive natural mineral nanozymes (MN-zymes), along with later generations of organic small molecule hybridized nanozymes, played a central role in the emergence and evolution of life. According to this idea, these materials were especially important during the earliest stages of life's development, helping generate the first biologically relevant molecules from nonliving substances.

Under primitive Earth conditions, MN-zymes may have gradually converted prehistoric inert chemicals (gases) into increasingly complex molecules through a combination of chemical (and physical) processes. The author proposes that this transformation occurred primarily through a process described as "inorganic photosynthesis."

Multiple Roles in Early Chemical Evolution

The nanozymes hypothesis assigns several important functions to natural MN-zymes. These include (a) catalysis, (b) surface binding/confinement, (c) anti-UV irradiation, (d) (photo-)selection, and (e) energy flow management.

By carrying out these roles, MN-zymes may have influenced early chemical reactions using natural sources of energy such as light, heat, and electricity. The hypothesis further suggests that they helped convert energy into molecular information stored in molecules (and entities) that could be read, written, and duplicated. Such capabilities are considered essential prerequisites for the emergence of living systems.

Earth as a Giant Natural Laboratory

The hypothesis views Earth itself as capable of gradually producing an organic world from an initially all-inorganic environment under harsh primordial conditions, an idea broadly consistent with earlier abiogenesis concepts.

In this framework, Earth functioned as a natural "all-in-one" chemistry laboratory operating over immense periods of time. Natural pressure gradients and temperature gradients throughout the planet (from mantle to crust), particularly near active volcanos and geothermal hot springs, may have provided ideal conditions for high temperature/high pressure lava reactions and hydrothermal reactions.

These environments could have generated the earliest MN-zymes, including metals/noble metals, metal oxides, and sulfide NPs. Notably, similar approaches are widely used today in laboratories to synthesize artificial nanozymes.

Over billions of years, this primordial collection of MN-zymes may have slowly evolved, renewed itself, and become increasingly sophisticated. Some may even have become incorporated into living organisms. According to the hypothesis, this process contributed to mineral evolution and gradual environmental changes that improved conditions for the survival and development of prebiotic molecules and primitive life.

Abundant Mineral Nanoparticles on Earth

Mineral NPs are already widespread throughout Earth's natural environments. Every year, thousands of terragrams (Tg) (1 Tg = 10¹² g) of these particles circulate through ecosystems. Some possess natural enzyme-like activity and are therefore classified as MN-zymes.

These materials are found in oceans, waters, the atmosphere, and soils, where they play important roles in environmental biogeochemical cycles.

Recent discoveries also suggest that nature may produce MN-zymes more easily than previously thought. Studies have shown that NMs can form spontaneously through the weathering of natural minerals in charged water microdroplets or under UV irradiation. Sunlight and lightning may further provide the photocatalytic and electrocatalytic conditions needed to support the large scale production of both primordial nanozymes and later organic hybrid nanozymes, along with a rich supply of prebiotic molecules on Earth's surface.

The Proposed "Au World"

A particularly notable aspect of the hypothesis involves monolayer-protected gold NPs (AuNPs).

The author argues that these particles may have been among the most effective MN-zymes and could have occupied a central place in the evolutionary history of nanozymes during the OoL on Earth. He refers to this concept as the "Au world."

Although AuNPs are commonly regarded as artificial nanozymes today, the hypothesis suggests they were geologically plausible under a variety of natural Earth conditions.

Free AuNPs may have struggled to remain stable in the original soup because they generally require organic surface coatings. However, once small molecules such as thiols and amines were produced (by other MN-zymes) and accumulated in certain locations, AuNPs may have persisted in (thiols/amines) monolayer-protected forms. In this way, they could have participated in the broader network of reactions that contributed to life's emergence.

Four Key Conditions for Life Molecules

To further explain how life molecules may have been naturally selected and stabilized, the author identifies 4 essential elements and conditions related to the OoL on Earth:

• Wet-dry cycling and amphiphilism
• Self-assembly and self-organization
• Catalytic and protoenzyme activity
• Pairing symbiosis and stabilization

Together, these factors are proposed as fundamental requirements for the survival and evolution of early life related molecules.

Looking Ahead

The review extends beyond nanozymes themselves and explores several other major questions related to the OoL on Earth. These include the water paradox, the importance of the micro-nano structure of Earth's surface, and the unique physicochemical properties of water and dry-wet cycling environments that may have influenced prebiotic chemistry.

The author also discusses molecular cooperation and co-evolution during the earliest stages of life's emergence, as well as additional physical perspectives on the OoL, including ideas related to the chiral origin of biomolecules.

Ultimately, the nanozymes hypothesis is intended to provide a broader framework that may help reconcile long standing disagreements among competing origin-of-life theories. The author hopes it will shed new light on one of science's most enduring mysteries while also encouraging further research into the possible role of nanozymes in the emergence of life on Earth.