One of the biggest questions in science is how life began on Earth. Researchers generally agree that the appearance of the first biopolymers and their building blocks marked a key step in the origin of life (OoL). But scientists still don’t know exactly how prehistoric collections of inert chemicals (gases) transformed into the first living systems.
The mystery remains difficult to solve because the sequence of events that led to the birth of life is impossible to directly observe and extremely difficult to reproduce. Over the past century, scientists have proposed numerous hypotheses, most of which focused on chemical evolution occurring on Earth or in space. However, each explanation has limitations and often relies on specific experimental results or theoretical assumptions.
Several well-known models have attempted to explain (terrestrial) chemical OoL, including the Metabolism-First World (FeS World), the Zinc World, the Thioester World, the RNA World, and the Lipid World. Each provides valuable insights, but none provides a complete explanation of how life arose from inanimate matter. No single theory has succeeded in integrating all aspects of the process into a unified and convincing scenario.
A new framework built around nanozymes
To address this challenge, Professor Yongdong Jin from the Department of Biomedical Engineering at Shenzhen University in China proposed the “nanozyme hypothesis” for OoL on Earth.
This hypothesis suggests that primitive natural mineral nanozymes (MNzymes), along with later generations of small organic molecule hybridized nanozymes, played a central role in the emergence and evolution of life. According to this idea, these substances are particularly important during the early stages of life’s development, helping to generate the first biologically relevant molecules from nonliving materials.
Under primitive Earth conditions, MNzymes may have gradually transformed prehistoric inert chemicals (gases) into increasingly complex molecules through a combination of chemical (and physical) processes. The authors propose that this change occurred primarily through a process called “inorganic photosynthesis.”
Multiple roles in early chemical evolution
The nanozyme hypothesis assigns several important functions to natural MNzymes. These include (a) catalysis, (b) surface binding/confinement, (c) anti-UV irradiation, (d) (photo)selection, and (e) energy flow management.
By playing these roles, MNzymes may have influenced early chemical reactions that used natural energy sources such as light, heat, and electricity. This hypothesis further suggests that they help convert energy into molecular information stored in molecules (and entities) that can be read, written, and replicated. Such abilities are considered to be an essential prerequisite for the emergence of living systems.
Earth as a giant natural laboratory
This hypothesis holds that the Earth itself could, under harsh primordial conditions, gradually give rise to an organic world from an initially all-inorganic environment, an idea broadly consistent with the concept of early abiogenesis.
In this framework, Earth functioned as nature’s “all-in-one” chemical laboratory operating over vast periods of time. Natural pressure and temperature gradients throughout the Earth (from the mantle to the crust), especially near active volcanoes and geothermal hot springs, may have provided ideal conditions for high-temperature, high-pressure lava and hydrothermal reactions.
These environments likely produced the earliest MNzymes, including metal/noble metals, metal oxides, and sulfide NPs. Remarkably, similar approaches are widely used in laboratories today to synthesize artificial nanozymes.
Over billions of years, this primitive collection of MN enzymes may have slowly evolved, updated, and become increasingly sophisticated. Some of them may have been taken up by living organisms. Hypothetically, this process contributed to the evolution of minerals and gradual environmental changes, improving conditions for the survival and development of prebiotic molecules and primitive life.
Mineral nanoparticles abundant on earth
Mineral NPs are already widespread throughout Earth’s natural environment. Thousands of teragrams (Tg) (1 Tg = 1012 g) of these particles circulate through ecosystems each year. Some have natural enzyme-like activity and are therefore classified as MNzymes.
These substances are present in oceans, water bodies, the atmosphere, and soil, and play important roles in the biogeochemical cycles of the environment.
Recent discoveries also suggest that nature may produce MNzymes more easily than previously thought. Studies have shown that NMs can be formed spontaneously in microdroplets of charged water or through weathering of natural minerals under UV irradiation. Sunlight and lightning, along with the abundant supply of prebiotic molecules at the Earth’s surface, may also provide the necessary photocatalytic and electrocatalytic conditions to support large-scale production of both pristine nanozymes and subsequent organic hybrid nanozymes.
“Au World” proposed
A particularly noteworthy aspect of this hypothesis concerns monolayer-protected gold NPs (AuNPs).
The authors argue that these particles are among the most effective MNzymes and may have held a central place in the evolutionary history of nanozymes during Earth’s OoL era. He calls this concept “Au World.”
Although today, AuNPs are generally considered as engineered nanozymes, this hypothesis suggests that AuNPs are geologically plausible under various natural conditions on Earth.
Free AuNPs generally require organic surface coatings and therefore may have struggled to remain stable in the original soup. However, when small molecules such as thiols and amines were generated (by other MNzymes) and accumulated at specific locations, the AuNPs could have survived in a protected form with a (thiol/amine) monolayer. In this way, they may have been able to participate in a broader reaction network that contributed to the emergence of life.
Four important conditions for life molecules
To further explain how life molecules are naturally selected and stabilized, the authors identify four essential elements and conditions associated with OoL on Earth.
- Wet/dry cycle and amphiphilic properties
- Self-assembly and self-organization
- Catalytic activity and protoenzyme activity
- Symbiosis and stabilization of pairings
Taken together, these factors have been proposed as fundamental requirements for the survival and evolution of early life-related molecules.
For the future
This review goes beyond nanozymes themselves and explores several other key questions related to OoL on Earth. These include the water paradox, the importance of micro-nanostructures on the Earth’s surface, and the unique physicochemical properties of water and wet/dry cycling environments that may have influenced prebiotic chemistry.
The authors also discuss additional physical aspects of OoL, including ideas related to molecular cooperation and coevolution during the early stages of the emergence of life, as well as the chiral origin of biomolecules.
Ultimately, the nanozyme hypothesis aims to provide a broader framework that may help reconcile long-standing discrepancies between competing theories of the origin of life. The authors hope this book will shed new light on one of science’s most enduring mysteries and encourage further research into the possible role of nanozymes in the emergence of life on Earth.
