The Origin of Life: Was the Earth Clear or Cloudy?
In discussions about the origin of life, there is debate on how organic compounds necessary for life were generated in the early Earth environment. This is because if basic organic compounds were not sufficiently generated in the natural environment, there would be no material for life to originate from.
Previously, it was thought that organic compounds required by life could only be produced by living organisms. However, with the chemical experiments of Stanley Miller, it was proven that basic organic compounds could be synthesized naturally from inorganic substances. Subsequent research has shown that various organic compounds can be synthesized under different conditions.
On the other hand, it is not well understood how much of these organic compounds were produced in the early Earth environment. Therefore, discussions about the origin of life often implicitly assume that such organic compounds were rare.
However, I question this assumption. Rather, it is conceivable that the early Earth environment was filled with far more abundant organic compounds than it is today. This is based on an estimate derived from what I call the “Opaque Atmosphere Hypothesis.”
Qualitative Estimation of the Total Amount of Matter
The total amount of existing matter changes based on the rates of production and decomposition. If the decomposition rate is faster, the total amount of matter increases over time. As the total amount of matter increases, there is less material available for production, or the amount decomposed per unit time increases, leading to a balance between production and decomposition rates over time.
It is important to recognize that many substances do not decompose without external factors. This is also true for basic organic compounds. By organizing the factors for production and decomposition, and qualitatively assessing the balance between production and decomposition rates, we can estimate the total amount of organic compounds in the early Earth environment to some extent.
Early Earth Environment
In the early Earth, volcanic ash and fine dust from meteorite impacts would have been suspended in the atmosphere.
It is believed that the Earth, along with the solar system, was formed about 4.6 billion years ago. Initially, the Earth was a scorching planet with exposed magma, which eventually cooled, and by around 4 billion years ago, solid rock landmasses and liquid water seas had formed.
The atmosphere of this period was characterized by having much less oxygen compared to the current atmosphere. Oxygen molecules are thought to have been produced and accumulated in the atmosphere by living organisms.
During this time, volcanic activity was very active, and volcanic ash was dispersed throughout the atmosphere. Frequent meteorite impacts would have also contributed to fine particles from meteorites and rocks being suspended in the atmosphere.
While volcanic ash and rock particles eventually settled on the surface over time, they could remain suspended in the atmosphere for extended periods due to air currents. Even today, a significant volcanic eruption can cause fine ash to linger in the atmosphere for months to years, slightly reducing the amount of sunlight reaching the Earth’s surface and impacting crops.
Considering this, it is plausible that the early atmosphere was covered by particles from volcanic ash and meteorite impacts, making it opaque, unlike the present atmosphere.
Synthesis of Organic Compounds on an Opaque Atmospheric Surface
Volcanic ash and rock particles suspended in the atmosphere are minerals. Some of these contain materials that can become part of organic compounds, and some have catalytic properties that promote organic compound synthesis.
Although there were no oxygen molecules in the atmosphere, it contained carbon dioxide (CO2), nitrogen (N), hydrogen (H2), and water vapor (H2O), providing most of the elements necessary for organic compounds.
Additionally, during the day, the atmosphere received ultraviolet rays and solar heat from the sun as energy sources. Given the opacity of the atmosphere, it is likely that volcanic ash and rock particles in the outer layers of the atmosphere were able to utilize these energies extensively.
It is highly likely that a large amount of organic compounds were synthesized on this opaque atmospheric surface. This is the basis of the Opaque Atmosphere Hypothesis.
In addition to synthesis in the atmosphere, organic compounds on early Earth might have been obtained from meteorites, synthesis on mineral surfaces or hot springs on the surface, and synthesis at hydrothermal vents underwater.
However, the surface of the opaque atmosphere had an overwhelmingly larger usable space, and the amount of available material, catalysts, and energy was also significantly greater. Therefore, I believe it is highly likely that the surface of the opaque atmosphere functioned as a major factory for the large-scale production of basic organic compounds.
Decomposition of Organic Compounds
Basic organic compounds, while being produced in large quantities on the opaque atmospheric surface, were also decomposed.
However, as mentioned earlier, many organic compounds do not decompose automatically. They can persist if no decomposition factors are present.
The main factors for the decomposition of basic organic compounds are ultraviolet radiation, high temperatures, and specific enzymes.
In the early Earth environment, the atmosphere was thought to be opaque. Therefore, ultraviolet radiation did not reach the lower layers of the atmosphere or the surface and sea. Thus, organic compounds would be protected from ultraviolet radiation if they were not exposed on the surface of the opaque atmosphere.
Also, the presence of liquid water seas on early Earth suggests that, due to atmospheric pressure, temperatures were generally not above 100 degrees Celsius. Since basic organic compounds decompose at temperatures above 150 degrees Celsius, decomposition due to heat would be unlikely except in special places like volcanic craters.
Regarding enzymes, they did not exist in the early Earth environment due to the absence of life. Therefore, decomposition by enzymes would not occur.
Organizing these factors, it is evident that the possibility of decomposition of basic organic compounds in the early Earth environment was extremely limited.
Estimation of Organic Matter Quantity in Early Earth Environment
Despite the very limited opportunities for basic organic matter to decompose, if large-scale production was occurring, it would imply that a significant amount of organic matter accumulated in the early Earth environment.
Considering that the solidification of the Earth’s surface and the formation of land and seas occurred around 4 billion years ago, and that early life is estimated to have emerged between 3.8 and 3.5 billion years ago, there was a potential for organic matter to accumulate over hundreds of millions of years.
Some organic production and accumulation could have occurred even before the Earth’s surface stabilized, and there might have been a stage where enzymes capable of decomposing organic matter formed before the emergence of life, which introduces some variability into this timeframe. Additionally, the degree of atmospheric opacity due to volcanic activity and meteorite impacts could have varied over time.
Nonetheless, it is reasonable to assume that organic matter could have accumulated over hundreds of millions of years.
If these conditions were consistently met, then during this period, ultraviolet rays from the Sun that reached Earth would have been absorbed by atmospheric dust and particles. A certain proportion of this energy could have been used for organic synthesis. From an energy perspective, this means that some of the ultraviolet energy striking Earth was used for organic synthesis and converted into chemical energy in the organic matter.
Therefore, estimating the amount of organic matter synthesized can be approached from the perspective of energy conversion.
The calculation formula will be provided at the end of this article, but assuming that 100% of the ultraviolet energy reaching Earth was used for organic matter production, approximately 37 mm of organic matter could be accumulated on the Earth’s surface per year.
Of course, it is not feasible to utilize 100% of the ultraviolet energy for organic synthesis. It is necessary to consider what proportion would be reasonable.
This can be seen as a problem of how efficiently natural energy can be converted into other forms of energy. Relevant information for this perspective includes solar power, photosynthesis, and experiments on organic synthesis using electrical discharge under simulated early Earth conditions.
Solar panels for residential use have a conversion efficiency of about 15%. In photosynthesis, high-efficiency C4 plants have about 3%, while common C3 plants range from 0.1% to 2%. The Miller-Urey experiment, which simulated early Earth’s atmospheric conditions with electrical discharge, estimated a conversion efficiency of about 0.1%.
In the case of organic synthesis using ultraviolet light in an opaque atmosphere, ultraviolet rays are mostly absorbed by particles and converted into heat. Therefore, the actual conversion efficiency is likely to be an order of magnitude or more lower than in plants or the Miller-Urey experiment.
Assuming a conservative estimate of 0.001% efficiency, applying this to the previous calculation means that instead of 37 mm per year with 100% efficiency, it would be 0.37 μm with lower efficiency.
However, this organic matter production continues over hundreds of millions of years. Even at 0.37 μm per year, over 100 million years, it would accumulate to 37 meters. Over 300 million years, it would exceed an average thickness of 100 meters.
Without decomposition factors such as ultraviolet rays, high temperatures, and specific enzymes, this large amount of organic matter would remain mostly undisturbed. Consequently, it could have accumulated on the surface like snow, dissolved in water, settled at the bottom, or drifted near the surface in the wind.
Certainly, the actual conversion efficiency might be lower or occasionally higher than this estimate. However, even with a hundredth of the efficiency, over 300 million years, the Earth would still accumulate organic matter to an average thickness of 1 meter. Thus, if organic synthesis occurred under an opaque atmosphere as assumed, the concept of scarce organic matter on early Earth as intuitively understood might need reconsideration.
Energy Accumulation
The opaque atmosphere hypothesis also explains the accumulation of energy alongside the production of basic organic matter. As compared with solar power or photosynthesis, the energy used for organic synthesis is also chemical energy that can be retrieved when the organic matter is decomposed.
Therefore, if a large amount of organic matter accumulated in Earth’s environment, it also implies that a significant amount of available chemical energy was accumulated.
An energy model can be considered where energy is collected in the upper atmosphere and delivered to the surface, where it can be utilized. Abundant organic matter serves as both material for life and as an energy source for chemical reactions leading up to the emergence of life.
Conclusion: From Startup Model to Conglomerate Model
The abundant material and energy estimated by the opaque atmosphere hypothesis fundamentally alter the image of early Earth as a premise for the origin of life.
Traditionally, the image was of a startup company struggling with scarce resources and relying on external energy sources, where life emerged only through a stroke of luck amidst harsh conditions.
However, the opaque atmosphere hypothesis significantly changes this image.
It suggests an image more akin to a conglomerate company, with abundant resources and energy, advancing multiple ventures simultaneously and efficiently giving rise to life.
Revisiting the origin of life with this conglomerate model changes several premises. The need to assume life originated in specific localized areas is eliminated because the basic materials and energy required for life were widespread on Earth. Therefore, there is no need to restrict the emergence of life to places with abundant materials or easily accessible energy.
This allows for simultaneous chemical evolution in diverse locations, with numerous attempts occurring even if some fail. Once a viable mechanism emerged, it could rapidly accelerate or amplify using the abundant materials and energy.
Thus, under an opaque sky generating massive amounts of organic matter, the process leading to the emergence of life could have progressed concurrently.
Supplement
The calculation for estimating the accumulation of organic compounds if 100% of the ultraviolet energy irradiated to Earth were utilized for organic matter production is as follows:
<Constants used in the calculation>
Solar constant (S): 1,000 [W/m²]
UV percentage of solar radiation (f_UV): 7 [%]
Earth’s radius (R): 6,378,137 [m]
Average molecular weight of organic compounds (MW): 100 [g/mol]
Energy required to form one mole of organic compounds (E_mol): 1,000,000 [W/mol]
Density of organic compounds (ρ): 1,500 [kg/m³]
<Calculation>
Cross-sectional area of Earth
(A = π * R²): 127,801,973,348,953 [m²]
UV energy reaching Earth per second
(E_UV = S * f_UV * A): 8,946,138,134,426,690 [W]
Number of moles of organic compounds generated per second
(mol_per_sec = E_UV / E_mol): 8,946,138,134 [mol/s]
Mass of organic compounds generated per second
(mass_per_sec = mol_per_sec * MW): 894,613,813 [kg/s]
Total mass generated per year
(total_mass = mass_per_sec * 365.25 * 24 * 3600): 28,231,864,879,098,400 [kg/year]
Volume of organic compounds
(volume = total_mass / ρ): 18,821,243,252,732 [m³/year]
Thickness if spread evenly over Earth’s surface
(thick = volume / (4 * π * R²)): 36.8 [mm/year]
