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The Alternate View

Life, RNA, and Asteroids

by John G. Cramer

The origin of life on primordial Planet Earth, which occurred around four billion years ago, is a deep mystery. It seems almost impossible that random molecular events on the cooling planet could have led to the intricate living plants and animals that we see around us today. Yet here we are! Nature somehow produced us from the hydrogen leftovers from the Big Bang and the nuclear ejecta of supernovas. How could that have happened?

Ribonucleic acid, or RNA, is believed to be the key intermediate step in that transition from inert matter to self-replicating life. The “RNA World” hypothesis, first proposed by Rich in 1962, suggests that self-replicating RNA molecules were present as the Earth was cooling, well before the appearance of DNA or proteins, and this RNA provided the essential bridge to the development of life, and, in particular, to the development of more complex prebiotic self-replicating systems.

So what is RNA? It is a long molecular chain, with chain links made from four possible nucleotide sub-molecules (nucleobases), the single-ring pyrimidines cytosine (C) and uracil (U), and the double-ring purines adenine (A) and guanine (G). The RNA chain strings a combination of these ingredients together like beads on a sugar-phosphate "backbone" of the sugar ribose. Note that the structure of DNA is quite similar, but DNA has a sugar-phosphate backbone of deoxyribose (ribose with one oxygen missing), it incorporates the pyrimidine thymine (T) in place of uracil (U), and it forms an information-redundant double-helix, while RNA does not.

Important active forms of RNA, called ribozymes, were discovered in 1982. These special RNA molecules, acting as enzymes, can initiate and enhance biochemical reactions and can replicate RNA itself. Thus, RNA serves double duty, acting both as a genetic material (like DNA) to store biological information and as a biological catalyst for the self-replication of more RNA molecules. That reproductive capability, given a few billion years of natural selection, led to DNA, proteins, cells, and ultimately complex living organisms. But where did the starting point, primordial RNA, come from in the first place?

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The geological record suggests that RNA first appeared near the end of Earth’s Hadean Epoch, about four billion years ago, when the cooling Earth had an atmosphere composed of mainly of hydrogen, helium, water vapor, carbon monoxide and dioxide, and nitrogen. The question is, did primordial RNA somehow assemble naturally in that environment, was it seeded from some external source, or did both processes play a role?

The famous Miller-Urey Experiment (1952) attempted to investigate whether in the Hadean Epoch a combination of warm pools of water, a reducing atmosphere, and strokes of lightning might produce the organic molecules needed as the precursors of life. They placed their guess for the components of Hadean Earth’s atmosphere (water vapor, methane, ammonia, and hydrogen) in a warmed reaction vessel. Then they fired continuous high-voltage electrical sparks between two electrodes in the vessel to simulate lightning. They reported that twenty complex organic compounds had been produced, including five amino acids. Later more sophisticated analysis of Miller’s archived samples revealed that well over twenty amino acids had actually been produced.

However, Miller’s guess about the Hadean atmosphere was wrong. Present estimates of Hadean Earth’s atmosphere indicate that it did not contain much methane or ammonia. Present estimates suggest that the dominant Hadean atmospheric components were molecular hydrogen, helium, water vapor, carbon dioxide, and some carbon monoxide, cyanide, and molecular nitrogen, to which volcanoes may have added sulfur dioxide and hydrogen sulfide.

Later work along the same lines, with better atmosphere replication, has shown that variants of the Miller-Urey experiment produce many of the amino acids, hydroxy acids, purines, pyrimidines, and sugars. Note that the latter three are the building blocks of RNA. Further, a Miller-Urey-inspired experiment on a mixture of ammonia, carbon monoxide, and water, using spark discharges to simulate lightning and laser-induced shock waves to simulate asteroid impacts, demonstrated that all four of the nucleobases (GACU) needed for RNA were formed, along with many amino acids. Thus, there is some plausibility to the conjecture that RNA may have occurred naturally in the conditions of late Hadean Earth.

However, of the 20 “natural” amino acids, the ones that are encoded for protein formation in our DNA, only 12 of them are produced in the best Miller-Urey follow-on experiments. The missing eight natural amino acids that were not produced are cysteine, histidine, lysine, asparagine, proline, arginine, tryptophan, and tyrosine.

Interestingly, a DNA study of the most primitive genes found in common in a broad range of living organisms found that the gene coding for protein synthesis in these genes focused on these same 12 amino acids. This suggests that the original genetic protein-formation code may have specified fewer amino acids than the 20 natural ones that are presently coded for in today’s organisms. Perhaps evolution has been adding more amino acids to the protein repertoire along the way.

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Besides natural synthesis on Hadean Earth, there is another possible source of the nucleobases needed for assembling RNA. The time at which primordial RNA first appeared on Earth coincides roughly to the period that astronomers call the Late Heavy Bombardment. This was a relatively short geological period in which many massive planetesimals from the outer Solar System were somehow dislodged from stable orbits there, possibly as a result of a transient orbital resonance between Jupiter and Saturn that shook things up gravitationally. (See AV 151 in the March 2010 Analog).

Some fraction of these dislodged planetesimals fell inward. The icy and rocky infalling objects bombarded the cooling inner planets of the Solar System. One of them, a Mars-size planetesimal, made a glancing blow with Earth to produce the Moon. Other planetesimal collisions later produced most of the Moon’s craters, gave the Earth more mineral-rich mass, and supplied water for Earth’s developing oceans.

In the context of RNA formation, one sub-class of stony meteorites that participated in the Late Heavy Bombardment is particularly interesting. These are the carbonaceous chondrite meteorites of type CM2. Meteorites of this type have been found to contain a rich mixture of complex organic compounds, including amino acids and purine/pyrimidine nucleobases. Curiously, of the five RNA/DNA component nucleobases (GACTU), some CM2 meteorites contained around 300 parts per billion of adenine and guanine and around 50 parts per billion of uracil, but there was no trace of cytosine or thymine.

In an attempt to understand the extraterrestrial production of RNA components, Pachen and coworkers have recently simulated the environment of the parent carbonaceous chondrite planetesimals, those objects in which the nucleobases were formed. They assume these to be carbonaceous asteroids with radii between 4 km and 150 km that, before the Late Heavy Bombardment, had been orbiting the Sun at orbital radii of 2 to 5 AU. These parent planetesimals contain water, the basic pre-biotic molecules involving H, C, N, and O, and some supernova-produced short- and long-lived radioactive materials, particularly 26Al, 60Fe, 40K, 232Th, 235U, and 238U. The radioactive decay of these unstable isotopes produced internal heat over the timespan of the planetesimal, which is on the order of 1–10 billion years. There is a time period in which the parent planetesimals become quite hot (~450 K) at the center, cooler at the surface, and in the intermediate region there is a thermal region where water is a liquid, the pressure is fairly high, and the brewing of complex organic materials from the resident volatile initial content of water, carbon monoxide, molecular hydrogen, molecular nitrogen, hydrogen cyanide, ammonia, and methane is quite efficient.

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The Pachen simulation accurately predicts the concentrations of adenine and guanine observed in carbonaceous chondrite meteorites of type CM2. However, the simulation has a few problems. There is more uracil observed in the meteorites than the simulation predicts, and the absence of cytosine and thymine is not predicted. Note that the accurate predictions involve the double-ring purines adenine and guanine, while the problems involve the single-ring pyrimidines cytosine, uracil, and thymine. The authors speculate that some chemical process not included in their simulation may have converted all the produced cytosine and thymine into uracil, accounting for both the absence of C and T and the overabundance of U.

In any case, it appears that the Late Heavy Bombardment may have supplied Hadean Earth with a quantity of the nucleobases guanine, adenine, and uracil, but it did not provide any cytosine. Thus, while RNA chains could perhaps have been formed from these three components, they would contain only three of the four nucleobases present in contemporary RNA. Development of the full RNA chain would not have been possible without some chemical help.

So how could full RNA form? One possibility is that in the conditions of Hadean Earth, some of the asteroid-derived U may have been converted back into C. To my knowledge, this has not been analyzed. The other possibility, perhaps the more likely one, is that the original nucleobases that produced the RNA leading to organic life forms was derived from those provided from both asteroid bombardment and lightning-induced Miller-Urey chemical processes.

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In any case, we are gaining understanding of how life initially formed on Earth. From that understanding we can develop a better idea of how life might form elsewhere in the galaxy and how likely that is. Most extrasolar Earth-size developing planets in a parent star’s habitable zone would pass through a phase in which the conditions would be much like those of Hadean Earth. If that is sufficient for the formation of RNA, then the occurrence of at least primitive life on such planets may be fairly common.

On the other hand, if the RNA components must be brewed up in the inner recesses of carbonaceous asteroids and delivered by an equivalent of the Late Heavy Bombardment, then the odds of extrasolar life become much longer. Not many developing planetary systems would experience an orbital resonance between two gas giants that would produce a system-wide displacement of planetesimals, leading to the equivalent of a Late Heavy Bombardment. If that is required, then the occurrence of life like that of Earth may be fairly rare.

NASA will be looking for evidence of life on Mars, moons of our Solar System, and on extrasolar planets. Watch this column for further developments.

 

References:

The RNA World:

Alex Rich, “On the problems of evolution and biochemical information transfer,” Horizons in Biochemistry, edited by M. Kasha and B. Pullman, Academic Press, New York, 103-126 (1962).

M. Neveu, H.-J. Kim, and S. A. Benner, “The ‘Strong’ RNA World Hypothesis: Fifty Years Old,” Astrobiology 13 (4) 391-403 (2013).

Miller-Urey-Derived RNA Components:

Stanley L. Miller and Harold C. Urey "Organic Compound Synthesis on the Primitive Earth", Science 130 (3370), 245-251 (1959).

A. Lazcano; J. L. Bada “The 1953 Stanley L. Miller Experiment: Fifty Years of Prebiotic Organic Chemistry,” Origins of Life and Evolution of Biospheres 33 (3), 235—242 (2004).

Asteroid-Derived RNA Component Simulation:

Klaus Paschek, et al, “Meteorites and the RNA world II: Synthesis of Nucleobases in Carbonaceous Planetesimals and the Role of Initial Volatile Content,” preprint arXiv:2112.09160v1 [astro-ph.EP], (2021).

 

Copyright © 2022 John G. Cramer

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