The primitive Earth

The age of meteorites tells us that the solar system - and therefore the Earth - was born roughly 4.6 billion years ago. The oldest terrestrial rocks are zircone crystals (zirconium silicates) which are 4.2 billion years old, but these stones do not tell us much apart from their age, because they are igneous, or magmatic, rocks whose melting processes have erased any trace of history. Much more interesting are the sedimentary rocks, because these were formed by materials that sank to the bottom of ancient seas, and may still contain remnants of the past. The oldest sediments have been found at Isua, in Greenland, and are 3.8 billion years old, which means that there were immense streches of water on our planet at that time, and that the first oceans had originated many millions of years earlier. 

But what was there, in addition to water, on the primitive Earth The four outer planets of the solar system (Jupiter, Saturn, Uranus and Neptune) are still made up mainly of hydrogen, helium, methane, ammonia and water, and it is likely that those same chemicals were abundant everywhere else in the solar system, and therefore even in its four inner planets (Mercury, Venus, Earth and Mars). These were too small to trap light chemicals, such as hydrogen and helium, but the Earth had a large enough mass to keep all the others. It is likely therefore that the Earth s first atmosphere had great amounts of methane (CH4), ammonia (NHJ and water, and was, as a result, heavy and reducing, like Jupiter s. 

The Isua sedimentary beds, on the other hand, contain iron compounds that could have formed only in the absence of oxygen, which means that the Earth did not have an oxidizing atmosphere for hundreds of millions of years. Those same sediments, however, contain also many types of carbon compounds, and this shows that, by Isua s times, there were in the atmosphere substantial quantities of carbon dioxide (C02), and probably nitrogen (N2), two gases which are neither reducing nor oxidizing. 

It seems therefore that the primitive atmosphere did change considerably in the first billion years, and from a highly reducing state, dominated by methane and ammonia, slowly turned into a slightly 

Another result of great interest is that the four components of the RNAs (adenine, guanine, cytosine and uracil) have all been obtained in abiotic conditions, but thymine has never been found, which means that primitive nucleic acids could have been RNAs but not DNAs. 

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In tracing the evolutionary development of iron ligands it is of interest to examine the machinery employed by organisms which carry out reactions on those substances believed to have been present on the primitive Earth. Specific substrates acted on by this group include, besides ferrous iron itself, hydrogen sulfide, hydrogen gas, methane and reduced nitrogen compounds. Species which perform photosynthesis may be presumed to have the capacity to synthesize protoporphyrin IX since this substance is an intermediate in chlorophyll biosynthesis (43). 

Once chirality is induced and amplified by some mechanism, the excess must first persist and then propagate in order to survive. A distinctive characteristic of homo-chiral protein and nucleic acid biopolymers is that they function within the enclosed environment of cells, which provide a membranous boundary structure that separates the intracellular components from the external environment. It has accordingly been postulated frequently that analogous but simpler enclosed environments must have been available and operative on the primitive Earth. 

The question as to the potential availability of the requisite amphiphilic precursors in the prebiotic environment has been addressed experimentally by Deamer and coworkers, [143,145] who looked into the uncontaminated Murchison chondrite for the presence of such amphiphilic constituents. Samples of the meteorite were extracted with chloroform-methanol and the extracts were fractionated by thin-layer chromatography, with the finding that some of the fractions afforded components that formed monomolecular films at air-water interfaces, and that were also able to self-assemble into membranous vesicles able to encapsulate polar solutes. These observations dearly demonstrated that amphiphiles plausibly available on the primitive Earth by meteoritic infall have the ability to self-assemble into the membranous vesides of minimum protocells. ... 

Cloud PE (1968) Atmospheric and hydrospheric evolution on the primitive Earth. Science 160 729-736... 

The energy from the decay of radioactive elements was probably not an important energy source for the synthesis of organic compounds on the primitive earth since most of the ionization would have taken place in silicate rocks rather than in the reducing atmosphere. The shock wave energy from the impact of meteorites on the earth s atmosphere and sur-... 

This synthesis of amino acids, called the Strecker synthesis, requires the presence of NH4+ (and NH3) in the primitive ocean. On the basis of the experimental equilibrium and rate constants it can be shown16 that equal amounts of amino and hydroxy acids are obtained when the NH4+ concentration is about 0.01 M at pH 8 and 25°C with this NH4+ concentration being insensitive to temperature and pH. This translates into a pNH3 in the atmosphere of 2 x 1(U7 atm at 0° and 4 x 10-6 atm at 25°C. This is a low partial pressure, but it would seem to be necessary for amino acid synthesis. Ammonia is decomposed by ultraviolet light, but mechanisms for resynthesis are available. The details of the ammonia balance on the primitive earth remain to be worked out. 

In a typical electric discharge experiment, the partial pressure of CH4 is 0.1 to 0.2 atm. This pressure is used for convenience, and it is likely, but never demonstrated, that organic compound synthesis would work at much lower partial pressures of methane. There are no estimates available for pCH4 on the primitive earth but 10-5 to 10-3 atm seems reasonable. Higher pressures are not reasonable because the sources of energy would convert the CH4 to organic compounds in the oceans too rapidly for higher pressures of CH4 to build up. 

A mixture of CH4, N2, and traces of NH3 and H20 is a more realistic atmosphere for the primitive earth because large amounts of NH3 would... 

The problem with sugars on the primitive earth is not their synthesis, but rather their stability. They decompose in a few hundred years at most... 

Our ideas on the prebiotic synthesis of organic compounds are based largely on the results of experiments in model systems. So it is extremely gratifying to see that such synthesis really did take place on the parent body of the meteorite, and so it becomes quite plausible that they took place on the primitive earth. 

For these reasons, it is generally felt that the interstellar molecules played at most a minor role in the origin of life. However, the presence of so many molecules of prebiotic importance in interstellar space, combined with the fact that their synthesis must differ from that on the primitive earth where the conditions were very different, indicates that some molecules are particularly easily synthesized when radicals and ions recombine. Another way of saying this is that there appears to be a universal organic chemistry, which shows up in interstellar space, in the atmospheres of the major planets, and in the reducing atmosphere of the primitive earth. 

The facts that basic proteinoids catalyze the synthesis of both peptide and internucleotide bonds and that they do this in suspensions of microspheres, i.e. in the same microlocale has led to the suggestion that microspheres on the primitive Earth were the site of development of the coded genetic mechanism 53). 

Miller SL. The atmosphere of the primitive earth and the prebiotic synthesis of amino acids. Orig Life 1974 3 139. 

Suess HE. Remarks on the chemical conditions on the surface of the primitive earth and the probability of the evolution of life. Orig Life 1975 6 9. 

Fig. 2 All terrestrial carbon was initially delivered to the primitive Earth during accretion. Much of the carbon was degraded to simple carbon compounds that could then undergo synthetic geochemical reactions to produce more-complex species. However, a fraction of the delivered organic carbon was likely to survive intact, especially during late accretion. This fraction had the potential to be incorporated into the molecular systems that gave rise to the origin of life...

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