Early Earth chemistry

The objective of the preceding equilibrium calculation has been to determine the state of a molecule such as an amino acid in the conditions that prevailed on the early Earth. The pH, degree of dissociation and the extent of the reaction all have a direct effect on the population of the species present. Temperature and cooperative effects have not been considered but serve to complicate the problem. Any prebiotic reaction scheme must take account of that troublesome restriction to chemistry - the second law of thermodynamics. 

The current models of the Sun suggest that its luminosity would have been some 20-30 per cent lower than its present value during the early part of the formation of the Earth. After the enormous temperatures of the Hadean period, the early precambrian may have been cooler, requiring prebiotic chemistry to occur below a layer of ice, perhaps heated by volcanic activity such as that found in geothermal vents. A layer of ice several hundreds of kilometres thick may have formed over the entire surface of the early Earth, providing protection from UV radiation and some global warming - conditions such as these may exist on the Jovian moon Europa. 

The recent developments and ideas in the field of prebiotic chemistry can be combined with the concepts noted here to produce what we regard as a research outline, rather than a detailed hypothesis, directed toward a coherent theory of the origin of complex self-contained, self-replicating chiral assemblies. In what follows we present one possible scenario that is consistent with our current knowledge of chiral induction and amplification and with the nature of early Earth as well as early life. It is exciting that this fundamental question can be formulated in a way that allows systematic experimental testing as we enter the next century. 

Silver, R. Form of Phosphate an the Early Earth in Proceedings of the Fourth Conference on Origins of Life Chemistry and Radiochemistry, Margulis, L., Ed. Springer-Verlag New York, New York, 1973, pp 215-216. 

A, 250 kPa. However difficult early-Earth atmospheric chemistry may prove to be, this particular problem is a simple one. Dalton s law states that the total pressure is simply the sum of the partial pressures of the component gases ... 

An alternative to the terrestrial synthesis of the nucleobases is to invoke interstellar chemistry. Martins has shown, using an analysis of the isotopic abundance of 13C, that a sample of the 4.6 billion year old Murchison meteorite which fell in Australia in 1969 contains traces of uracil and a pyrimidine derivative, xanthine. Samples of soil that surrounded the meteor when it was retrieved were also analyzed. They gave completely different results for uracil, consistent with its expected terrestrial origin, and xanthine was undetectable [48], The isotopic distributions of carbon clearly ruled out terrestrial contamination as a source of the organic compounds present in the meteorite. At 0°C and neutral pH cytosine slowly decomposes to uracil and guanine decomposes to xanthine so both compounds could be the decomposition products of DNA or RNA nucleobases. They must have either travelled with the meteorite from its extraterrestrial origin or been formed from components present in the meteorite and others encountered on its journey to Earth. Either way, delivery of nucleobases to a prebiotic Earth could plausibly have been undertaken by meteors. The conditions that formed the bases need not have been those of an early Earth at all but of a far more hostile environment elsewhere in the Solar System. That environment may have been conducive to the production of individual bases but they may never have been able to form stable DNA or RNA polymers this development may have required the less extreme conditions prevalent on Earth. 

If the global structures of terran life reflect origins, we must turn to models of early Earth and consider its chemistry in an effort to guess the potential for life in the solar system. Other planets and moons in our solar system have had histories different from Earth s. We would be called on to creatively define chemistries that might have originated in the early histories of those other bodies. 

Allamandola and Hudgins have considered the formation of complex organic species in ice matrices and provided a summary of the photochemical evolution on those ices found in the densest regions of molecular clouds, the regions where stars and planetary systems are formed 42 Ultraviolet photolysis of these ices produces many new compounds, some of which have prebiotic possibilities. These compounds might have played a part in organic chemistry on early Earth. 

The opportunity to study isolated molecules also offers an avenue into questions of prebiotic chemistry. To determine the rules of chemistry that may have been important on an early earth, before the onset of life, one needs to observe interactions between biomolecular building blocks in the absence of biology. 

One of the enticing consequences of the excited state dynamics of base pairs is the possible role this property may have played in chemistry on the early earth. Prior to the existence of living organisms photosynthesis would have been absent. Consequently there would have been no free oxygen in the atmosphere and no ozone layer would have existed. The earth s surface would have been exposed to deeper (more energetic) UV irradiation than is the case today. Therefore UV photochemistry is part of the set of rules that may have governed the chemistry that could take place at that time. 

The implication is that under the conditions of deep UV irradiation, which likely existed on the early earth, selective chemistry may have taken place in favor of species with the shortest excited state lifetimes. Benner and coworkers, for example, have proposed a molecular lexicon of 12 alternate bases that can produce 6 base pairs with virtually identical geometries as the guanine-cytosine (GC) triply hydrogen bonded base pair [1], Several of these alternate bases have been observed as products in simulation experiments that test the feasibility of synthesis of such compounds under primitive conditions [77, 78], This raises the... 

Since the terrestrial rock record has hitherto failed to furnish unequivocal evidence of prebiotic organic chemistry, early chemical evolution was almost certainly confined to the Hadean era bracketed by the time of Earth s formation ( 4.5 billion years ago) and the onset of the sedimentary record about 3.8 billion years ago (Figure 29). 

Clark PD, Dowling NI, Huang M. (1998) Comments on the role of H2S in the chemistry of Earth s early atmosphere and in prebiotic synthesis. J Mol Evol 47 127-132. 

Similarly, banded iron formation (BIF), a sedimentary rock produced by chemical precipitation, is extremely rare in the Phanerozoic but common in the Archaean and Proterozoic record. Explaining its origin in terms of the atmospheric or ocean chemistry of the early Earth is an important part of recovering the history of early Earth. This is discussed in Chapter 5 (Section 5.4.3.2). 

In this section we first explore the likely timing of ocean formation and then investigate the nature of ocean water chemistry in the early Earth. 

Life is also complex and the detailed chemistry of how the organic molecules which sustain and regulate life were formed is yet to be worked out. However, there was process in which simple molecules were synthesized to form complex polymers useful in the process of replication and the transmission of genetic information. Many believe that the nucleic add RNA was a precursor to the modem DNA and that life first developed in an RNA world. In addition the relevant energy sources were harnessed to facilitate life. Now these come mostly from sunlight, but in the early Earth they were probably chemical. 

Interstellar PAHs, Ices, and Chemistry on the Early Earth... 

To better understand the role these interstellar/precometary residues might have played in the chemistry on the early Earth, we have also investigated both their bulk or collective chemical properties and searched for specific molecules such as amino acids. Examples of both will be given here. 

A growing body of data on Antarctic ice cores shows that the history of volcanic eruptions is resolvable year-by-year (38). The record includes dated explosive events such as Cerro Hudson in 1991, Pinatubo-1991, Agung-1963, Krakatoa-1883, Tambora-1815, and a 1259 AD eruption of unknown location. Analysis of the oxygen and sulfur isotope composition of aerosol particles frozen in the polar refrigerator provides a proxy of atmospheric chemistry applicable to early Earth. Aerosols from Cerro Hudson, the smallest of the explosive eruptions, have normal sulfur isotope compositions with no anomalous fractionation of (39). Cerro Hudson s eruption cloud did not break... 

What did you think about using the context of the chemistry of early Earth and the origin of life as a way to understand and appreciate chemistry ... 

I think a (at least basic) understanding/appreciation for chemistry is essential to really understanding early earth / origins of life, and understanding where we came from is a good reason to study chemistry. ... 

---