"The high oxygen content of the Earth’s atmosphere is unique among the planets of the Solar System and could have been tied up with the composition of the core and its crust.
Interesting organic molecules such as sugars and amino acids can be formed from laboratory ‘atmospheres’ of different proportions of CO2, H2O, N2, NH3, H2, CH4, H2S and CO. This happens only in the absence of free O2.
Oxygen is highly reactive, breaking chemical bonds by removing electrons from them. A reducing gas (H2, CH4 or CO) is therefore thought to be essential for the successful synthesis of prebiotic organic molecules.
It has been generally accepted that at about 1.5 Ga [Giga annum = billion years ago] the oxygen content of the air rose at least 15-fold. (Note that evolutionary/uniformitarian ‘ages’ are only used here for argument’s sake.)
Before this, the oxygen had been reduced by Fe(II) in sea water and deposited in enormous bands as oxides or hydroxides on the shallow sea floors. The source of the ferrous iron was hydrothermal vents in the company of reducing gases such as hydrogen sulphide (H2S).
In 1993 Widdel and his team cultured non-sulphur bacteria from marine and freshwater muds. These anoxygenic, photosynthetic bacteria use ferrous iron as the electron donor to drive CO2 fixation. It was a signal discovery that oxygen-independent biological iron oxidation was possible before the evolution of oxygen-releasing photosynthesis. Quantitative calculations support the possibility of generating such massive iron oxide deposits dating from Archaean and Early Proterozoic times, 3.5–1.8 Ga.
It is fair to conclude that the Earth’s early atmosphere before 3.5 Ga could have significant quantities of oxygen. This should discourage the sort of hypothesising on abiotic monomer and polymer syntheses so often assumed to have occurred in Archaean times. Robert Riding says that the Grypania discovery
“ … could spell the end of BIF-dominated models of oxygen build-up in the early atmosphere … The cat really will be put among the pigeons, however, if [further] fossil discoveries extend the eukaryote record back much beyond 2200 million years ago, into what is still widely perceived to have been an essentially anaerobic world.”
A number of revised textbooks on molecular biology came out in 1994–1995 which, while conveying the standard arguments for origin-of-life hypotheses, are cautious in their affirmation.
Rightly so, because advances in the field have uncovered exquisite details of intracellular processes. These challenge superficial explanations that their origin and subsequent refinement were fed by randomness. After mentioning the famous simulation by Miller and Urey of prebiotic synthesis of organic compounds, Voet and Voet handle the riddle of the formation of biological monomers with a caveat. They write:
“Keep in mind, however, that there are valid scientific objections to this scenario as well as to the several others that have been seriously entertained so that we are far from certain as to how life arose.”The text of Molecular Cell Biology in its second edition was well indexed on the evolution of cells, describing the Miller experiment in detail.
The third edition has dropped the chapter on evolution of cells found in the second edition.
Similarly, Stryer’s fourth edition of his textbook on biochemistry makes no mention of the abiotic synthesis of organic molecules.
“Doubt has arisen because recent investigations indicateClassical mechanisms generally rely on chance for the selection of l-amino and d-sugars by self-replicating systems.
the earth’s atmosphere was never as reducing as Urey and Miller presumed. I suspect that many organic compounds generated in past studies would have been produced even in an atmosphere containing less hydrogen, methane and ammonia. Still, it seems prudent to consider other mechanisms for the accumulation of the constituents of proteins and nucleic acids in the prebiotic soup.“For instance, the amino acids and nitrogen-containing bases needed for life on the earth might have been delivered by interstellar dust, meteorites and comets.”
Mason has put forward the tantalizing speculation that a weak nuclear interaction will stabilize the l-amino acids and their polypeptides over their d-forms.
This electroweak advantage is considered too weak to affect the outcome of biochemical evolution. An imaginary flow reactor of a kilometer in diameter and four meters deep would be needed to autocatalyze a change of 10–2 to 10–3 moles of one isomer over 10,000 years if the temperature is kept at ambient. Admittedly a good thought experiment “but it will find no popular primitive Earth scenarios.”
A pillar of “prebiological evolution” has been the long period of time supposedly available for the emergence of “protocells” whose development in turn profoundly altered the climate of the planet and its geology.
For an estimated age of the Earth of 4.6 Ga this seemed initially to pose no problem. However, the discovery of stromatolites in Western Australia and in South Africa upset the timetable severely.
Iron is taken out of the earth,.. Job 28:2 |
Pari passu the list of processes thought to occur abiotically has
CMI