This section attempts to clarify Origins of Earth and Life, Section 4.7.  The course of evolution of Archaean prokaryotes is inferred to have been something like this (possibly!):

Heterotrophic organisms feeding on abiogenically produced amino acids and other organic molecules may have been the very first life, near the end of the Hadean/beginning of the Archaean (c. 3.8 Ga).

Anaerobic photosynthesizers (autotrophes) displaced any preceding heterotrophes and came to dominate the Archaean.  In the absence of free oxygen these autotrophes obtained energy by a 2-stage chemical process linking anaerobic photosynthesis with fermentation. Anaerobic photosynthesis produced carbohydrates using CO2 (which was abundant in the atmosphere), H2 as a reducing agent (possibly supplied by hydrogen sulphide, H2S, at hydrothermal vents) and energy from sunlight: [Equation 4.2] Note that the sulphur (2S) liberated in the above equation may be the source of the sulphates mentioined on p. 117 as being found in association with the oldest stromatolites. Fermentation then broke down the carbohydrates to supply th energy needed to build amino acids and proteins: [Equation 4.5]

Aerobic (oxygenic) photosynthesis arose during the Archaean and began to displace anaerobic photosynthesis.  The oxygen so released was the Earth's first experience of air pollution.  Aerobic photosynthesizers would also have used a 2-stage process of photosynthesis and fermentation: For aerobic photosynthesis, they would have obtained hydrogen from water (H2O), not from hydrogen sulphide (H2S): [Equation 1.4] So aerobic photosynthesizers produced O2 but did not use it. Note that this form of photosynthesis would have been sustainable because the abundant reduced iron (Fe2+) in seawater and iron in surface rocks would have acted as a sink, removing any free oxygen from the environment by oxidation reactions.  That's why these aerobic photosynthesizers did not poison themselves! Fermentation broke down the carbohydrates to release energy: [Equation 4.5]

Aerobic respiration, which replaced fermentation in the 2-stage process, arose when organisms evolved which could use the free oxygen liberated as a by-product of aerobic photosynthesis.  So the 2-stage process of liberating energy became: aerobic photosynthesis as before: Respiration (the reverse of aerobic photosynthesis) instead of fermentation: [Equation 4.6]

The coupling of aerobic photosynthesis with respiration set the stage for the evolution of eukaryotes.

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FOR	AGAINST
Carbonate globules in fissures. Interpreted by NASA to have been precipitated 3.6 Ga by water from the surface seeping into fractures in the Martian crust and fossilizing nanobacteria living in the fractures.	Sr/Rb ratios in the meteorite indicate a young age of 1.39 Ga, long after liquid water had ceased to be present at the Martian surface. Carbonate globules might have been deposited by hot CO2-rich fluid flung out by the impact that sent ALH84001 into space.
Tiny tubes on the surfaces of carbonate globules interpreted by NASA as microfossils of nanobacteria. Ovoid & tube-shaped bodies typically 380 nm long (250 nm is the minimum for terrestrial bacterial to contain the necessary genetic material). Smaller ovoids & tube-shaped bodies (40-80 nm) may be fragments of larger units (such fragments are observed in terrestrial bacteria). The tubes are strikingly similar in size & shape to bacterial microfossils in the 17 Ma Columbia River Basalts from Washington State.	Small size of "fossils" (c. 200 nm) puts them at the limit of being able to accommodate genetic material. No cell walls or internal structures have been observed in the "fossils".
Enrichment of carbonate globules in 18O compared to the surrounding rock, cited by NASA as evidence that the globules were formed from a water rich fluid at temperatures of 0° C - 80° C.	NASA results failed to take into account the loss of light oxygen isotopes from the Martian atmosphere by "sputtering" (removal of lighter isotopes in the atmosphere by the solar wind). Adjustment for sputtering suggests that the carbonate globules formed at temperatures of 40° C to 250° C.
Enrichment of the carbonate globules in 12C (i.e. a negative value of d13C), taken as evidence that the carbon is organic.	Enrichment in 12C is what would be expected from the carbon isotope ratios in the Martian atmosphere.

Magnetite (Fe3O4) & iron sulphide particles found in the carbonate globules have a size, purity, morphology & crystal structure typical of those left by terrestrial bacteria. Some magnetites in ALH84001 arranged in chains precisely like those left by terrestrial bacteria, when magnetite crystals produced by biological processing of iron and oxygen in water align themselves with the Earth’s magnetic field.	Imaging of 50 nm slices of magnetite particles reveals that the crystals grew like a staircase around a spindle. This form of growth is unknown in magnetite left by terrestrial organisms. It is only known in magnetite crystals that grow at fumaroles by the condensation of gases at temperatures of 500° C to 800° C.  Sulphur isotope ratios from the iron sulphides indicate an inorganic origin.
Polycyclic aromatic hydrocarbons (PAHs) like those left by the decay of terrestrial micro-organisms are found concentrated deep inside ALH84001, but not on the surface. Cited as evidence that the PAHs are not the result of terrestrial contamination. PAHs are always located in carbonate-rich regions of ALH84001. This distribution rules out terrestrial contamination or an extraterrestrial source other than Mars. The carbon isotopic composition of the PAHs is consistent with output by methanogenic bacteria.	PAHs probably got into ALH84001 by contamination on Earth. The same PAHs found in the meteorite have been found in the ice at the discovery site. The meteorite is black, and would heat up in sunlight, melting the ice around it and allowing water containing PAHs to seep inside. PAHs are found experimentally to collect on the surfaces of carbonate globules in the presence of water. PAHs are found in other non-Martian meteorites found in Antarctica, and in meteorites found elsewhere which are not fragments of planets. PAHs are destroyed by ultraviolet light. UV light is strong in Antarctica. This may explain why PAHs are found only on the inside of ALH84001.
The occurrence of PAHs, carbonate globules, magnetite and iron sulphide particles, and "nanofossils" all together can best be explained as the result of biological activity, even though inorganic explanations are possible for each feature by itself.	Alternative, inorganic explanations are available for each feature.
And now the latest twist in the argument - from a report in Nature, Vol. 410, 8 March 2001
Chains of magnetite crystals in ALH84001, identified on high-power electron microscope images, are said to resemble chains of magnetite used by terrestrial bacteria to orientate themselves in the Earth's magnetic field.  The chains are said to be too orderly to have been formed other than by living organisms.  Furthermore, individual magnetite crystals are claimed to show characteristics of crystals produced by living organisms.	The magnetite crystals may have been produced by the decomposition of iron-rich carbonate minerals.

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What is the difference between Ccarb and Corg? Which one is implicated in the control of atmospheric oxygen?	Ccarb is oxidized carbon, such as the CaCO3 in limestones. Corg is reduced carbon, such as coal, peat and charcoal. Corg is implicated in the control of atmospheric oxygen because when carbon is buried before it has a chance to be oxidized, the oxygen that would have been consumed remains free in the atmosphere. The burial of Corg breaks the cycle of photosynthesis balanced by respiration (see Origins of Earth and Life, p. 119). Back to question
Why do the  d13C  values for Ccarb plot at around 0?	Ccarb represents oxidized carbon and so reflects the  d13C  value of the atmosphere. Back to question
Why do the  d13C  values for Corg plot as negative (c. -260/00)?	Corg represents reduced carbon in which the fractionation into organic tissue has been preserved, i.e. it was buried before it could be oxidized. Back to question
Does Figure 3.7a give any grounds for confidence that rocks dated at 3.5 Ga really do contain organic carbon?	At 3.5 Ga, the mean value and range of values of Corg are like those of younger rocks.  The gap between the Corg and Ccarb values of d13C  are nearly constant from the present back to 3.5 Ga. Back to question
Looking at the Isua rocks, why is the mean value of Corg for Isua rocks more positive than that for more recent rocks, and why is Ccarb more negative?	The original values may have been changed by metamorphism. Back to question
What do the blips at 2.7 Ga and 2.2 Ga represent on Figure 3.7a?	Stronger fractionation of carbon. Cause unknown, but see p. 58. OEL suggests a second cycle of organic processing of organic carbon, perhaps in buried sediments by methanogenic bacteria; so the blips might represent carbon that has been fractionated twice. Back to question
Why do the  d13C values for the Isua rocks provide compelling evidence of life at 3.8 Ga?	Either, The original  d13C values may have been the same as for younger rocks, implying organic processes like those that came later. Metamorphism has altered the ratios at Isua but has not obliterated the signal. Or, the  d13C values at Isua are real (unaltered by metamorphism). They would imply organic activity by organisms that fractionated carbon less strongly than "modern" oxygenic photosynthesis. Back to question
With reference to Atmosphere, Earth and Life, explain why d13C values become less negative when the rocks are metamorphosed.	Preferential loss of 13C-depleted material (because the lighter 12C is fractionated into volatiles) shifts the ratio in what remains in favour of 13C, raising the d13C values. Back to question
Can you recall from Video 2, The K/T Event, that d13C values for the shells of Cretaceous marine phytoplankton are positive? Can you recall the reason given?	Organisms preferentially use 12C in their living tissues, leaving an excess of 13C to be incorporated in their shells. Back to question
Does the Isua data give any grounds for arguing that anoxygenic photosynthesis prevailed at 3.8 Ga?	The Isua Corg values might indicate organisms that fractionated carbon less strongly than "modern" oxygenic photosynthesis. Look at green sulfur bacteria and methanogenic bacteria on Figure 3.7b. Back to question

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