366 PRECAMBRIAN/Prokaryote Fossils last common ancestor of all living organisms was a hyperthermophile, adapted to hot hydrothermal springs or to life deep in the Earth’s crust One explanation for this is that life originated in such conditions Another explanation is that hyperthermophiles were pre-adapted to survive the catastrophic period of meteorite bombardment between 4500 Ma and 3800 Ma Biochemical evidence can be taken to suggest the following evolutionary sequence of autotrophic prokaryotes, each of which used carbon dioxide as their sole source of carbon Anaerobic chemolithotrophic prokaryotes, which mainly use hydrogen produced from inorganic reactions between rock and water as their main electron source Anaerobic anoxygenic prokaryotes such as green and purple sulphur bacteria, which use photosynthesis to reduce carbon dioxide to form organic matter, with hydrogen sulphide as the electron source, in the absence of oxygen Oxygenic cyanobacteria, which use photosynthesis to reduce carbon dioxide to form organic matter, with water as the electron source, releasing oxygen These must have had an enormous impact on Earth surface processes and the biosphere, and considerable interest has been focused by astrobiologists upon their first appearance in the rock record At the time of NASA’s Viking missions to Mars in 1976, it was such photosynthetic autotrophy that scientists were hoping to find Heterotrophic prokaryotes not synthesize organic matter Like us, they use preformed organic matter as their source of carbon and can use a range of oxidants to break it down and release the energy bonds Methanogenic Archaea are among the most primitive heterotrophs alive today, living in highly reducing sediments (such as peat bogs) and releasing methane gas Sulphate-reducing bacteria use seawater sulphate (SO4) ions in the absence of oxygen, but require a highly oxidized form of sulphur (SO4), which may not have been widely available in the early ocean Aerobic heterotrophic bacteria use freely available atmospheric oxygen and are unlikely to have radiated before the so-called Great Oxygenation Event, 2450–2200 Ma ago, when various indicators of the weakly reducing planetary surface (banded iron formations (see Sedimentary Rocks: Banded Iron Formations), detrital pyrite, uraninite, and siderite) begin to disappear from the rock record and red beds start to appear This oxygenation event may relate in part to increasing rates of carbon burial in expanding cratonic basins and subduction zones and in part to the irreversible loss of hydrogen to space from the upper atmosphere While oxygen producers and consumers could have existed prior to 2450 Ma, they were probably restricted to rather local oases of oxygenation This inferred evolutionary sequence of methanogenic to sulphate-reducing to aerobic heterotrophic prokaryotes is likely, on the basis of evidence from living bacteria, to have been accompanied by an increasing yield of energy from the same amount of carbonaceous ‘food’ Significantly, this evolutionary succession closely resembles the modern distribution of prokaryotic populations within marine muds, with methanogenic Archaea lying deep within the sediment pile, aerobic heterotrophs and photoautotrophs in the upper layers of the sediment, and sulphate reducers in between Evidence for the Earliest Biosphere Biogeochemistry The fossil evidence for life on Earth gets increasingly scarce as the age of rock units increases This is because older rocks have suffered more exposure to erosion and have experienced a greater degree of alteration by metamorphism Hence, the oldest rocks on Earth (approximately 3800–3700 Ma), from Isua and Akilia in Greenland, have been too heavily metamorphosed to yield morphological evidence of life Possible traces of life must therefore be explored using biogeochemical techniques Stable isotopes of carbon from Isua and Akilia, for example, are somewhat lighter than usually expected from an inert world (ca À18% d13 CPDB cf Pee Dee Belemnite standard) This has been taken to imply that life was able to self-organize and survive the period of catastrophic meteorite impacts before about 3800 Ma (Figure 1) Such a view is now controversial for a variety of reasons The sedimentary origin of the carbonaceous grains is questionable: fractionation of carbon compounds could also have resulted from abiogenic processes or even from carbonaceous meteoritic debris The carbon may also be younger than claimed While light carbon isotopes (ca À40% to À25% d13 PDB) are commonly encountered in rocks younger than about 3500 Ma, some of these hydrocarbon compounds may also have an abiogenic origin, and precise discriminators, such as C–H ratios, H isotopes, and aliphaticity, are needed to discount this possibility Even so, most scientists assume that this 25–40% difference in carbon isotopes of carbonates and organic matter seen after about 3500 Ma provides key evidence that biological metabolic pathways (i.e autotrophic fractionation of carbon isotopes) were in place by this time