Earliest Life Forms

Voronkov, Zelchan, and Lukevits (4a) have summarized all the beliefs and hypotheses of the last 2000 years on this subject, including the possibilities of other life forms in the universe based on Si-Si, Si-N, or Si-C polymers. 

By far the oldest fossil remains of living organisms are the blue-green algae embedded in chert (microcrystalline silica), discovered by Barghoorn and Tyler (12) and 

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The ability to catalyse the evolution or oxidation of H2 may have been exploited by the earliest life forms as H2 would have been present in the early prebiotic environments. The origins of the proton-dependent chemiosmotic mechanism for ATP synthesis may also reflect the formation of proton gradients created by hydrogenases on either side of the cytoplasmic membrane. In addition, it has been speculated that the coupling of H2 and S metabolisms was also of fundamental importance in the origin of life. These two processes seem intimately coupled in the bifunctional sulfhydrogenase found in Pyrococcus furiosus (a combination of subunits for hydrogenase and sulfite reductase) which can dispose of excess reductant either by the reduction of protons to H2 or S° to H2S (Ma et al. 1993 Pedroni et al. 1995). 

Although these issues could, in principle, aU be covered in this chapter, some are dealt with in more detail in other chapters and, therefore, are given only cursory treatment here. Furthermore, there are major gaps in our knowledge that render a comprehensive overview unworkable. The nature of the early crust (item (vii)) is poorly constrained, although some lines of evidence will be mentioned. The nature of the earliest life forms... 

Tungstoenzymes play an important role in the metabolism of carbohydrates and peptides in the hyperthermophiles. Hyperthermophily is regarded as an ancient phenotype. It is assumed that the earliest life forms were hyperthermophilic and W-dependent, which later evolved into mesophilic molybdenum-dependent species and now are the overwhelming majority. As to the reason why W was required and not Mo, there exist two assumptions W is present in volcanic areas where hyperthermophiles can be found (e.g., deep-sea hydrothermal vents) or alternatively, the better temperature stability of the valence state in question of the W compound. 

The evolution of aerobic respiration was predated by the development of photosynthesis as this increased the concentration of carbon dioxide in the atmosphere (see Section 3.6.1). Indeed, respiration of some sort must have been characteristic of the earliest life forms because they required energy to survive and reproduce. The evolution of the eukaryotes also appears to have been dependent on earlier life forms such as the archaea and bacteria. As discussed in Section 3.4.1 cell organelles in bacteria, such as chloroplasts and mitochondria, became isolated as they developed membranes around them and subsequently the bacteria were incorporated into organisms to produce eukaryotes. This will be considered in Chapter 4. 

It is significant that the earliest records of life on Earth start shortly after the period of impact frustration. Apparently life formed as soon as the conditions permitted it. Life originated from compounds produced by prebiotic organic chemistry. The source of the molecules included those produced on Earth by energetic processes such as impacts and electrical discharges as well as those that fell in from space. Whatever processes occurred, they would have had to happen either in the deep ocean or in what might have been rare regions of land and shallow water. 

Catalysis is not a new phenomenon, but its wilful applications by humans has really begun in this century. Enzymatic catalysis is necessary for all living mater. Most essential of all catalysis processes is photosynthesis which underlay most of the simplest and earliest evolved life forms. 

The earliest known form of elective surgery was the operation of circumcision. Blood loss, although usually minimal, could be dramatic if the mohel, or priest, was unskilled. Such catastrophes provided an early focus on the critical role of blood in life and the therapeutic power conferred by transfusion. 

All cancers are diseases of abnormal cell proliferation, development and death. During the earliest stages of human life all of the embryonic cells divide constantly and differentiate to form the specialised tissues and organs. 

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