Ill 



include those on organic geochemistry and isotope analyses, miner- 

 alogical and elemental analyses, and micromorphological and ana- 

 logue community analyses. 



Fossils are the records of historical events important in the 

 origin and evolution of life. They exist in three forms: (1) embedded 

 in rocks, (2) inherent in the complex, metabolic patterns of living 

 organisms, and (3) recorded in the sequences of amino acids in 

 proteins and in the sequences of nucleotides in RNA and DNA. 



The knowledge we have gained from the rock record, in recent 

 years, has improved measurably. In particular, photosynthesis, as 

 represented by the photosynthetic bacteria, appears very early in the 

 rock record. These findings are consistent with the hypothesis that 

 autotrophic (or photosynthetic) organisms are the original life forms. 

 This would upset the current conjecture that the original organisms 

 were heterotrophic and hence, would change our perspective on the 

 problem of the origin of life. The search for older rocks should be 

 actively pursued. 



To supplement the microfossil records, isotope fractionations of 

 lighter from heavier isotopes of carbon and sulfur have been used. In 

 the case of carbon the fractionation of isotopes has been interpreted 

 as due to the fixation of carbon dioxide in photosynthesis by the 

 Calvin cycle. The sulfur isotope fractionation is related to the reduc- 

 tion of sulfate to sulfide by various sulfur bacteria. 



In the realm of metabolism, the fixation of carbon dioxide by 

 various organisms by metabolic pathways other than the Calvin cycle 

 and the related fractionations of isotopes has not been extensively 

 studied. Examples are the fixations by Chlorobium (photosynthetic 

 green sulfur bacterium) of carbon dioxide by a reverse citric-acid 

 cycle, and the fixation by the methanogens of carbon dioxide by 

 metabolic pathways still unknown. Thus, the interpretation of the 

 isotope fractionation in the geological record is woefully weak on the 

 biological side. A much more systematic study of carbon dioxide 

 fixation is in order. 



In the case of sulfur isotopes, the role of sulfide to sulfur 

 metabolism in photosynthetic bacteria has been ignored in recent 

 years in spite of the postulated early appearance of H 2 S photosyn- 

 thesis in the biosphere. Again this is evidence of a poor liaison 

 between biologists and isotope geochemists. 



