SIMPLE LIFE 



127 



like organism viruses become active. The virus at- 

 taches to the host's surface and injects its nucleic 

 acids into the host. Once inside the host cell the 

 nucleic acids use the cell's materials to form identical 

 nucleic acids. Next the nucleic acids apparently com- 

 bine into groups and each group creates a protein 

 covering. Therefore, each grouping of nucleic acids 

 within a protein covering is a new virus. Finally, the 

 viruses leave the host cell, often destroying the host 

 in the process. 



Another problem is how viruses fit into the scheme 

 of past evolution. Biologists generally agree that 

 modern forms are relatively complex and that archaic 

 viruses probably were far simpler, perhaps no more 

 than pure groupings of nucleic acids. However, their 

 origin is a subject of debate. Some biologists believe 

 viruses are living descendants of the early evolu- 

 tion stage prior to the organization of cell-like struc- 

 ture. This would mean that viruses are older than 

 monerans. Other biologists, perhaps most, do not 

 place viruses in the main evolutionary stream of life. 



Rather, these biologists assume that viruses repre- 

 sent an early escape of nucleic acids from living 

 organisms, perhaps archaic monerans. In spite of 

 this disagreement, it is regularly assumed that a 

 nucleic acid or virus-like stage did occur during the 

 origin of life. Therefore, to some biologists viruses 

 probably are descendants from that stage; to other 

 biologists viruses just happen to resemble the nucleic 

 acid stage. 



Even if one could assume that viruses do represent 

 a step in evolution prior to the origin of Monera, 

 one still does not solve the problem of their being 

 living or nonliving. Possession of the life char- 

 acteristics due to the presence of nucleic acids is 

 counterbalanced by their lacking either the chemical 

 complexity associated with definite living substance, 

 protoplasm; the cell-like structure (other than nuclear 

 material) found in definite organisms; or the organi- 

 zation one can call an organism. Moreover, this 

 statement would be accepted by most biologists who 

 prefer to consider viruses living. 



MONERA 



The Monera are the oldest known kind of life. The 

 oldest known fossils seem to be monerans, perhaps 

 blue-green algae. These early forms probably were 

 different in many respects from any living group of 

 organisms, even living blue-green algae (Cyano- 

 phyta). However, the gross structure of the oldest 

 fossils, dated at no less than 2.7 billion years old,* 

 is said to be much the same as that of present 

 Cyanophyta. -Likely fossils of organisms that appear 

 to represent bacteria, blue-green algae, and fungi 

 have also been found; they are slightly less than 2 

 billion years old. The bacteria and blue-green algae 

 are fairly well represented in deposits from this lat- 

 ter time onward. 



Because the archaic Monera seem complex, the 

 origin of life might date back some 4 billion years. 

 According to recent approximations this would place 

 the origin some one billion years after the formation 

 of our planet, perhaps soon after the origin of the 

 oceans. As indicated on the geological calendar 

 (Table 7.1, p. 109), the earliest conditions of the earth 

 often are assumed to have been quite unlike those of 

 today. When the earth first solidified, the land was 



•Ahrens, L. H., 1955. Gcol. Soc. America, Special Paper, 62. 



bare rock and the atmosphere probably had only a 

 trace of oxygen. The air, perhaps, was primarily 

 poisonous ammonia and hydrogen sulfide, but had 

 carbon dioxide and water vapor as well. When these 

 gases cooled and the first oceans formed, the chem- 

 icals necessary for life to form most likely were 

 present. 



There is good evidence to suggest that this radically 

 different set of geological conditions, the Preactualis- 

 tic Phase, continued in progressively less intense 

 form up to a little less than 2 billion years ago. 

 At this time the light ammonia and hydrogen sulfide 

 gases already had been used by organisms or lost to 

 space and probably little more than the present con- 

 centrations of these gases remained. 



Because the Preactualistic atmosphere had little 

 or no oxygen, the earliest forms of life had to be much 

 like some present-day bacteria. These modern bac- 

 teria form their own foods from basic materials with- 

 out using oxygen, and some cannot even exist in 

 oyxgen. If the first life was the same as this group of 

 living bacteria, during the Preactualistic Phase the or- 

 ganisms could have lived in direct contact with the 

 early, oxygen-free atmosphere. 



The loss of ammonia and hydrogen sulfide from the 



