40 



Apart from those generalities, the details of all three groups differ. 

 The proteins, especially enzymes and their consequent metabolic 

 pathways, the cell walls, and most of the membranes differ. This 

 suggests that the three great groups diverged at the earliest stage, 

 with not much but the building blocks, the genetic code, and its ribo- 

 somal machinery in common. 



More than that we have not been able to read from the details 

 of the similarities of living forms. Perhaps we should have expected 

 no more. For all the forms we know now are cellular, compact; the 

 earlier forms we are looking for have still to depend for some neces- 

 sity upon features of the nonliving environment, whether it be for a 

 source of chemical free energy, the means of replication and varia- 

 tion, or the simple preservation of high concentrations of key con- 

 stituents. All the primitive cells, we infer, are microbial, enclosed, 

 and can replicate using the full normal apparatus of bacteria; they 

 derive free energy either from some part of incident sunlight or from 

 an organic-rich world or both. There is still a big gap to explore. For 

 that we must turn to the laboratory. 



SUGGESTIONS FOR FURTHER READING 



L. Margulis, Symbiosis in Cell Evolution, W. H. Freeman Co., 

 San Francisco, 1981. 



J. W. Schopf, editor, The Earth's Earliest Biosphere, Princeton Uni- 

 versity Press, 1983. 



C. W. Woese, Sci. Amer., "Archaebacteria," vol. 244, no. 6, 1981, 

 pp. 98-121. 



