102 



Gradients of other ions such as sodium, potassium, and calcium 

 can be modulated by carriers or channels. Relatively simple organic 

 compounds are known which act as carriers for these ions, and it is 

 possible that analogous compounds could have arisen abiogenically. 

 It should be noted, however, that in contemporary cell membranes 

 channels rather than carriers appear to be the main mediators of spe- 

 cific ion permeabilities. 



Closed membrane compartments play, of course, a crucial role 

 in the two most important contemporary energy conversion sys- 

 tems — photosynthesis and respiration. Their evolution has been 

 extensively discussed and will not be further pursued here. However, 

 cells presumably existed before either of these mechanisms evolved 

 and our main purpose is to discuss the problems of evolution from 

 abiogenic organic matter to protocells. The speculations advanced 

 here are necessarily based on our knowledge of present-day living 

 cells. Unfortunately, fossils tell us little about the chemistry of 

 Precambrian life. 



Thus, we have been taken full circle back to the Precambrian. 

 We have traced our knowledge from the fossils dated 3.5 b.y. 

 through our speculations about the early Earth from 4.5 b.y. until 

 3.5 b.y. We must note here that this gap of almost 1 billion years is 

 slowly being filled by our experiments in the laboratory and by our 

 expanding knowledge of our Earth and solar system. We have there- 

 fore deemed it necessary to outline in the concluding chapter the 

 next steps in our quest to close the gap and elucidate the nature of 

 the origins of life on Earth. 



SUGGESTIONS FOR FURTHER READING 



A. G. Cairns-Smith: Genetic Takeover. Cambridge University Press, 



London, 1982. 

 W. Day: Genesis on Planet Earth. Talos Press, East Lansing, Mich., 



1979. 

 R. E. Dickerson: Scientific American, vol. 239, no. 3, 1978, 



pp. 70-87. 



