112 



Thus, the lack of a coherent attack on the problem is evident 

 in carbon and sulfur isotopic fossil geochemistry and a close collab- 

 oration between the biochemist and the isotope geochemist is 

 imperative to solve this important problem. 



THE CELL 



Studies in recent years have shown that the halophiles (e.g., 

 Halobacterium halobium) create a proton gradient across their mem- 

 branes after the absorption of light by bacteriorhodopsin. This is 

 considered by many to be the most primitive photosynthetic model 

 which we have available for study. As noted in chapter III the halo- 

 philes belong to the archeabacteria. These organisms which include 

 the methanogens have very different cell membranes and 

 coenzymes, and a protein-synthesizing machinery which has proper- 

 ties intermediate between procaryotic and eucaryotic cells. A cur- 

 rent conjecture is that the archeabacteria branched off from other 

 bacteria about 3.4 b.y. ago. Therefore, a detailed comparison of the 

 two should push our knowledge of the biological record back. A 

 coherent and systematic use of sequences of amino acids and nucleo- 

 tides to clarify this split has just begun. The continual investigation 

 of these sequences will be important in elucidating the nature of 

 early life for us. 



Among the kinds of questions or problems that should be 

 addressed is a general one — the description and reconstruction of 

 the universal common ancestor. At what stage in evolution did the 

 entity exist? Was it preprocaryotic? Why was there a common 

 ancestor? Was it chance? Was it necessary? What are the salient 

 differences among the three major lines of descent? 



In addition, the problem of evolution of metabolism needs to 

 be addressed. What was the nature of pregenetic "metabolism"? 

 Was it a basically dark reaction, solution biochemistry as is com- 

 monly believed? Or was it basically a "membrane" (surface) chemis- 

 try? Although pregenetic, was it nevertheless cellular? Did a primi- 

 tive, pregenetic metabolic network develop any refining quality; i.e., 

 did it tend to organize itself, become restrictive, more specific? What 

 were the primitive catalysts? Reaction centers? How do they relate 



