EVOLUTION OF PHO TOSYNTHETIC MECHANISMS MELVlN CALVIN* I. Introduction The planning ofthis discussionhas turned out to be particularly difficult, perhaps more difficult than any other I have undertaken. The reason for this, I console myself, lies in the very nature ofthe evolutionary process itself. In physical science (and particularly in mathematical sciences) we are accustomed to a single sequence ofevents, each idea being precursor to the next, until gradually a whole pattern ofthought is developed—a whole notion from beginning to end. Those ofyou who are more familiar with the way biological material has evolved will know that this is not really the way the living organism can be described in its evolutionary history. The subject ofthis discussion, the problem ofphotosynthesis, is especially difficult to trace. It turns out, you will see as we go along, that the evolution ofphotosynthesis entails the fusion of a number of quite independent threads of evolution at some point in time to give rise to the modern process and the modern apparatus as we know it. As I try to describe this pattern ofevents, my respect for authors of historical novels increases greatly. They have many apparently independent chains ofevents that give rise to a particular incident at the end, or perhaps at the beginning, ofthe novel and are very skilful at starting each thread and jumping from one thread to another, bringing them along so they all come together at the right time and in the right place. I haven't yet been able to move smoothly among the various evolutionary threads that are involvedhere, which ultimately fuse together to give rise to the very complex process ofphotosynthesis. The story may * Department ofChemistry and LawrenceRadiation Laboratory, University ofCalifornia, Berkeley 4, California; Research Professor ofChemistry 1960-61 in the Miller Institute for Basic Research in Science, University of California, Berkeley. The preparation ofthis paper was sponsored by the U.S. Atomic Energy Commission. 147 appear, therefore, more confused than it really is, since I mustjump back and forth among separate evolutionary threads and try to indicate their points offusion. II. Modem Photosynthetic Processes With this apology over, I will begin this study ofthe evolutionary history ofphotosynthesis by first describing what we think we know ofthe modern process at which we must eventually arrive. The process ofphotosynthesis is the process by which living organisms are able to transform electromagnetic energy into chemical energy by inducing the reaction between carbon dioxide and water to evolve molecular oxygen and reduced carbon: C02+ H20 —^-+(CH2O)nO2. This is the over-all process ofphotosynthesis which has long been recognized as a processfor transforming electromagneticenergy, hererepresented by the quantum, into chemical potential, represented by oxygen in the elementary form and the elements ofcarbon and hydrogen largely in the oxidation level ofcarbohydrate (1-3). This is the stage that was available to us roughly one hundred years ago. Ifthis were all we knew about the process ofphotosynthesis, we would be hard pressed to try to reconstruct an evolutionary history which might have given rise to this process. Fortunately, in the last decade or two we have learned perhaps more about the process ofphotosynthesis than in the previous one hundred years. Our chemical knowledge of photosynthesis increased only slowly untiljust prior to World War II, beginning in the middle thirties, and then at an increasingly rapid rate after the war. What do we know today about the process ofphotosynthesis? Rather than try to give you a history of how the knowledge has evolved, I am going to (a) put down some ofthe established things that we know about photosynthesis, represented by the over-all reaction, (b) examine the types oforganisms which perform this process, (c) determine what the biological apparatus is within some ofthe organisms (so far as we can), and (d) finally go further on down to the molecular level. The question ofthe evolution ofa process ofthis sort also raises others: What level shall we deal with? Shall we deal with photosynthesis at the level ofthe whole organism, the cell, the subcellular particles, the macro148 Melvin Calvin · Photosynthetic Mechanisms Perspectives in Biology and Medicine · Winter 1962 molecules, or at the level ofthe small substrate molecules that are involved? We should, in...