16 The Nature of Biological Diversity 



history. The problem with which I am dealing, the problem of photo- 

 synthesis, is especially difficult to trace. 



It turns out, as I shall try to point out in more specific detail as we 

 go along, that the evolution of photosynthesis 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. In trying to describe that sequence of events, I find myself 

 greatly increasing my respect for the novelist who writes historical 

 novels. You know how he does it: He has many chains of events, 

 giving rise to a particular incident at the end, or perhaps at the begin- 

 ning of the novel, and he is very skillful at starting each of these 

 threads and jumping from one thread to the next, bringing them 

 along so they all come together at the right time and in the right place. 

 I haven't yet been able to do that, and what I am going to have to do 

 is to jump back and forth among the various evolutionary threads that 

 are involved here, which ultimately fuse together to give rise to the 

 very complex process of photosynthesis. The story may appear, there- 

 fore, more confused than it really is, since I must jump back and forth 

 between separate evolutionary threads and try to indicate their points 

 of fusion. 



Modern Photo synthetic Processes 



With this apology, let us begin our study of the evolutionary history 

 of photosynthesis by first describing what we think we know of the 

 modern process toward which we must eventually come. Most of you 

 know that photosynthesis 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: 



C0 2 + HoO X (CHoO) „ + 2 



This is the overall process of photosynthesis which has long been 

 recognized as a process for transforming electromagnetic energy, 

 here represented by the quantum, into chemical potential, represented 

 by oxygen in the elementary form and the elements of carbon and 

 hydrogen largely in the oxidation level of carbohydrate. 1 ' 2,3 



If this were all that we know about the process of photosynthesis, 

 we would be hard pressed to try and predict an evolutionary history 

 which might give rise to this process. Fortunately, in the last decade 

 or two we have learned perhaps more about the process of photo- 



