82 THE FOSSILS 



Because of the similarity with deposits of lime-secreting Algae 

 found in later geologic history and at present, the Rhodesian deposits 

 are commonly called 'algal limestones'. This similarity has even led 

 some scientists to look for later algal lime deposits of exactly similar 

 build, and on that feature alone to conclude to a definite taxonomic 

 relationship of the organisms which formed these deposits. 



ANOXYGENIC METABOLISM OF EARLIEST KNOWN ORGANISMS 



Although it is by way of this close similarity in structure between the 

 Rhodesian deposits and later algal deposits that we are justified to 

 conclude that the Rhodesian deposits, too, are formed by organisms, 

 that they really are biogenic, any further conclusions on taxonomic 

 relationship to later forms, or even a designation as 'Algae' is un- 

 warranted. 



2.7 Billion years ago, there must still have existed the pre-actualistic 

 atmosphere. MacGregor's lime-secreting organisms were therefore 

 anoxygenic, and consequently far removed in systematic position 

 from all later oxygenic Algae of the actualistic atmosphere. We can 

 but guess at the peculiar anoxygenic metabolism which already led 

 to lime secretion by these early organisms, but they cannot have been 

 related at all to any of the systematic groups in which the present 

 oxygenic life and its fossil parentage are cut up in taxonomic pro- 

 cedure. Biochemically, it seems that lime secretion in anoxygenic 

 metabolism presents no difficulties. There are even various an- 

 oxygenic metabolistic schemes possible which would yield energy to 

 the organism and at the same time secrete lime (Kluyver, in lit.). 



STRUCTURES IN BIOGENIC AND INORGANIC LIMESTONE DEPOSITS 



It is well, at this point, I think, to clarify why we are so emphatic 

 that the remains described by MacGregor are biogenic; that they 

 really are deposits formed by organisms. They are not real fossils, 

 because they are not the structurally-preserved remains of the organ- 

 isms themselves. Just as with the later lime-secreting Algae, we may 

 even presume that these early organisms did not have a definite 

 morphological structure, but instead formed more or less irregular 

 masses. It is the finely lamellar structure of these deposits which is 

 so typically biogenic. This not only closely resembles the structure of 

 lime deposits formed by later Algae and other organisms, but it is, 



