CHAPTER EIGHT 



The Origin of Life and 

 the Differe7itiafion of the 

 Plant and Animal Kingdoms 



The expression "from ameba to man" is commonly used as though this 

 encompassed the grand extent of evolution. Yet this cannot be true, for 

 there is a vast realm of life on more primitive levels of organization than 

 the ameba. Within the Protozoa, the Flagellata are now universally recog- 

 nized as being more primitive than the Rhizopoda (the class to which 

 ameba belongs ) , and probably ancestral to it. Indeed, many of the Flagel- 

 lata have chlorophyll (as, for example, the well known Euglena) and 

 other typical plant characters, and thus form a link between the Plant and 

 Animal Kingdoms. But the Flagellata are already highly complex organ- 

 isms, hardly a starting point on the scale of life. The Cyanophyta, or blue- 

 green algae, are still more primitive. In these, there is typically no mor- 

 phological separation of nucleus and cytoplasm, but, rather, the chromatin 

 is distributed throughout the cell. Nonetheless these organisms are still 

 significantly advanced beyond the bacteria, for the blue-green algae do, 

 by virtue of the catalytic properties of chlorophyll, synthesize sugar from 

 carbon dioxide and water in the presence of sunlight. Yet the bacteria 

 themselves can hardly be called simple. The chemical analysis of their 

 protoplasm leads to a result not too different from that obtained by the 

 analysis of the protoplasm of higher plants and animals. And their mor- 

 phology and colonial characteristics are sufficiently distinct to serve as 

 guides to identification even where the smallest bacteria are concerned. 

 But there are pathogenic agents so small that they will pass the finest fil- 

 ters, and they are invisible with the best light microscopes. Yet they mul- 

 tiply within the protoplasm of an appropriate host, and the products of 

 their metabolism cause the production of disease symptoms in the host 

 organism. These are the viruses. Viruses have been crystallized, and the 

 crystals are nucleoproteins— very simple when compared to typical proto- 

 plasm, but very complex when compared to inorganic, or to most organic, 

 molecules. It is debatable whether the viruses are living. But they are the 

 simplest things with respect to which any such debate might be possible, 

 and so they bring us squarely before the problem of the origin of life. 



89 



