XX 



Life in the Universe 



H 



We started with simple atoms and molecules — the building bricks of 

 all the complex patterns which have been discovered in living things. 

 We then worked our way upwards through proteins and enzymes to 

 the larger aggregations, such as nucleoproteins, which occur in 

 chromosomes and viruses and in the cytoplasm of cells. The living 

 cell, taken as a whole, represents a much higher level of complication 

 still, since many of these structures must be present in every living 

 cell in a precisely organized manner. It is only at this stage that we 

 reach a level which is recognizably living, in the sense that everything 

 necessary for continuing life in a natural environment is present. 



It is probable that the smallest visible cell contains about a quarter 

 of a million protein molecules of many kinds and larger cells many 

 more. Taking the average protein molecule as containing about 

 20,000 atoms, we see that the smallest independently living units con- 

 tain something like five thousand million (5,000,000,000) atoms, 

 united into molecules of great complexity and all the molecules org- 

 anized into a single functioning whole. 



There are as many or more cells in the human body than atoms in 

 a single cell, so that the cell comes midway in the scale of complexity 

 between atoms and man. If we were to take another step of the same 

 order of magnitude, we should have to include a great deal of the 

 visible universe! Man is therefore in a midway position between the 

 minute atoms of which the universe is constructed and the almost 

 unimaginable whole. It is clear from this that human beings and 

 other large animals are of colossal dimensions on an atomic scale and 

 hardly insignficant even when judged on a cosmic scale (see Fig. 28). 



In a broad sense we have been tracing the great landmarks in the 

 ascent of life. They are four in number, although no doubt many 

 intermediate gradations could be recognized. The first stage, the com- 



