104 Inside the Living Cell 



only say that if such circumstances exist they remain to be discovered. 



We can obtain some clues about the circumstances under which 

 living things originated from our knowledge of their chemical com- 

 position. 



As I mentioned above, all living things are optically active, i.e. 

 when two forms of a compound are possible only one is present. Thus 

 all the amino-acids present in proteins are of the /-configuration. It 

 follows that at some stage or other something must have happened 

 to decide that living things should use compounds with the / (left- 

 handed) rather than the reverse d (right-handed) configuration. 



There are few natural ways which discriminate between these two 

 forms of a compound. As I mentioned above, one of these is crystal- 

 lization. Some minerals crystallize quite naturally into optically 

 active forms, e.g. quartz crystals are found in either one or the other 

 form and this is also true of other minerals. It is thus possible that 

 life acquired its preference for one configuration (in the case of pro- 

 teins, the /-form) by being associated with a particular kind of crystal. 

 Thus if the original germ of life started in contact with a crystal of 

 quartz (or clay minerals) which had optical activities, it may well be 

 that one of the possible forms of amino acid was made use of and, 

 having been established, the pattern involving this type was con- 

 tinued to the present day. Another way in which this could come 

 about has been suggested by Dr J. H. Northrop, who pointed out 

 that, when organic substances crystallize, they may give optically 

 active crystals (in equal quantities). But when pairs of crystals of 

 opposite sign are formed, they may easily be separated from each 

 other by perfectly natural agencies like the wind, so that localities 

 may occur where an excess exists of organic material of one confi- 

 guration. It must have been some such accident as this which im- 

 printed on living things their preference for /-amino acids. The natur- 

 ally occurring sugars also are found with particular configurations. 



It is probable that there could be a similar family of living things 

 making use of the J-compounds — in which all the important com- 

 pounds were of the opposite configuration. There is no sign of this 

 family on this world, but it might exist on another planet. 



It is, however, difficult to reconcile the idea of a primitive organic 

 environment with the amount of oxygen now in the earth's atmo- 

 sphere. There is in fact plenty of oxygen to burn all the carbon com- 

 pounds completely to carbon dioxide and water. Under the condi- 

 tions suggested for the elaboration of complex compounds, oxida- 

 tion would certainly occur. Even in the present world, few organic 

 substances can remain exposed to the atmosphere and sunshine and 



