THE OXYGENIC ATMOSPHERE OF THE PRESENT 47 



impossible, because only living matter in its turn can synthesize or- 

 ganic compounds. Only living matter can produce other living matter. 



THE OXYGENIC ATMOSPHERE OF THE PRESENT 



The crucial point lies not, however, in the fact that such an origin 

 is altogether impossible, but only in that it is impossible under present 

 circumstances. The most essential of these is that present environ- 

 mental conditions for life on earth are all based on the fact that 

 our present atmosphere contains an appreciable amount of free 

 oxygen. 



The free oxygen of our atmosphere, and the free oxygen dissolved 

 in most of our hydrosphere, enables the breathing by most plants and 

 animals. This is the most important part of their energy cycle, and 

 as such forms the basis for poetic quotations about the 'life-giving' 

 oxygen. There is, however, another important aspect to this free 

 atmospheric oxygen. An aspect much less easily perceptible in our 

 every day life than breathing, but still of equal, or even higher 

 importance. This is that the free oxygen in the higher reaches of the 

 atmosphere forms a relatively thin layer of ozone. Ozone absorbs 

 most of the ultraviolet light emitted by the sun and consequently 

 shields us from these rays. A small amount of the longer ultraviolet 

 rays can be sustained, and may even produce a healthy sunburn, but 

 the full spectrum of the ultraviolet rays, in the intensity as produced 

 by the sun, is more deadly to present-day life on earth than any 

 radiation due to radioactive decay. 



Of the two effects of free atmospheric oxygen, the possibility of the 

 breathing and shielding from the ultraviolet rays, the second only 

 is a conditio sine qua non for the present life on earth. We know 

 a wide variety of anaerobic microbes, which have quite a variation 

 of metabolism cycles, in which free oxygen plays no part. Free oxygen 

 is even deadly to most of them, and because our atmosphere contains 

 free oxygen in large quantities, these forms can only live when shut 

 off from the atmosphere so well that they are able to reduce small 

 amounts of oxygen reaching them. So there is at present life on earth 

 which lives and propagates under the exclusion of free oxygen. 



However, both aerobic and anaerobic forms would be killed off 

 by the shorter ultraviolet sunrays if these were not filtered out high 

 up in the atmosphere by the ozone layer. This in turn depends on 



