Conditions for the Appearance of Life 171 



lution of the atmosphere. By causing the dissociation of water it enabled free 

 oxygen and hydrogen to be formed. The oxygen liberated from the water of 

 the air enabled further evolution of the atmosphere to occur. It took part in the 

 formation of organic compounds and served as a screen. Radiations and, in 

 particular, short-wave radiations, while making possible the synthesis of organic 

 compounds (as is mentioned in the communications of Miller, Bahadur, Pav- 

 lovskaya and Pasynskii to this Symposivmi), überated, at the same time, free 

 oxygen and thus gave rise to a carrier to their own action, which carrier absorbs 

 the excess of ultraviolet radiations that might otherwise have set a limit to the 

 further evolution of organic substances. 



However, other substances in the hydrosphere and atmosphere could also act 

 as a screen. As a number of workers have shown, nitrogen, as well as oxygen, 

 has absorption bands in the ultraviolet part of the spectrum. Compounds of 

 nitrogen and oxygen and carbon dioxide can be decomposed by irradiation. 



At my request K. S. Shifrin calculated the thickness of a layer of carbon 

 dioxide and that of a layer of water sufficient to prevent the passage of the short- 

 wave radiations which are absorbed by ozone if it is present in the atmosphere 

 in an amount equivalent to a layer 3 nmi thick at N.T.P. The thickness of such a 

 layer of CO2 is just 500 m and that of a corresponding layer of clear water is 

 10 m. If the water is turbid the thickness might be less. 



According to Urey the transition from the reducing to the oxidizing atmos- 

 phere was accompanied in particular by the formation of CO2. Chamberlin 

 maintains that the aggregate of CO2 laid down in sedimentary deposits, used up 

 in the production of coal, peat, bitumen, humus, etc., and now present in 

 the hving material of the biosphere, is about 400 times greater than that now 

 contained in the atmosphere and oceans. If this is true, then, of course, this 

 carbon dioxide could have acted as a screen. 



Water is an unsatisfactory screen, for its superficial layer is constantly being 

 mixed. The turbulent hydrosphere could not serve as a screen for plankton. 



Water could only act as a screen for deep-living forms of Ufe. 



Shifrin's calculation shows that Hving things attached to the floor of shallow 

 basins would have been shielded from ultraviolet radiations even in the absence 

 of any atmospheric 'protoscreen'. This calculation is further confirmation of 

 Markov's hypothesis that Ufe originated on the bottoms of shallow lagoons. 

 However, an atmospheric 'protoscreen' did, in fact, exist. 



The Earth is an open system. The energy of the Sun is the cause of the 

 migration of matter which, at first, can only have been a hindrance to the 

 origin of life. In the course of time, owing to the grinding up of rock 

 formations and the appearance of a triphasic system on the surface of the Earth, 

 there arose obstacles to the turbulent movement of the air and water. This 

 created the conditions required for the origin of life. The surface of the 

 land, the weathered superficial layers of the crust of the Earth, shallow conti- 

 nental basins and lagoons of the ocean, were the arenas in which life came into 

 being on our planet in the form of a multifarious host of germs. 



