304 ESSENTIALS OF BIOLOGY 



ocean was very saline, for sodium chloride and some other haiides 

 are volatile in steam at a high temperature. As the volume of 

 the ocean increased it must, then, have become less saline. 



The original atmosphere. We have some knowledge of the 

 composition of the gases liberated when molten magma solidifies. 

 We find, in the following order of abundance : 



CO,, CO, S, so,, CI, HCl, No, H, ; 



and such may have been the composition of the original atmo- 

 sphere. The sulphur would condense as solid ; the SO 2, CI and 

 HCl would combine with the solid materials of the earth-crust 

 and there would remain CO 2, CO, N, and Ho. There would 

 be no oxygen at first since all of it would have been used in 

 oxidative processes. The first atmosphere that endured would 

 therefore contain COo, CO, N2 and Ho. 



Thus the earliest protoplasmic organisms probably evolved in 

 small, shallow, highly saline seas and not on the surface of dry 

 land since that had still to undergo erosion and mature weathering. 

 The gases in the atmosphere and in solution in sea-water were as 

 above stated. 



loih. The Original Modes of Metabolism. Such physical 

 conditions present no difficulties for living things. Even at 

 present we know organisms (Algae, Fungi, Bacteria, etc.) that can 

 (i) utilize CO 2 in the absence of chlorophyll, (2) can assimilate, 

 or utilize, Ng and also inorganic N-compounds, (3) can utilize, 

 or assimilate S-compounds, probably carbon-compounds, such 

 as coal, and also Fe-compounds, and (4) are anaerobic, that is 

 can live and function in the absence of free oxygen. These earliest 

 organisms were probably unicellular ones, or they may have been 

 plasmodic, that is, masses of protoplasmic material containing 

 nuclei but undifferentiated into cells. In structure they were 

 probably similar to the organisms we know, for there are no 

 apparent cytoplasmic or nuclear differences between say, iron, 

 sulphur or CO 2 assimilating bacteria : the original " life-sub- 

 stance " was capable of all these modes of metabolism. We do 

 not know when chlorophyllian organisms first evolved and this 

 was probably very early in earth-history. When they did evolve, 

 the atmosphere would come to contain O2 and this would oxidize 

 the H2 to water and the CO to CO 2. The water would condense 

 into the ocean and the COo would tend to decrease in amount as 



