LIFE m THE UNIVERSE — ^HUANG 243 



which supplies most of the body's needs for energy, yields about 700 

 kilogram-calories of free energy per mole. On the other hand, in a 

 reducing atmosphere the free energy has to be derived from fermenta- 

 tion of glucose to ethyl alcohol and carbon dioxide, according to 



CsHi^Oe >2C2H50H+2C02, 



which amounts to only about 60 kilogram-calories per mole. 



Consequently, under reducing conditions a living being has to 

 consume more than 10 times as much food as in an oxidizing environ- 

 ment in order to derive the same amount of free energy. Therefore, 

 it is doubtful that a mind such as man's would appear through evolu- 

 tion in a reducing atmosphere, because living beings would be too 

 preoccupied with seeking food. 



If hydrogen must first escape from the air before a high form of 

 life emerges, the planet must not be too large. Plausible values for 

 the radius would be between 1,000 and 20,000 kilometers, which in- 

 cludes the moon and Mercury. The former could hold air if its density 

 were high, and the latter would have a suitable atmosphere if its 

 distance from the sun were greater. 



The problem of life on other worlds is ultimately related to the 

 formation of the complex molecules that are essential to life processes. 

 Life on the earth, as we all know, depends upon carbon-containing 

 molecules and on water. The fundamental question of bioastronomy 

 is whether living beings elsewhere must also depend on the carbon 

 bond, with water as a solvent. Although a definite answer cannot be 

 provided, I have several arguments in favor of an affirmative one. 



From what other element can complex molecules be built? A 

 glance at the periodic table shows silicon, located directly below 

 carbon, to be a likely candidate. Indeed, silicon is largely responsible 

 for the great variety of molecules fomid in the earth's crust. How- 

 ever, silicon appears to have a higher affinity for fluorine and other 

 halogens than for hydrogen. While its cosmic abmidance is as much 

 as one-fifth that of carbon, the percentage of halogens in the cosmos 

 is negligible compared with hydrogen. As a result, complex com- 

 pounds of silicon have much less chance to form than do those in- 

 volving carbon. 



There are several empirical results favoring carbon as an essential 

 life constituent. M. Calvin and his associates made the first successful 

 experiment in prebiological chemistry when they obtained formalde- 

 hyde and formic acid in a cyclotron from a mixture of carbon dioxide 

 and water. In 1953, S. L. Miller found that the amino acids — the 

 building blocks of proteins — are formed, together with other organic 

 compounds, when an electric discharge is passed through a mixture of 

 methane, ammonia, and water vapor, in concentrations approximately 



