8 



TABLE I-l.-SOME ELEMENTARY RECIPES FOR LIFE FORMS 



is comprehensible once the special properties of the carbon atom 

 referred to above are considered. It becomes almost expected in the 

 light of recent discoveries showing that carbon compounds complex 

 enough to have biological importance are reasonably abundant in 

 carbon-containing meteorites (carbonaceous chondrites), even in 

 vast, cold, and dilute interstellar clouds. Analysis of carbonaceous 

 chondrites has revealed the presence of numerous amino acids, 

 including at least eight of the amino acids of living proteins. It has 

 been generally concluded that these amino acids are of extraterres- 

 trial and nonbiological origin. Equally remarkable is the demon- 

 stration, by microwave spectroscopy, of a variety of organic com- 

 pounds in interstellar space, in association with dust clouds rich in 

 molecular hydrogen. Among the substances identified in these 

 clouds are intermediates familiar in the synthesis of amino acids and 

 of purines, pyrimidines, and sugars — in short, precursors of the 

 genetic system, albeit very dilute within those enormous astronomi- 

 cal volumes. 



It is clear from these discoveries that nonbiological reactions 

 leading to the formation of biologically interesting molecules have 

 occurred and are still occurring in the universe on a grand scale. 

 This suggests that wherever life may be found it will be carbon-based, 

 not greatly different in chemistry from our own. 



