involve substrate to the extent that a replication of pattern is itself 

 seen as beginning first on a mineral surface. 



None of these schemes is yet much supported by experiment or 

 quantitative theory, but given that life arose in an abiotic environ- 

 ment, some stable and yet not immutable spatial ordering was a key 

 part of the process. Was that ordering all spatial, structural, in the 

 sense of the formation of discrete phases beyond the molecular 

 scale? Or was it in part temporal, kinetic in the sense of variable 

 reaction rates among molecules? Those rates could be self-controlled 

 by catalytic feedback loops, as well perhaps as by temporal, even 

 cyclic, changes in the external chemical environment, e.g., dry-wet, 

 or light-dark. That both possibilities might have been of importance 

 is not to be overlooked. In the present state of our knowledge we 

 would hope for small steps along any of these paths. 



THE CHEMISTRY OF LIFE: WHY CARBON? WHAT ELSE? 



The capability for generating, storing, replicating, and finally 

 utilizing large amounts of information implies an underlying molecu- 

 lar complexity that is known only among the compounds of carbon. 

 The special properties of the carbon atom that make it suitable for 

 the construction of large, complex, three-dimensional molecules 

 which are, in fact, thermodynamically unstable but kinetically 

 metastable, are discussed in textbooks of organic chemistry. While 

 living forms contain significant amounts of hydrogen and oxygen (in 

 the form of H 2 0), no other element enters as many and complex 

 compounds as carbon. As every student knows, there are more com- 

 pounds of carbon known than there are of all the other elements 

 put together. 



Table 1-1 lists the relative number of atoms of the chemical ele- 

 ments which comprise the particular sample of life described. The 

 data give in round numbers the number of atoms for each important 

 element (not the weight) among 100 atoms of the sample. Note small 

 round-off errors. 



Life on our planet is carbon-based despite the fact that carbon 

 is a minor element in the Earth's crust (about 0.5% by weight). This 



