XI 



have yet found in the cosmos allows the presence of liquid water. 

 Mars long ago had great rivers, we think. Water and carbon- 

 containing molecules fit for living forms go tightly together. The con- 

 clusion which everyone accepts is that small carbon-containing 

 molecules form spontaneously, under the right ambient conditions, 

 whether or not life is directly involved. If not the true precursors of 

 life, these molecules are at least patterns for the loom of life. They 

 can be made by a variety of synthetic processes; they differ indeed 

 only in detail from nonorganic molecules, copious but biologically 

 insignificant in the atmospheres of the cooler stars. The molecule is 

 plainly implicit in the atomic world. 



We will press the rock record back to its first pages. We will 

 improve the planetary and meteorite studies forward to the Earth in 

 birth. But can we look squarely into that half-billion year gap 

 between the birth of the planet and the oldest relics of life? 



THE ESSENTIAL MEMORIES 



The extension of 35 years of molecular biology involving the 

 idea of the genetic code and its work, has shown us much of the 

 inner nature of life, especially of microbial life. The effort to frame a 

 general definition of life, which might seem at the core of the matter, 

 is in fact not so salient in the search for life's origin. For what we 

 seek must be the long chain of events which gave rise to that specific 

 complex web of present-day life on Earth of which we are a part. 

 Even if quite other forms are possible, they would seem less relevant 

 to the quest for what happened here. It is no surprise, then, that the 

 key questions have been sought on theoretical grounds. For 20 years 

 people have looked for conditions which, under the little-known 

 natural circumstances of 4 b.y. ago, could plausibly lead to the rise 

 of proteins — the working molecules of life which marshal the linked 

 chemical reactions to build living systems and lend them function. 

 Parallel effort has been spent with the nucleic acids, which on Earth 

 alone form those subtle, long molecules (DNA and RNA), as well as 

 the various copying peripherals that embody and reproduce the 

 instructions for the proteins which implement the order, even the 



