I. TOWARD THE ORIGINS OF LIFE 



A DEFINITION OF LIFE? 



The search for a clear definition of life is in itself a scientific 

 problem of real depth. In the present context, that search appears a 

 little peripheral, however deep and urgent the issue may be to a full 

 understanding of biology. For our task is not at all to examine every 

 path that the lifelike might walk. The classical metaphors of the 

 flame, the whirlpool, the growth of a crystal, even so speculative a 

 diversion as the notion of life based on silicon, whether within some 

 computer mainframe or in the creaking motion on some far-off 

 planet, are somewhat beside the point. The task is set for us sharply 

 by the historical event: The life whose beginnings we seek is that of 

 our own planet, carbon-based, molecularly elaborated, and capable 

 of evolving into the web of life shown in the geological record up to 

 our own time. 



The attributes by which we recognize living things as alive — 

 their capacity to grow, replicate, and repair themselves, to produce 

 elaborate and seemingly purposeful structures and behaviors, to 

 adapt to the most varied conditions — are derived ultimately from 

 the genetic properties of living matter. By genetic properties we 

 mean these two: self-duplication and discrete change. That is to 

 say, living systems are systems that reproduce themselves closely, 

 but that mutate as well, and then can reproduce their mutations. 

 These properties define the living state. Such systems, mutating — 

 albeit blindly — in many directions, will evolve through the process 

 of natural selection. In time, they can yield the living world in its 

 endless variety and complexity. 



Within the notion of self-duplication, so intrinsic to life, there 

 once seemed to rest a logical disaster. A mechanism which must 

 reproduce itself needs to be complicated. But the more complicated 



Electron micrograph of DNA by Lome MacHattie, in collab- 

 oration with Ken Berns and Charles A. Thomas, Jr. 



