Session I. DISCUSSION 



N. H. Horowitz: 



On Defining ''Life' 



Several speakers at this Symposium have brought up the question of how life is to be 

 defined. As a biologist, I am interested in this question, because it is a test of our under- 

 standing of the nature of hving matter, and because at a symposium on the Origin of 

 Life it seems important to try to reach a common understanding on this basic concept. 

 I would like to consider the problem first in rather general terms and later narrow it to 

 the forms of life that we know. I shall first ask: What are the minimum properties of a 

 living system? And then: What is the simplest chemical system known to exhibit these 

 properties? I do not claim any special originality for the ideas I shall present; in one 

 form or another, they have been current among geneticists for some time. 



Some biologists and biochemists tend to regard the question of the definition of life as 

 essentially meaningless. They view living and non-living matter as forming a continuum, 

 and the drawing of a line between them as arbitrary. Life, on this view, is associated 

 with the complicated chemical apparatus of the cell — with enzymes, membranes, meta- 

 bohc cycles, etc. — and it is said to be impossible to decide at what point in its evolution 

 such a system becomes ahve. I do not accept this point of view, because I have not been 

 convinced that the postulated continuum actually exists. 



Others define life in terms of metabohsm and energy flux, or in terms of the ability to 

 reproduce. Such definitions fall short of the mark. A steam-engine has metabolism, and 

 many simple examples of self-reproduction are known; the hydrogen ion, for example, 

 can catalyse its own production from many substances (e.g., ethyl acetate). That familiar 

 analogy of a living system, a flame, shows both metabolism and the ability to grow and 

 perpetuate itself. 



Self-duplication and metabolism of a sort are important in the definition of life I shall 

 propose, but they are not sufficient. In addition, we must provide our system with the 

 impulse to evolve. This is essential, because in specifying the minimal properties of a 

 primitive living system, or eobiont, we must include the potentiality of evolving into the 

 countless forms that we recognize as ahve. To the property of self-duplication, I therefore 

 add the ability to tmitate randomly and to reproduce in the new form. This property of 

 mutability implies that the reproductive process in Hving things involves much more 

 than the simple autocatalysis we find in ordinary chemical systems. It involves a copying 

 mechanism which insures that mutations occurring in the parents will be faithfully 

 reproduced in the oflfspring. 



With the ability to mutate and to reproduce by copying, evolution becomes inevitable, 

 and our system is almost alive. It is necessary to add a final element, however, and that 

 is the ability to influence the environment in such a way as to insure a supply of the materials 

 necessary for the perpetuation of the system. I suggest that these three properties — 

 mutability, self-duplication, and heterocatalysis — comprise a necessary and sufficient 

 definition of living matter. Any system endowed with these properties must, given the 

 right conditions, evolve under the pressures of random mutation and natural selection. 

 In time, all the complexity of structure and function that wc associate with living things 

 might conceivably develop by a kind of logical necessity. Some years ago, I attempted 

 to show how the synthesis of biochemically important substances might evolve in such 

 a system [i]. 



I am familiar with the argvmient that since not all living things can reproduce them- 

 selves the mule is often cited as an example— it is not permissible to include self- 

 reproduction in a definition of life. It is true that mules rarely reproduce, but this is for 

 trivial reasons having to do with the mechanics of gamete formation. That the cells of 

 which the mule is composed arc capable of reproducing is evident from the fact that 

 they are products of the division of one original cell and are undoubtedly capable of 



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