Genetic Nucleic Acid: Key Material in the Origin of Life

PERSPECTIVES IN BIOLOGY AND MEDICINE Volume V · Number ? · Autumn 1061 GENETIC NUCLEIC ACID: KEY MATERIAL IN THE ORIGIN OF LIFE H. J. MULLER* I. How Do Living Things Differfrom Lifeless Ones? The differences between living and lifeless things that are big enough to be seen are for the most part so far-reaching and manifold that even primitive man, the world over, despite his prevalent limitations in coming to generalizations, has tried to divide all nature into these two categories. The category of the living, containing himself, he tends to endow with the main attributes supposedly characteristic of himself, including especially the possession ofa sentient spirit, capable ofacting upon matter yet also, so he thinks, ofexisting apart from it. Sometimes the distinction between living and lifeless becomes blurred when these spirits are thought of as coming to dwell not only in plants, animals, and human beings but also in sticks andstones, the earth, the wind, andheavenly bodies: a widespread doctrine known as animism. Yet despite this extension, his cleavage between living and non-living usually remains for him a fundamental one. And even scientists find it useful and sensible today, both on operational and theoretical grounds, still to make this distinction, although with rare exceptions they no longer think ofit as based in any inherent spirit or vital force. * Department of Zoology, Indiana University, BIoomington, Indiana, Contribution No. 708. This articlerepresents a lecture given at a Symposium on the Origin and Nature ofLiving Matter, held at the Albert Einstein Medical College onJune 8, 1959, in connection with the graduation ofits first medical class. Ever since our range ofview has been increased so as to include objects of microscopic and submicroscopic sizes, it has remained evident that, with only a limited set ofpossible exceptions, living things form a natural group, for all their enormous diversity, that is set apart in deep-seated and remarkable ways from lifeless things. When now we seek to define the difference between the living, or animate, and the non-living, or inanimate , in some exact yet general way, we find that some ofthe most outstanding characteristics by which we recognize many animate bodies to be such—for instance, motion, contractability, conduction of the impulses to react, sensitivity to light, sound, touch, etc., specific powers of digestion or conversion, even adjustment to such features ofthe environment as heat or drought—are present only in some types oforganisms or in particular stages and under given conditions. Under certain circumstances some organisms—even human cells—can remain quite dormant. Even most ofthe peculiar chemical substances such as carbohydrates, fats, proteins, and vitamins found, in many variant forms, in all except a few submicroscopic organisms do not actually define things as living. For organisms that are no longer living may still possess these materials, although doubtless with alterations in some components and in the manner in which they are fitted together that have so far eluded us. Still, all living things larger than viruses do have all these materials, and at some stage oftheir existence have a turnover of them, or metabolism, whereby they are manufactured from more or less different incoming materials and broken down into outgoing waste, so that the organism is constituted ofmaterials that are subject to flux. And in all cases, at some stage or stages, the income exceeds the outgo, so that the organism grows while maintaining in the main its own specific composition. And this growth is so ordered as to cause the individual cells to form two cells, that is, to undergo a binary replication. And when the individual consists of many cells, there is a stage when individual cells separated from the parent, or cells formed by the combination oftwo that have separated from the parent, redivide and redifferentiate to form a new multicelled individual. The balanced flux has sometimes been compared with a flame or a whirlpool , although it is inordinately more complicated. But nothing like the orderly growth ofsuch a complicated organization, leading to its regular binary replication, is known outside ofliving matter. Now, as we all know, living things differ from lifeless ones in that the H. J. Müller · Genetic Nucleic Acid Perspectives in Biology and Medicine · Autumn...