The Origin of Life on Earth and Elsewhere

THE ORIGIN OF LIFE ON EARTH AND ELSEWHERE MELVIN CALVIN, Ph.D., D.Sc* The particular sequence ofthoughts indicated by the title has its origin in my interest in the process of photosynthesis. It began when I asked myself the question: Which came first, the plants or the animals? This question had been answered in a variety ofways, but one can't help but wonder, as one learns more and more about the detailed mechanism by which living organisms store energy and use it, how they got started in the first place. Actually, these thoughts began long before we heard of Sputnik and all ofits successors. However, the fact is that it is now becoming possible for us to know, perhaps within five years, whether there are organic chemicals on the Moon and what the genetic nature ofthe living material on Mars might be. (I am assuming, now, that there is some.) Within the lifetime ofmost ofus we will know these things. It therefore becomes a much more pressing matter for us to surmise how life got here (on Earth) so as to have some clue as to what to look for in outer space. This is anothersource ofthe driving force for inquiry in this direction, and a very pressing driving force it is. Therefore, in order to surmise what we should look for, we ought to extract some basic idea of the essential features of living material as we know it on the surface ofthe earth, so we can devise the proper instrument to look for it elsewhere. You can see that there are all sorts ofpressures which stimulate—even compel—us to examine this subject in a very concrete and practical way, other than the much broader pressure ofpure hu- * Department of Chemistry and the Lawrence Radiation Laboratory, University of California, Berkeley, Calif.; Research Professor ofChemistry, Miller Institute for Basic Research, 1960-61. This paper waspresented before the California Section ofthe American Chemical Society, Berkeley, CaUf., September 19, i960, and before the Third Annual Basic Science Lecture ofthe Southern California Region of The American College of Physicians, November 18, i960. It is reprinted here, by kind permission, from Annals ofInternalMedicine, 54: 954, 1961. Preparation ofthis paper was sponsored by the U.S. Atomic Energy Commission. 399 man curiosity, which is the pressure that has existed since man began to think about the nature of life. I. What Is Life? It seems best to begin with a discussion of what the nature of living matter is and what properties we might expect to find, which had to be generated on the surface ofthe earth and which generation may, or may not, be going on elsewhere. Actually, what I am about to describe is nothing more than an extrapolation of the Darwinian idea. (It is rather impressive to find so many scientific people interested in a serious discussion ofthe origin oflife. This would not have been the case thirty or forty years ago. It was a disreputable subject then; in fact, it was a disreputable subject for almost fifty years—between 1870 and 1920. It is rather interesting to speculate why this heightened interest in the thinking about this subject has arisen.) Thoughts about the nature oflife itself—really the first serious ones in modern times—stem from Darwin himself. You will recall that the basic contention of Darwin was expressed in the title of the Darwin-Wallace paper of1858 which was titled "On the Tendency ofthe Species to form Varieties, and on the Perpetuation ofVarieties and Species by Natural Means of Selection" (1). Darwin had already recognized the significance of this basic notion of a variety of living matter to depart indefinitely from its original type and to become a new species, and he also recognized the significance ofa backward extrapolation ofthis notion. Thus ifyou start outwith two species, and ifyou acceptDarwin's notion that these two species were originally two varieties ofone species which, in turn, was once one ofa pair ofvarieties turned species, you can keep going back until eventually you must come—and Darwin recognized this as implicit in his basic notion—to a point where there was only one type, or species, ofliving thing. Eventually...