to initiate nuclear reactions, the contraction stops: the star has entered the main sequence. When all its fuel is burned the star dies as a white dwarf or explodes as a supernova, depending on its

mass.

Although hydrogen and helium were created in the big bang, the rest of the elements were formed inside stars and in stellar explosions. Thus, every one of the heavier atoms in our bodies, including the oxygen that we breathe, the carbon and nitrogen in our tissues, the calcium in our bones, and the iron in our blood, was formed through the fusion of lighter atoms either at the center of a star or during the explosion of a star.

Stars spend 99 percent of their active lives burning hydrogen. When the hydrogen in the core is converted to helium, contraction begins. Core contraction releases enough energy to ignite helium which then burns to carbon. While all stars may eventually contribute to the production of carbon, nitrogen, and oxygen, and other elements up to and including iron, the heavier elements are probably made by neutron capture and beta decay inside massive stars during their final stages of evolution or when these stars explode as supernovae. Supernovae, then, may be the primary means by which the elements which are created in stars are recycled back into the interstellar medium. Out of the material sprayed around the galaxy by these explosions, somewhere else a new star with rocky planets can form.

The Milky Way galaxy is one of some 10 billion galaxies in the presently observable Universe. and our Sun is one of some 300 billion stars in the galaxy. The striking fact is that our galaxy and our Sun, to which we owe our very existence, are not unusual in any fundamental way compared with other galaxies and stars. Astronomical data indicate no peculiarities in origin, location, mass, luminosity, age, or other characteristics. Our Sun, therefore, is a relatively common type G dwarf star situated in a typical part of the disk of a rather typical spiral galaxy.

Discoveries made during the last 30 years have resulted in increased support for the nebular theory of the formation of planetary systems. According to this theory, development of the solar nebula was the result of the collapse of a large, rotating interstellar cloud whose inner portion was then heated by absorption of infrared radiation emanating from the proto-sun. At this stage, the cloud would be greatly flattened by its rotation and would consist of a disk with a denser, rapidly rotating, hot central region. Irregular density distributions in the disk are believed to have produced nucleation centers for the accretion of material into planets. In the outer regions of the disk most of the primordial gases of the disk probably went into forming the dense atmospheres of Jupiter-type planets. In the inner part of the disk, terrestrial-type planets may have formed early atmospheres which were then dissipated by the central star and replaced by outgassing from the planetary interiors; or they may have formed without atmospheres, the hydrogen and helium having been blown out of the inner solar nebula by the young Sun.

This renewed scientific support for the nebular theory of planetary formation has rekindled interest in the existence of extraterrestrial life because it predicts that stars with planetary systems should be the rule rather than the exception. In contrast to other theories, which attribute planetary formation to catastrophic events like star explosions or collisions, the nebular theory

suggests that formation of planets will usually accompany the formation of a star. This implies that the galaxy and Universe should be replete with potentially life-supporting planetary sites.

Planets around other stars have never been observed directly with telescopes because of the great brightness difference between the star and planet, and the proximity of their images. Indirect evidence for the existence of another planetary system has been obtained by observation of the motion of Barnard's star over a 30-year period, but is inconclusive. The oscillation in its movement which, if real, could be accounted for by the presence of a planet of roughly Jupiter's size may be only instrumental error. At the present time, techniques are being developed which, in the near future, may allow direct observation of extra-solar planets (see Section II-3).

If the planets did condense from the proto-solar disk, their initial composition might be expected to reflect that of the Sun. The atomic abundances of some of the elements found in the Sun are, in order: hydrogen (87.0 percent), helium (12.9 percent), oxygen (0.025 percent), and nitrogen (0.02 percent). It should be noted that, with the exception of helium, these are the very elements which constitute 99 percent of living matter.

The giant planets appear to have a similar composition; that is, less than 1 percent heavy elements and the remainder hydrogen and helium. Furthermore, Jupiter contains methane, water, and ammonia, the reduced forms of carbon, and oxygen and nitrogen, as would be expected based on thermodynamic properties of these elements in the presence of a large excess of hydrogen. Hence, the present Jovian atmosphere may be in fact its primitive atmosphere, retained for billions of years because of the planet's mass. The inner planets, on the other hand, are composed almost entirely of heavy elements. With their present masses, these planets cannot prevent hydrogen or helium from escaping. Hence, the Earth's primitive atmosphere may have been transitory, with a more stable atmosphere arising as a result of outgassing of the crust by volcanic action. Volcanos discharge large quantities of water vapor and carbon dioxide, some nitrogen, and other gases. The carbon dioxide and water could be reduced to hydrogen and methane by free iron present in the early crust, and hydrogen and nitrogen could combine to form ammonia. Therefore, the Earth's early atmosphere may have consisted mainly of hydrogen, nitrogen, methane, ammonia, water, and carbon dioxide, with small amounts of other gases. It is believed that chemical evolution leading to life on Earth began in such an environment some 4.5 billion years ago.

The theory of chemical evolution was contemplated by Darwin, conceptualized by Oparin and Haldane, and tested experimentally by Miller and Urey. Simply put, the theory proposes that release of energy in the Earth's primitive atmosphere by various mechanisms resulted in the synthesis of simple organic molecules, which in turn were converted into molecules of greater complexity. New chemical reactions gradually came into being as a result of the increasing complexity, and new chemical order was imposed on the more simple organic chemical relations. At some point in this process, the first self-reproducing molecules appeared.

The energy sources available for the synthesis of organic compounds under prebiotic conditions were ultraviolet light from the Sun, electric discharges, ionizing radiation, and heat. Most of these have now been used in laboratory simulations to produce a wide variety of organic molecules from the presumed primitive atmosphere of methane, ammonia, water, and hydrogen.

These simulation reactions yield some compounds identical with those found in the complex biochemical structures of present-day organisms.

Among the compounds synthesized in these experiments are amino acids (the precursors to the proteins of living systems), purines and pyrimidines (monomer units of the nucleic acid genetic material), carbohydrates, hydrocarbons, fatty acids, and other compounds of structural and metabolic significance. Further prebiotic simulations have produced compounds with striking properties. For example, amino acids have been condensed to form proto-proteins which display low levels of enzymatic activity. Also, it has been shown that organic compounds formed under prebiotic conditions can aggregate into more complex structures that display chemical, structural, and physical properties remarkably similar to those of living cells. These discoveries are exciting because they provide models for the microenvironments in which specific compounds could be concentrated and reactions of importance to biological systems could more readily occur.

Besides these simulation experiments, two other lines of evidence support the theory of chemical evolution, and indicate that such syntheses are indeed universal. Recent radioastronomical observations have detected the presence, in the inhospitable environment of interstellar space, of ammonia, water vapor, formaldehyde, carbon monoxide, hydrogen cyanide, cyanoacetylene, acetaldehyde, formic acid, methanol, and a host of other compounds that are known precursors or intermediates in the chemical evolution simulation experiments. There is even a suggestion of highly polymeric material, like porphyrins or polyaromatic hydrocarbons. Cometary spectra indicate that a variety of organic compounds may be present. Since comets are considered to be similar in composition to the primordial material of the solar nebula, this constitutes evidence of organic matter in the very material from which the solar system was formed.

The analysis of meteorites has very clearly indicated that organic matter is present, with carbonaceous chondrites containing as much as 5 percent. Sophisticated organic chemical analyses have identified amino acids and other biologically significant compounds in meteorite samples. The nature of the compounds, their optical properties, and the distribution of isotopes within the molecules all argue conclusively for their being indigenous to the meteorite and therefore of extraterrestrial, abiological origin. Organic matter appears to be common in the cosmos.

To this point, then, the theory of chemical evolution is reasonable, understandable, and well-supported by experimental evidence. However, the sequence of events between the time when only a mixture of organic precursors existed in the primitive seas of the Earth and the time when, according to the geological record, the first living cell appeared some 3 billion years ago, is still a mystery. It is the only portion of the entire chain of events, culminating in man, for which substantive theories and data are lacking. And it is the crucial step for it marks the transition from nonliving to living systems. Somehow, organic molecules in the primitive ocean were assembled into that complex unit of life, the cell. The prebiotic simulation experiments and the terrestrial fossil record do, however, provide one significant inference: Processes leading from organic chemicals to living systems may take place over a relatively short period of time in the lifetime of a planet.

Over the next 3 billion years, the primitive organisms on Earth slowly evolved into the vast array of living systems we see today. Two cornerstones in the evolution of life were the development of photosynthetic capability, which is thought to have resulted in conversion of the atmosphere to its present oxidized state and which permitted cells to derive a great deal more energy from nutrient molecules, and sexual reproduction, which allowed advantageous mutations to be combined in a single individual.

The basic mechanism underlying biological evolution is mutation, the modification of the structure of the genetic material, and the retention in the gene pool of favorable traits. A favorable mutation confers a greater chance of survival since the cell or organism can now compete more successfully for energy sources and can better withstand environmental stresses. Over many generations the organisms possessing favorable mutations will gradually displace those without them. This is the essence of natural selection, originally proposed by Darwin and Wallace as a rational explanation for the whole history of the evolution of widely differing species that make up the plant and animal kingdoms. Experimental evidence from genetic research, the fossil record, and comparative biochemistry of present-day species supports the theory so completely that few have any major reservations as to its validity.

Man appeared very late in this sequence of events and, with his increased intelligence, came civilization, science and technology. Cultural evolution began and has proceeded very rapidly in the last few millenia (see Section II-2). An infinitesimal fraction of the matter of the Universe has been converted into the organic matter of the human brain. As a result, one part of the Universe can now reflect upon the whole process of cosmic evolution leading to the existence of human thought. We wonder whether this process is a frequent occurrence in the universe: in so doing we come to the postulate that life is widespread in the Universe, and at least in some cases, that this life may have evolved to the stage of intelligence and technological civilizations that it did on Earth. Some of these civilizations may be much more advanced than ourselves. They may have learned to communicate with each other, and achieved major advances in their own evolution as a result. Can we detect them?

Although many gaps, puzzles, and uncertainties still remain, this unifying concept, in which the expansion of the Universe, the birth and death of galaxies and stars, the formation of planets, the origins of life, and the ascent of humans are all explained by different features of the process of cosmic evolution, provides a sound scientific rationale on which to base a program to search for extraterrestrial intelligence (see Introduction and Conclusion 1 of Section I).