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).

2. CULTURAL EVOLUTION

How likely is the evolution of intelligent life and technological civilization? Assuming that some self-replicating molecule that we can call "alive" evolves on some planet, will biological evolution act over the eons to produce a diversity of living forms similar to that we see on Earth? Might many of those forms develop intelligence and technology, or is man but a fluke that arose on Earth only as a result of a combination of highly improbable circumstances? What factors determine whether or not biological evolution will lead to intelligent and technological species?

These questions have been asked many times, but satisfactory answers have not been forthcoming. Perhaps this is because we have lacked the data to enable us to understand evolution to the necessary degree. The past two decades have seen a revolution in our understanding of animal behavior and a deluge of paleontological discoveries, especially of fossil hominids. As a result, we are, perhaps for the first time, able to make a rough assessment of the probability that evolution will produce an intelligent, technological species, and have some degree of confidence that the assessment is correct. Our new knowledge has changed the attitude of many specialists about the generality of cultural evolution from one of skepticism to a belief that it is a natural consequence of evolution under many environmental circumstances, given enough time.

On November 24 and 25, 1975, NASA sponsored a Workshop1 on the evolution of intelligent life and technological civilizations which brought together prominent physical, biological, and social scientists (see Section III-15). That workshop endorsed the estimate that the combined probability that both intelligent life and technological civilization would evolve, assuming the origin of life on an arbitrary planet, is at least 102. The workshop considered the opinion of George Gaylord Simpson, who was not present, that the evolution of intelligence was so improbable that perhaps it did not even happen on Earth. Albert Ammerman, geneticist from Stanford, pointed out that when Simpson formed his opinion, in the early 1950s, much less was known about such things as the evolution of the hominids, the social structure and communicative abilities of the chimpanzees, and the social behavior of other animals. At that time man appeared terribly unique; today this is no longer true. It is easy to imagine how other lines could have led or might yet lead to intelligence and civilization here on Earth. Intelligence, complex social organizations, and even the manipulative ability which makes possible the use of tools, have been demonstrated to have positive survival value; hence evolutionary pressures will tend to produce them.

A recurrent theme of the workshop was that it is wrong to focus on particular "crucial” events in the long evolutionary chain that has led to our modern technology. That these events appear by themselves to be improbable is misleading, since a low probability for a particular event does not imply a low combined probability for all possible events similar to it. Furthermore, that there can be a high probability of certain results, however improbable each of the innumerable ways they may come about, has been demonstrated many times by biological evolution itself.

1A Workshop on Cultural Evolution. Chairman, Dr. Joshua Lederberg. Held at the Center for Advanced Study in the Behavioral Sciences, Palo Alto, Calif.

Hans Lukas Teuber, neurophysiologist from MIT, pointed out that many organs have polyphyletic origins. The eye has been invented at least three times. The cephalopod eye, the insect eye, and the vertebrate eye all have totally different, independent evolutionary histories, but these histories have converged to essentially the same result. The neural networks in each of these eyes are shockingly similar. Indeed, any life-form evolving in an environment where the optical spectral band is important might well develop a light sensing organ with similar nerve structure. Nevertheless, octopi and vertebrates cannot see the same things. Octopi cannot distinguish mirror images. Therefore different evolutionary pathways cannot be expected to produce identical results. It is not unlikely that technological species are abundant in our galaxy, but it is very unlikely that elsewhere we will find men.

There was a wide range of opinions on what evolutionary factors were responsible specifically for hominid intelligence; probably many were important. Joshua Lederberg, geneticist from Stanford, thought that intraspecific warfare played a significant role. War seems to require rapid invention. Strategy discussions that are connected with the planning of warfare tend to involve a kind of verbal competition that is highly inventive. Furthermore, intraspecific conflict makes special demands on organisms that their battle with the environment does not: the difference is between intelligence against intelligence on the one hand, and intelligence against mere nonintelligence on the other. Finally, warfare seems to involve the young; organisms not suited to it suffer the consequence that their genes are eliminated from the breeding population. However, the necessity of the evolution of such an institution as warfare cannot be said to be established. Territoriality is a common trait among Terran animals, and some of the social mammals, such as man and the hyena, exhibit, as one form of territorial behavior, organized violent conflict between social groups. But territoriality is not a basic biological trait; many species do not exhibit it. Bernard Campbell, anthropologist from UCLA, thought it was but a result of the primary need to compete for resources, and that other means of allocating resources might be possible.

Probably the most important stimuli to the development of intelligence in early hominids was the demands of communication and language. J. Desmond Clark, paleoarcheologist from the University of California at Berkeley, pointed out that an increased rate of evolution of the brain set in about 3 million years ago, and this was correlated with increased use of stone tools. Manufacture of artifacts is evidence of complex social structure which in turn implies need for improved communication; at the very least the techniques of manufacture must be taught to the young. But John Eisenberg, ethologist from the Smithsonian Institute, thought that some of the hominids' increased cranial capacity was related to a general increase in motor coordination. The "text book idea" that man is a puny beast is simply not true. He is a fantastically powerful and coordinated organism, especially in the hands and limbs. He has subtle and accurate motor control which gives him great physical ability. He has independent control of his fingers and motor control of vocalization. And he has a very complex feedback system which enables him accurately to determine the course of thrown projectiles, with a little practice.

There was general agreement that the need to adapt to a predatory way of life on the savannah stimulated at least the early development of manipulative ability, motor coordination, and complex social organization in the hominids. The arboreal environment of the hominids' ancestors cannot have produced these traits. No monkey or ape can control a thrown projectile the

way a man can; independent finger control is a uniquely hominid characteristic. Moreover, chimpanzees and other apes, though they use natural objects such as sticks for tools, have never developed a systematic tool-making ability. In captivity they can be taught to chip flakes from stone, but that they do not do this in the wild means nothing less than that their arboreal environment makes no demand on them to do it. Correspondingly, they have never evolved the motor control of the throat that is seen in the hominids, though chimpanzees have great natural ability to communicate by gestures. They cannot develop spoken language because they are physically incapable of pronouncing words.

Thus the demands of the savannah environment were probably responsible for the development of intelligence and technological society in man. But it does not follow that this type of environment is a necessary prerequisite to the development of these characteristics. Bernard Campbell thought it was crucial that an animal well adapted to life in the complex forest environment proved to be pre-adapted to an ecological niche on the savannah and was successfully able to invade it. This opened many new possibilities. Moreover, any animal is likely to expand its range into new environments given the necessary amount of preadaptation. Thus what appears most important in stimulating evolution is the presence on a planet of a large diversity of environments readily accessible to the inhabitants of each. The larger number of possibilities inherent in this situation will result in the greatest diversity of species, and will speed the development of traits having survival value, such as intelligence.

This requirement allows us to characterize planets likely to produce intelligent, technological civilizations. In the first place they must have heterogenous and time-variable environments. Bernard Campbell pointed out that in a very stable, homogenous environment no evolution at all occurs, even over indefinitely long time scales. On Earth the deep-sea echinoderms are evidence of this. John Eisenberg pointed out that on isolated land masses intelligence appears to have developed very slowly. For example, in Australia, Madagascar, and pre-Pliocene South America, there was development of many mammalian species, independent of that which took place on the contiguous land masses. On none of the isolated land masses did there develop mammals with large cranial capacity. We see this today in the mammals of Australia and Madagascar. This was also true in South America, although the development of these species was interrupted in the Pliocene, when the northern and southern continents were joined. Only mammals on large contiguous land masses developed both large bodies and large cranial capacities. It is unlikely that planets with limited contiguous land area will evolve intelligent terrestrial life very rapidly, while an aqueous environment is not conducive to the evolution of manipulative ability and hence technology. However intelligent dolphins may ultimately prove to be, they will never build radio telescopes or spacecraft.

Almost certainly once a species with the requisite intelligence, manipulative ability, and complex social organization has evolved, technological civilization will develop. Modern man is little different biologically from Cro-Magnon man. To go from a stone age culture to our present level of technological development required no biological evolution. All that was needed was the development of ideas, and their testing by trial and error. Philip Morrison pointed out that Turing had shown that once yes-no choices can be recognized in large numbers, one can program any mathematical computation, given enough time. Thus, once a system capable of conceptualizing