SETI: A Critical History: Cover
There was not a great deal of profound thought
or anything involved in putting it together.
– Frank Drake
When it was first suggested in the late 1950s that the still-new technology of radio astronomy could be used to communicate with ETIs, a number of scientists excitedly turned to their radio telescopes to begin searching for incoming messages. The National Academy of Sciences saw NASA as a potential institutional home for a SETI project and invited a group of interested scientists, including most of those who went on to spearhead America's SETI effort, to demonstrate that a SETI-style search was plausible. Using a simple decision model and bold assumptions, SETI's architects did so and then claimed that they were establishing the ETI discourse on a rigorously experimental basis for the first time.
Neither Cocconi nor Morrison was an astronomer. According to historian Steven Dick, they "stumbled onto the subject of interstellar communication as an aside to their primary research."2 One night some years before they published their famous SETI paper, the pioneer radio astronomer Frank Drake was conducting observations for his doctoral dissertation when he found a radio signal that appeared to be anomalously coherent. The fact that the signal had a pattern suggested to Drake that it might be of intelligent origin. It initially looked to be coming from a distant galaxy. This got Drake thinking, too, about the possibility of using radio waves to communicate across interstellar distances.3
Drake was not the first in his profession to be tempted by objects that appeared to be the product of intelligence to speculate about the existence of ETIs. In the early modern period Johannes Kepler was one of the first astronomers to turn an optical telescope onto the Moon. When he did so he concluded that the objects we today call craters were so "perfectly round" that they had to be of intelligent origin. (He thought they might be forts.5) Then, around the turn of the 20th century Percival Lowell was inspired by Giovanni Schiaparelli's intimations that the long, straight lines he observed on Mars were the product of intelligence to speculate extensively about what Martians might be like.6
The signal Drake found turned out to be of terrestrial origin. But when Drake got a job at one of America's first big radio astronomy facilities, the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia, he convinced his boss to allow him to test his idea that radio waves might be conveying messages to Earth. When Cocconi and Morrison's paper was published, Drake was already quietly preparing to run the very kind of search the two recommended. As Drake later put it, "Morrison and Cocconi were thinking and we were building."7 From this first moment in SETI's history a subtle rivalry between reflection and action was established that reappeared regularly.
During the spring of 1960 Drake searched for messages from two nearby stars. He found none.
The first SETI conference was held the year after Drake's search, at the same site. It was convened by the Space Science Board of the NAS. J. P. T. Pearman from the Space Science Board published a summary of the meeting, "Extraterrestrial Intelligent Life and Interstellar Communication: An Informal Discussion."8 As the title reveals, the scientists who convened this conference considered it "informal." Proceedings of the meeting were never formally published; Pearman’s seven-page article is the most extensive record of the SETI project's genesis.
The NAS has, on two occasions, played a pivotal role in determining SETI's institutional status within American science. Convening this week-long conference at Green Bank, held soon after the possibility of a SETI-style search was suggested, was the first. The purpose of the meeting was to investigate whether SETI could be considered part of NASA's existing mission to search for extraterrestrial life.9 The ideas that the proper institutional home for SETI was NASA, where it would be part of the broader SETL effort, both had important consequences for SETI.
Cocconi and Morrison's paper and Drake's search had been catalysts around which an informal group of American-based scientists known to be interested in SETI had begun to take shape. The Green Bank meeting attracted many of them, including four who became key SETI figures: Morrison, Drake, Carl Sagan, and Bernard Oliver. Sagan was, at the time, a young astronomer whose interest in the possibility of ETIs had been kindled by several college and graduate-school mentors. Oliver was an electrical engineer and an early Silicon Valley entrepreneur. As the head of Research and Development at Hewlett Packard, he was responsible for the team that developed the hand-held calculator. He had heard about the Green Bank meeting and asked Drake if he could attend. Drake, who hosted the meeting, had no funding to fly Oliver in, so Oliver hitched a ride with a private pilot. It was not the last time Oliver made a resourceful and somewhat spectacular appearance in SETI's history.
The men who attended this conference were invited because they were already excited about SETI. They did not spend their time together thinking critically about whether a SETI-style search represented a good way to confirm the existence of ETIs; they wanted to develop a rationale to justify additional searches. And they believed the key to that was having a plausible rationale for why ETIs exist.
When Cocconi and Morrison wrote their paper about how distant beings might try to communicate with us, they began by acknowledging that reliable theories about planet formation, the origin of life, and the evolution of advanced scientific societies did not yet exist. The authors believed that without these it was impossible to make reliable estimates of the probability that ETIs exist.11 This did not, however, deter Drake, who set the agenda for the Green Bank meeting. He took the three variables Cocconi and Morrison identified and expanded them into seven. He then used them to frame the Green Bank discussions about the likelihood that ETIs existed. These seven variables subsequently became part of what is now known as the Drake Equation.
According to the Drake Equation, the number of civilizations that might be sending us messages was the product of the rate of star formation, the percentage of stars that form planets, the percentage of those hospitable to life, the percentage of those on which life actually emerges, the percentage of those on which life evolves to become intelligent life, the percentage of those capable of interstellar communication, and the longevity of such civilizations. It is often written as follows:
N = R* fp ne fl fi fc L
Despite its central importance to SETI, the Drake Equation was not the product of studied reflection. As Drake recalled, "I thought we should organize the meeting, and that caused me simply to think about what we needed to discuss – there was not a great deal of profound thought or anything involved in putting it together."12 Even more striking than the casualness with which Drake constructed his equation was the boldness of the conferees' probability estimates for each of its seven variables.
The attendees felt they were being conservative in estimating that the rate of star formation in our galaxy, R*, was about one per year. They acknowledged the uncertainty around their estimate of the percentage of stars that form planets, fp, but settled on 40%. For each star that forms planets, they settled on an estimate that between one and five of them would have environments suitable to life; this was based on "pure guesswork." Nor did the participants seem to struggle much when they concluded that the probability of life actually arising on such planets, fl, was "unity," or a certainty, probably because they could point to expert opinion to support their ambitious assumption.13 According to his biographers, one of the epiphanous moments in Sagan's intellectual development occurred when a University of Chicago mentor, Harold Urey, suggested that Sagan attend a colloquium given by Stanley Miller in 1953. There Sagan learned first-hand about the famous Miller-Urey experiment in which some of the building blocks of organic life, amino acids, were synthesized from conditions that were thought to prevail in the Earth's primordial biosphere. To Sagan the fact that the origins of terrestrial life could be replicated in a lab meant that life was something less miraculous and more commonplace than had been previously thought.14 Drake had a similar reaction to the Miller-Urey experiment. As early as 1960 he was discussing how Miller's "molecules, which are the basic building blocks of life, should provide for its emergence on other planets just as they apparently did on earth."15 Sagan and Drake were astronomers, and might have been inclined to latch onto evidence that suggested life arose elsewhere. However, they were hardly alone in reaching their conclusion. The chemist Robert Shapiro reported that, "The Miller-Urey experiment is now recognized as the single most significant step in convincing many scientists that life is likely to be abundant in the cosmos."16 In the same year the Green Bank attendees met, the Soviet biochemist Aleksandr I. Oparin, a co-founder of the theory that the Urey-Miller experiment was designed to test, claimed that "life ... results each time the requisite conditions for it are on hand anywhere in the Universe."17 Melvin Calvin, a Nobel laureate biochemist who was right there at the Green Bank meeting, argued "that the origin of life was a common and even inevitable step in planetary evolution."18
Thus, after considering the first four of the Drake Equation's seven variables, the conferees were inclined to attribute a relatively high probability to the proposition that ETIs existed. Their deliberations around the probability to assign to fi, the likelihood that intelligence would evolve once life originated, were less straightforward. They noted that, "The successful persistence of a multitude of simpler organisms from ancient times argues that intelligence may confer no unique benefits for survival in an environment similar to that of earth."19 They reasoned, however, that two other factors combined to suggest that intelligence was likely. First, although the Green Bank conferees did not cite it by name, the principle of convergence became an important part of SETI-Science because it established that any given adaptation could be repeated. Specifically, convergence suggested that intelligence did not have to be unique to humans.20
In the same way that Calvin served as an "expert witness" at the conference, establishing that life would inevitably arise on earthlike planets, another guest had been invited to establish that the convergence of intelligence had occurred on Earth and was thus likely to evolve elsewhere, too. In the 1960s John C. Lilly was the leading proponent of the idea that dolphins were intelligent (see dolphins as a form of alien intelligence).21 When recounting the proceedings of the conference, the NAS's Pearman reported that Lilly presented "evidence for the existence of a remarkable level of intelligence among the Cetacea" and that this evidence was "noted with great interest."22 As one of Sagan's biographers observed, "If Lilly was right, then the emergence of intelligence might be more common, less context-specific, than was generally assumed."23 In other words, if intelligence was found to have evolved more than once on a single planet, that would be a powerful argument in favor of its evolving elsewhere, too. Convergence became a key component of SETI-Science, and the idea quickly circulated. For example, the biologist Robert Bieri, writing in American Scientist shortly after the Green Bank meeting, used convergence to discuss the possibility of "Humanoids on Other Planets."
The phenomenon of convergent evolution ... shows that, again and again, animals and plants have independently evolved not only similar structures but also similar biochemical systems and similar behavioral patterns as solutions to the same fundamental problems.... If we ever succeed in communicating with conceptual beings in outer space ... in all probability they will look an awful lot like us.24
Convergence suggested that the evolution of intelligence elsewhere was possible. The conferees also discussed why it was probable. We know little about this aspect of their discussions beyond the conclusion they reached. The conferees reasoned that the fact that the human species had increased in both absolute numbers and geographic dispersion was evidence that "intelligence has definite survival value."25
The Green Bank group felt it was in a position to estimate the probability of fi by taking account of the facts that intelligence had apparently evolved at least twice on this planet alone and that the human species seemed to be thriving. Pearman reported that "Taking these factors into account and again with some appeal to the unexceptional nature of the sun and, by extension, the solar system, a value of about unity was postulated for fi."26
This was an extraordinary conclusion. The conferees had just decided that it was likely that intelligent life would evolve on planets with an environment similar to Earth's, and that there were hundreds of millions of these kinds of environments in the galaxy. They did so with no established theories to support their assumptions, a risk one of their own number, Morrison, warned against in the first sentence of his seminal SETI paper.
Two variables of the Drake Equation still remained to be addressed. The conferees assumed that 20% of intelligent species would develop the capacity to send radio messages. This was essentially a pure guess: they acknowledged that this assumption was "vulnerable" and "frankly anthropocentric."27
When the conferees turned to the last variable, L, or the longevity of a communicative civilization, the imprint of the Cold War was unmistakable. (See lifetime of extraterrestrial civilizations). Pearman reported that the participants believed "fears that the value of L on earth may be quite short are not groundless. However, there is a least the possibility that a resolution of national conflicts would open the way for the continued development of civilization for periods of time commensurate with stellar lifetimes." They concluded that L was the most influential of all the variables. In the end the conferees chose two values for L: less than one thousand years and more than one hundred million years. They obviously considered the resolution of nuclear conflicts to be a gating factor for a species, the successful negotiation of which could lead to something like immortality for a civilization.
Multiplying out the probabilities that the conferees assigned to each variable of the Drake Equation thus produced two estimates of the possibility that there were ETIs sending us messages. If species were doomed to annihilate themselves once they evolved the technology to do so (see hazards to extraterrestrial civilizations), there were "perhaps much less than 103 for the whole galaxy." If, on the other hand, the "more optimistic values of communicative lifetime" were chosen, there might be between one hundred thousand and a billion in our galaxy.28
By making this series of optimistic assumptions the Green Bank conferees were able to demonstrate to the satisfaction of the NAS that the idea that ETIs exist that might be sending us messages we could understand was, at the very least, a reasonable one. Although years would pass before NASA could begin to implement its SETI project, the Green Bank conference established that a SETI-style search was a plausible part of NASA's mission to search for extraterrestrial life.
Intelligent Life in the Universe
After reading Cocconi and Morrison's paper, Iosef S. Shklovskii, the "father" of the Soviet Union's CETI project, wrote, "We may consider that there are in the galaxy at least a billion planets, rotating around dwarf stars similar to our sun ... on which a highly organized and possibly intelligent life may take place."29 A year later Shklovskii was asked to contribute an article to an anthology commemorating the fifth anniversary of the 1957 flight of Sputnik. He wrote about the possibility that ETIs existed and how radio astronomy might be used to communicate with them. He then elaborated this into a book that was published in 1963.
Shortly thereafter Sagan wrote to Shklovskii asking that he be allowed to arrange for Shklovskii's book to be translated and published in the United States.30 Shklovskii agreed. He also invited Sagan, "a young and little known planetary astronomer" at the time, to edit the text where Sagan thought it appropriate to do so, in recognition of Sagan's superior background in biology.31 In 1966 Sagan published Intelligent Life in the Universe. It went through four re-printings, suggesting that the text was the way many Americans were introduced to SETI.
At Green Bank the core of SETI-Science had been constructed around four ideas: life will inevitably originate on Earth-like planets; nature repeats useful adaptations, the evidence of which we call the principle of convergence; intelligence is such a highly useful adaptation that it will inevitably evolve once life arises; and that radio astronomy is a logical way to communicate with these intelligences. Sagan edited Shklovskii's original text in such a way that Intelligent Life in the Universe became an elaboration of these four core ideas. Sagan also added a fifth that he believed was needed to complete SETI's conceptual foundation: uniformitarianism.
The Green Bank attendees spoke of "intelligence" as a generic kind of adaptation, and thus implicitly assumed that all intelligences would be fundamentally similar. This assumption was critically important to SETI-Science because it underwrote the assumption that we could understand messages composed by an ETI. Sagan developed an explicit rationale for the assumption. Although he did not use this term, philosophers of science call his rationale "uniformitarianism." Proponents of uniformitarianism claim that, because the universe is essentially one place, composed everywhere of matter and energy as we know it, and governed by natural laws that apply everywhere in it, natural selection will tend to produce similar solutions to adaptive pressures that are, themselves, similar.32
Sagan began the section of Intelligent Life in the Universe that dealt with uniformitarianism by reminding the reader that the matter found on Earth is identical to that in remote parts of the universe, and that "the same interactions occur, and the same laws of nature govern their motion." He later made the additional point that, although we are unable to know what the specific characteristics of an ETI might be, we do know that the laws of physics are universal in character.33 Sagan's point was that the evolutionary histories of all intelligent beings would have a common experience at their core because those evolutionary paths would be shaped by matter, energy, and forces that are the same everywhere. Thus, the kind of intelligence each of those species would evolve would be similar.
Indeed, because human intelligence is one example of this general phenomenon, Sagan implied that all intelligence would be humanoid. Sagan returned to this cornerstone theme of uniformitarianism repeatedly in his popular writings and television programs. In his Pulitzer Prize winning reflections on human intelligence, Dragons of Eden, Sagan spoke of a "resonance ... between our brains and the universe" that would also characterize intelligence everywhere because they "all must come to grips with the same laws of nature." Sagan believed that, as a result, "we will have little difficulty in understanding each other."34 A few years later, in his hugely popular television series Cosmos, Sagan acknowledged that ETIs are likely to be different from us and then asked, "How could we possibly understand their messages?" His answer was that the laws of nature, which "are everywhere the same," constitute "the language of science" and will thus serve as a kind of Rosetta Stone that will allow all intelligent beings to communicate with each other.35
For Sagan, these five basic tenets of SETI-Science supported an inevitable conclusion that he called the principle of mediocrity. This was the core idea Sagan wanted to disseminate in Intelligent Life in the Universe.
The principle of mediocrity
A key phrase in the deliberations of the Green Bank conferees was their reference to "the unexceptional nature of the sun and, by extension, the solar system." The idea that there is nothing exceptional about our sun, the planets of our solar system, or the intelligent life that evolved on Earth became the most important assumption of SETI-Science. Sagan called it the principle of mediocrity and devoted a chapter of Intelligent Life in the Universe to the idea. For Sagan mediocrity had the quality of a self-evident given. It was, simply, "the idea that we are not unique."36
In making the assumption of mediocrity Sagan was operating comfortably within the recent traditions of his field. A number of prominent astronomers had established a clear trajectory toward mediocrity during the 20th century. Sir Harold Spencer Jones, Astronomer Royal, published a popular exposition of ETI science in 1940, Life on Other Worlds, that Steven Dick called "the standard for a quarter century, read by scientists, lay persons, and students alike until superseded by Shklovskii and Sagan's Intelligent Life in the Universe."37 Initially Jones was reluctant to embrace the possibility of ETIs due to the "tidal pull" theory of how planets were formed. This theory, advanced by Sir James Jeans, was especially popular during the 1920s and 1930s (see Jeans-Jeffreys tidal hypothesis). It suggested that planets were rare occurrences. By the 1940s Jeans' theory had begun to fall out of favor. In 1942 Jeans himself discussed the possibility of life elsewhere and noted that, in light of the new insights astronomers were developing about the immensity of the universe, "although planetary systems may be rare in space, their total number is far from insignificant."38 By the time Jones released the second edition of his book in 1952, the year before Sagan's epiphany while listening to Stanley Miller, Jones had overcome his reticence about ETI:
Wherever in the Universe conditions are suitable for life to exist, life will somehow come into existence as it has come into existence on Earth.... With the Universe constructed on so vast a scale, it would seem inherently improbable that our small Earth can be the only home of life.39
On the eve of the publication of Cocconi and Morrison's paper Harold Shapley, Director of the Harvard Observatory, similarly wrote an "obituary ... of anthropocentrism in our description of the universe." He urged humans to prepare themselves for a "fourth adjustment." Having come to grips with the facts that the Earth orbits the sun instead of the other way around, and that ours was a relatively humble star in an inconspicuous corner of an unspectacular galaxy, and after having "accepted rather cheerfully the Darwinian evidence and argument of our animal origin," Shapley now urged us to get used to the fact that we are not the only intelligent beings in the universe. He claimed that "whenever the physics, chemistry and climates are right on a planet's surface, life will emerge, persist, and evolve."40
In one of his original passages in Intelligent Life in the Universe, Shklovskii credited the German astrophysicist and early SETI supporter Sebastian von Hoerner with being the first to make the mediocrity point in the context of searching for ETI. In 1961, the same year as Green Bank, von Hoerner wrote, "The one basic assumption we want to make can be formulated in a general way: Anything seemingly unique and peculiar to us is actually one out of many and is probably average."41
In the following year Ronald Bracewell, a radio astronomer who was one of SETI's friendlier critics for decades, wrote that, "Conditions suitable for intelligent life are believed to be widespread in our galaxy, and the main gap in our knowledge is that we don't know whether life has evolved on the planets where the conditions are favorable. But there is no reason to think it couldn't happen elsewhere if it has happened here." 42
Sagan was claiming that earthlike planets were likely in the universe and that, as a consequence, the evolution of earthlike beings was also common. In doing so he was not simply positioning himself on a recent trajectory, he was calling on a tradition that began millennia ago.
SETI constructed within the boundaries of the traditional ETI discourse
Like the traditional ETI discourse, SETI assumed that ETIs would be humanoid. Both did so by ignoring the question of what the precise "nature" of intelligent beings might be like, leaving the reader to assume it would be similar to the intelligence with which we are most familiar, viz., our own. Prior to the 1960s the ETI discourse is virtually silent on the questions of what an ETI's cognitive structure, consciousness, and intelligence might be like. There were frequent speculations about what ETIs' external appearances might be like, but questions about their possible cognitive structures, consciousnesses, and intelligences did not occur regularly until just about the same time the SETI project was getting underway. The work of one exceptional scientist suggests that this need not necessarily have been the case.
Bernard le Bovier de Fontanelle was the first scientist to offer plausible insights into possible non-humanoid ETIs. He was also the last to do so for almost three centuries. He employed analogical reasoning (see argument from analogy) to modify one aspect of human cognition and then followed the implications of that all the way to the possibility that intelligent beings may be inherently incapable of engaging each other. This is the very issue that later inspired the expansion of the ETI discourse while coming to haunt SETI. As Fontenelle wrote in Conversations on the Plurality of Worlds:
It's quite possible we're missing a natural sixth sense that would teach us many things we don't know. This sixth sense perhaps exists in some other world where they lack one of the five we possess. Perhaps there are really a great number of natural senses, but in the division we've made with the inhabitants of other planets only five have fallen to us, with which we're content because we don't know of the others. Our sciences have certain limits which the human understanding has never been able to pass; there's a point at which they suddenly fail us. The rest is for other worlds, where some of what we understand is unknown.44
The methodology Fontenelle used to construct his non-humanoid ETIs began with human cognition. He then tweaked this in some manner and imagined the changes that such a modification would produce in the new being's consciousness. This methodology was not used again to construct plausible ETIs until, ironically, the 1960s. Although the assumptions of SETI-Science rested comfortably within an established intellectual framework which assumed that intelligent extraterrestrials possessed a humanoid intelligence, that framework was about to be challenged, for the first time, at the very moment SETI's organizers invoked it.
By assuming that radio astronomy was a logical choice for any intelligent being that wished to communicate across interstellar distances, SETI's organizers proudly claimed that "for the first time in human history it has become possible to make serious and detailed experimental investigations of this fundamental and important problem." 45 Sagan argued that they had demonstrated that the question of whether ETI exists "is amenable to experimental testing. It has been removed from the arena of pure speculation. It is now in the arena of experiment." 46 The ETI discourse was "about to enter an experimental phase, after several millennia in which the subject was amenable only to somewhat murky speculation." 47 Sagan and his colleagues did not believe they were changing the ETI discourse; they thought they were testing the central question around which it had always been constructed.
At the seminal Green Bank meeting SETI's organizers used the Drake Equation to frame a conversation in which they informally concluded, on the basis of a small number of bold assumptions, that it was plausible to assume that ETIs exist and could be sending us messages we would understand.
The Green Bank discussion was elaborated and popularized by Sagan into the six core assumptions of SETI-Science: that the Drake Equation identified all the variables relevant to determining if ETIs exist; that life would inevitably arise on Earth-like planets; that nature repeats successful adaptations; that intelligence is a successful adaptation, if we don't use it to destroy ourselves; that all intelligences are similar enough to permit communication among them; and that communication was likely to be by radio when over interstellar distances. Sagan fashioned the middle four assumptions into the principle of mediocrity. SETI-Science was constructed within the boundaries of the traditional ETI discourse, which assumed that the "nature" of ETI's intelligence was similar to ours. Indeed, SETI-Science offered a formal rationale, uniformitarianism, for why this was likely. Although SETI's architects claimed they were engineering an important enhancement to the traditional ETI discourse by placing it on rigorously experimental grounds for the first time, they were leaving its most important assumption unchallenged. By establishing that SETI was plausible, SETI's organizers successfully negotiated the first phase of their project's institutionalization inside NASA, where SETI was thought of as a part of NASA's broader mission to search for extraterrestrial life. However, NASA's Apollo mission to land men on the Moon and Viking mission to land probes on Mars dominated NASA's SETL efforts until its success in 1976. Fifteen years passed before NASA turned serious attention to SETI.
2. Steven J. Dick, "The Search for Extraterrestrial
Intelligence and the NASA High Resolution Microwave Survey (HRMS): Historical
Perspectives," Space Science Reviews, 64 (1993): 100.
3. David W. Swift, SETI Pioneers – Scientists Talk About Their Search for Extraterrestrial Intelligence (Tucson: University of Arizona Press, 1990), 59.
4. http://scifun.chem.wisc.edu/Calendar/FrankDrake/FrankD_stars.jpg, accessed 10 April 2009.
5. George Basalla, Civilized Life in the Universe – Scientists on Intelligent Extraterrestrials (New York: Oxford University Press, 2006), 26.
6. See Percival Lowell, Mars (London: Longmans, Green and Co., 1895), and Mars as the Abode of Life (New York: The Macmillan Company, 1908). 7. Swift, 67.
8. J. P. T. Pearman, "Extraterrestrial Intelligent Life and Interstellar Communication: An Informal Discussion," in A. G. W. Cameron, ed., Interstellar Communication (New York: W. A. Benjamin, 1963), 287–93.
9. Steven J. Dick, The Biological Universe – The Twentieth-Century Extraterrestrial Life Debate and the Limits of Science (New York: Cambridge University Press, 1996), 427.
10. www.isepp.org/.../Phillip_Morrison.jpg, accessed 10 April 2009.
11. Giuseppe Cocconi and Philip Morrison, "Searching For Interstellar Communications," Nature 184 (September 1959): 844.
12. Swift, 73.
13. Pearman, 289.
14. See, e.g., William Poundstone, Carl Sagan – A Life in the Cosmos (New York: Henry Holt and Co, 1999), 25.
15. Frank D. Drake, "How Can We Detect Radio Transmissions From Distant Planetary Systems?," reprinted in Cameron, 167.
16. Robert Shapiro, Origins: A Skeptic's Guide to the Creation of Life on Earth (New York: Summit Books, 1986), 99.
17. A. Oparin and V. Fesenkov, Life in the Universe (New York: Twayne Publishers, 1961), 239.
18. Dick, Biological Universe, 430.
19. Pearman, 290.
20. Convergence is a well-established phenomenon in evolutionary biology. It is the separate evolution, in different locations or at different times, of the same adaptation in unrelated animals. Dramatic examples of convergence include the facts that vision has evolved independently at least forty times, employing nine different kinds of eyes, and that flight has evolved at least five times. See, e.g., Jack Challoner, The Science of Aliens (New York: Prestel, 2005), 55.
21. Not long after Green Bank, Lilly's research was questioned by some when he published articles about the insights that he believed were available by taking LSD. Although Sagan remained friendly with Lilly, visiting his dolphin research lab on a number of occasions, the proponents of SETI-Science stopped citing Lilly's work.
22. Pearman, 290.
23. Keay Davidson, Carl Sagan – A Life (New York: John Wiley & Sons, 1999), 129.
24. Robert Bieri, "Humanoids on Other Planets," American Scientist, 52 (1964): 457.
25. Pearman, 290.
26. Ibid., 291.
28. Ibid., 292.
29. Iosef S. Shklovskii, "Is Communication Possible with Intelligent Beings on Other Planets," reprinted in Cameron, 11.
30. Davidson, 150.
31. Iosef Shklovskii, Five Billion Vodka Bottles to the Moon – Tales of a Soviet Scientist, Mary Fleming Zirin and Harold Zirin, trans. (New York: W.W. Norton, 1991), 251.
32. Nicholas Rescher, The Limits of Science (Berkeley: University of California Press, 1984), 174.
33. I. S. Shklovskii and Carl Sagan, Intelligent Life in the Universe – Being a Translation, Extension, and Revision of I .S. Shklovskii's Universe, Life, Mind, Paula Fern, trans. (New York: Delta, 1966), 357, 359.
34. Carl Sagan, The Dragons of Eden – Speculations on the Evolution of Human Intelligence (New York: Ballantine, 1977), 242–3.
35. Carl Sagan, Cosmos, Public Broadcasting System (1980), Chapter XII, "Encyclopedia Galactica," minute 28.
36. Shklovskii and Sagan, 357.
37. Dick, The Biological Universe, 54.
38. James Jeans, "Is There Life on the Other Worlds?" Science 95 (1942): 591.
39. Harold Spencer Jones, Life on Other Worlds, 2d ed. (London: English Universities Press, 1952), v–vi.
40. Harlow Shapley, Of Stars and Men (New York: Beacon Press, 1958), 1, 98.
41. Sebastian von Hoerner, "The Search for Signals from Other Civilizations," in Cameron, 272.
42. R. N. Bracewell, "Life in the Galaxy," reprinted in Cameron, 232.
43. http://thecia.com.au/reviews/c/images/close-encounters-of-the-third-kind-7.jpg, accessed 10 April 2009.
44. Bernard le Bovier de Fontenelle, Conversations on the Plurality of Worlds (1686), H. A. Hargreaves, trans. (Berkeley: University of California Press, 1990), 46.
45. Carl Sagan, ed., Communication With Extraterrestrial Intelligence (CETI), (Cambridge: MIT Press, 1973), 353.
46. Carl Sagan, Cosmic Connection (New York: Cambridge University Press, 1973; reprinted as Carl Sagan's Cosmic Connection – An Extraterrestrial Perspective (New York: Cambridge University Press, 2000), 195.
47. Carl Sagan, "A Discussion on the Recognition of Alien Life," Proceedings of the Royal Society of London 189 (6 May 1975): 143.