Are We Alone?
The Sagan–Mayr Debate on Intelligent Life in the Universe
A comprehensive treatise, in celebration of Carl Sagan’s birthday — born November 9, 1934.
I. Prologue: A Birthday and a Question
On the ninth of November, 1934, in a working-class corner of Brooklyn, a boy was born who would spend his life asking the oldest question humans have ever raised to the night sky: are we alone? Carl Sagan did not invent the question, but no one in the twentieth century did more to make it respectable, urgent, and thrilling. He turned a childhood dazzled by “the ampleness of counting” — a boy who learned from his mother that beyond every large number there is always a larger one, that there is no end — into a scientist who insisted that the cosmos was not made for our benefit, and that the only way to learn our place in it was to look.
This treatise is offered in his memory, and it takes as its subject one of the most illuminating arguments of his career: a courteous but profound debate, conducted in 1995, between Sagan and the great evolutionary biologist Ernst Mayr over whether the Search for Extraterrestrial Intelligence — SETI — had any real hope of success. It is a debate worth revisiting not because either man “won,” but because the two of them, between them, mapped the entire terrain of the question so clearly that we are still walking the ground they charted. One spoke for the physical sciences and their cautious optimism; the other for biology and its hard-won sense of how contingent, how improbable, how unrepeatable the history of life really is.
What follows is organized in three movements: an account of the debate itself and its setting; a full presentation of Carl Sagan’s case; and an equally full presentation of Ernst Mayr’s. Around these we place the longer story — how the argument began, how it has aged, and why, three decades later, neither the silence of the sky nor the sudden abundance of discovered worlds has settled it.
II. The Setting: 1995, and a Quiet Newsletter
The debate appeared in an unlikely venue: a modest newsletter called Bioastronomy News, published by The Planetary Society for Commission 51 of the International Astronomical Union, and edited by the Argentine physicist Guillermo A. Lemarchand. It was Lemarchand who had the inspiration to invite the two men to state their positions in print, and who framed the exchange for readers with a question that has lost none of its force: which of the two positions will prove the more palatable to your taste? Read on and decide for yourself.
The timing was not accidental. In 1992, NASA had at last begun a serious, well-funded search — the High Resolution Microwave Survey — with a ten-year, roughly hundred-million-dollar budget. Barely a year later, in 1993, the United States Senate killed it; a single amendment ended federal funding for SETI, and the search survived only on private money. The argument between Sagan and Mayr therefore took place in the shadow of a political defeat, when the question “is this search worth doing at all?” was being asked in Washington as sharply as in any seminar room.
The earlier bout: Mayr versus Drake
Mayr had fired the opening shot two years earlier. In a 1993 letter to the journal Science, he called the search “a deplorable waste” of resources and found it astounding that so dubious an endeavor should be supported “in this time of appalling federal debt.” His scientific point, beneath the budgetary one, was that the kind of intelligence capable of building a civilization was an evolutionary fluke — not an inevitable outcome of life, but an accident so rare that searching for its like elsewhere was close to hopeless. Frank Drake, the founder of the field and author of the famous equation that bears his name, replied that the fact we are the first technological species on Earth does nothing to exclude the independent rise of such species on other worlds. Two years later, Sagan took up the same cause in Bioastronomy News, and the exchange widened into the fuller debate studied here.
The shared scaffold: the Drake Equation
To see what the two men were really arguing about, it helps to recall the framework both of them accepted. In 1961 Frank Drake had written down a simple chain of multiplied factors to estimate the number of communicating civilizations in our galaxy: the rate at which suitable stars form, the fraction with planets, the number of habitable worlds per system, the fraction on which life actually begins, the fraction of those on which intelligence evolves, the fraction of intelligent species that develop a communicative technology, and finally the average lifetime of such a civilization. The power of the equation is that it decomposes an unanswerable question into seven smaller ones.
The crucial thing to grasp — and the key to the entire debate — is that Sagan and Mayr did not disagree about the early terms. Both believed planets were abundant; both believed the origin of life was relatively easy and probably common. Their quarrel lived entirely in the last three factors: whether intelligence evolves, whether it builds radio technology, and how long such a civilization survives. Everything that follows is, in effect, a struggle over the tail of Drake’s equation.
Figure 1. The Drake Equation decomposed. Sagan and Mayr agreed on the opening, planet-and-life terms (teal) and divided sharply on the final, biological-and-social terms (rust) — where the whole debate actually lives.
III. The Two Antagonists
A debate is partly a meeting of arguments and partly a meeting of temperaments. It is worth pausing on who these two men were, because each spoke from the deepest instincts of a different science.
Ernst Mayr (1904–2005)
Mayr was arguably the most important evolutionary biologist of the twentieth century — a principal architect of the “Modern Synthesis” that united Darwin’s natural selection with Mendelian genetics, the man who gave us the modern biological definition of a species, and a scholar whose career spanned the better part of a century; he died in 2005 at the age of one hundred. Mayr’s lifelong theme was the contingency and particularity of living things. Where physicists saw universal laws unfolding toward predictable outcomes, Mayr saw a branching, opportunistic, unrepeatable history in which almost everything that happened might easily have happened otherwise. He came to the SETI question not as an enemy of wonder but as a biologist convinced that his physicist colleagues had simply never reckoned with how strange and unlikely their own existence was.
Carl Sagan (1934–1996)
Sagan was an astronomer and planetary scientist, a pioneer of the young discipline of exobiology, and — uniquely among research scientists of his generation — a public teacher of genius. He helped design the plaques aboard the Pioneer spacecraft and the golden record aboard the Voyagers, humanity’s messages to whatever might one day find them. His television series Cosmos introduced hundreds of millions of people to the scale and beauty of the universe. Mentored by the geneticist H. J. Muller and inspired by the physicists Giuseppe Cocconi and Philip Morrison, who in 1959 first proposed searching for interstellar radio signals, Sagan made the search for other minds the central cause of his scientific life. He would not live to see the debate’s later chapters; Sagan lived to see the discovery of 51 Pegasi b, the first confirmed planet orbiting a Sun-like star, announced in October 1995. He died in December 1996. The abundance of worlds he predicted began to be confirmed only months after his death.
With the stage set and the antagonists introduced, we turn to the arguments themselves — first Mayr’s case for pessimism, then Sagan’s for hope — each presented as its author would have wished it presented, before we weigh them against each other and against the evidence of the intervening thirty years.
IV. Ernst Mayr’s Case: The Argument from Improbability
Mayr’s essay carried a blunt title: the search, he argued, was very unlikely to succeed. His method was disciplined and characteristically biological. Rather than argue from a single grand principle, he broke the probability of success into a chain of separate questions and examined each in turn, asking at every link how confident we could really be. Multiply a series of small probabilities together, he warned, and you arrive very quickly at a number indistinguishable from zero.
Where even the skeptic is an optimist
Mayr began by conceding a great deal. Does life exist elsewhere? Almost certainly, he said. The building blocks — amino acids, the precursors of nucleic acids — are found in interstellar dust; the origin of life somewhere else seemed to him entirely conceivable, perhaps even likely. And where would life arise? On planets, of which the galaxy surely holds millions or billions. On the first two terms of the problem, the evolutionist and the astronomer stood together.
His caution entered with the environmental conditions. A life-bearing world needs a tolerable and stable temperature, the right distance from its star, enough mass to hold an atmosphere, protection from lethal radiation, and liquid water — a demanding combination. Of the planets of our own solar system, exactly one drew the winning hand, and that, Mayr insisted, was a matter of chance. What fraction of other systems would hold such a world? One in ten, one in a hundred, one in a million? The honest answer, he said, is that we are guessing from a single example.
The pivot: the singularity of higher intelligence
Here the argument reached its hinge, and here the biologist parted company with the physicist. Physicists, Mayr observed, tend to think deterministically: grant them life, and they expect intelligence to follow in due course, as though drawn out by a law of nature. The biologist is impressed instead by how improbable such a development is. Evolution does not move in a straight line toward a goal. It branches like a tree, responding only to immediate circumstance, with no foresight and no destination.
Mayr then walked the long road of life’s history to show how lonely the path to intelligence has been. For more than two billion years, life on Earth consisted of nothing but simple prokaryotic cells; none became intelligent. The eukaryotic cell arose once, in a singular and still-mysterious event. Of the sixty to eighty animal phyla that branched out in the Cambrian, only one — the chordates — led toward real intelligence. Among the chordates, only the vertebrates; among the vertebrates, only one line of mammals; among the roughly two dozen orders of mammals, only the primates; among the primates, only a small branch of hominids. If there have been perhaps fifty billion species in the history of the Earth, exactly one acquired the kind of intelligence that can build a civilization. Nothing, Mayr wrote, demonstrates the improbability of higher intelligence better than the millions of evolutionary lineages that failed to achieve it.
Figure 2. Mayr’s funnel. At every level of the tree of life — phylum, class, order, family — only a single narrow branch continued toward technological intelligence. To Mayr, this cascade of near-misses was the heart of the matter.
Two explanations, and the verdict “better to be stupid”
Why should so useful a thing as intelligence be so rare? Mayr offered two possibilities. Perhaps higher intelligence is simply not favored by natural selection: the millions of species that get along without it are proof that it is no prerequisite for success. Or perhaps it is extraordinarily difficult to acquire, demanding a brain so costly in energy, and so improbable a sequence of preconditions, that evolution almost never assembles it. Either way, the conclusion is the same. On the evidence of our own planet, where the overwhelming mass of living things is microbial and does quite well without a single neuron, it is — from the gene’s point of view — very often better to be stupid than to be smart.
Intelligence is not civilization; civilization is not radio
Even granting intelligence, Mayr pressed on, several further filters remain. A degree of intelligence appears in crows, parrots, dolphins, and apes, yet none founds a civilization. And civilization itself does not guarantee the relevant technology: in the last ten thousand years the Earth has borne at least two dozen high civilizations — Sumer, Egypt, Greece, the Indus Valley, the Maya, the Aztec, the many civilizations of China and India — and exactly one of them developed the ability to send and receive signals across space. Nor is there any guarantee that an alien biology would possess sense organs tuned to our electromagnetic signals at all.
The short open window
Finally, and for Mayr decisively, every civilization is brief. He illustrated the point with a small fable. Suppose intelligent beings on a distant world had detected the young Earth a billion years ago and, judging it promising, beamed signals toward us for a billion years, hearing nothing. Discouraged, they resolve to try for one final century — and they choose, say, the years 1800 to 1900. They would have switched off their transmitter precisely before humanity learned to listen. Because the window during which any two civilizations are both technological and close enough to communicate is so narrow, the probability of overlap — even in a galaxy full of civilizations — is vanishingly small. And since our instruments can reach only a small part of our own galaxy, the endless profusion of other galaxies offers no help at all.
Conclusion: an improbability of astronomical dimensions
Mayr’s summation was that at least six of the eight conditions required for SETI to succeed are individually improbable, and that multiplying six small probabilities together yields an improbability of astronomical dimensions. Why, then, he asked, are the advocates of SETI so numerous and so confident? Because, he answered pointedly, they are almost all astronomers, physicists, and engineers, who do not realize that the success of their search turns not on physics and technology but on biology and sociology — precisely the factors their calculations leave out.
V. Carl Sagan’s Case: The Argument from Abundance and Humility
Sagan’s reply was not a point-by-point refutation so much as a reframing — a patient insistence that the question was empirical, not to be settled by calculation from a single example, and that the universe was turning out to be far richer than the skeptics supposed. He answered under the banner of abundance, but his deepest theme was humility: a warning against the long human habit of assuming we are the measure of all things.
The dawn of other worlds
Sagan wrote at the precise moment when the first planets beyond our solar system were coming into view. He could point to Earth-mass bodies detected around a pulsar, to Jupiter-like worlds found around nearby Sun-like stars, and to the discovery that roughly half of young, Sun-like stars are girdled by disks of gas and dust of the very kind from which planets form. He described how the Galileo spacecraft, swinging past the Earth on its way to Jupiter, had successfully detected the signatures of life on our own planet from space — oxygen, methane in disequilibrium, the red edge of vegetation, even the radio emissions of a technological species. And he foresaw, correctly, that space-based instruments would soon be able to find Earth-sized planets around other stars. We were, he wrote, probably entering a golden age in the study of other worlds.
Life comes quickly
On the origin of life, Sagan turned the geological record into an argument. The young Earth was savaged for hundreds of millions of years by a rain of impacts violent enough to boil away oceans and strip away atmospheres. Yet almost as soon as the bombardment eased — within a geological eye-blink — life appeared. A process that begins the moment conditions permit it, he reasoned, is unlikely to be a freak; the swiftness of life’s arrival on Earth suggests that the origin of life is a probable event wherever the chemistry and the conditions allow. He granted that this was an argument from a single case — but insisted it was the best the evidence allowed, and that it pointed toward abundance rather than rarity.
Not humanoids, but functional equivalents
Against Mayr’s funnel, Sagan deployed his sharpest conceptual move. The error of the pessimists, he argued, was to picture extraterrestrial intelligence as something like ourselves — to ask the vanishingly small odds that evolution would retrace the exact path from Cambrian worm to human being. But SETI does not require humans, or anything resembling them. It requires only the functional equivalent of humans: any creature, of whatever shape, chemistry, or history, capable of building a radio telescope. There may be a great many separate evolutionary roads toward such a capacity, each one individually improbable, yet together summing to a substantial likelihood. Evolution is indeed opportunistic and without foresight — on this Sagan and Mayr agreed — but across millions of species and billions of years, a general drift toward greater intelligence can be discerned, and on some worlds the selection pressure toward it may run stronger than it did here.
Convolving the odds with deep time
Sagan then added the dimension Mayr’s static accounting left out: time. Imagine two curves. The first is the probability that a given world has evolved a technological civilization, rising from near zero to something appreciable over billions of years. The second is the distribution of ages of Sun-like stars, many of which are billions of years older than our own. Put the two curves together, Sagan suggested, and an intriguing possibility emerges. If technological civilizations can remain detectable for long periods, the first one we find could be considerably older than ours. Each such civilization would be the improbable winner of its own evolutionary lottery, and some of its members, Sagan noted wryly, might take as much pride as Mayr does in being the only intelligence in the universe.
Figure 3. Sagan’s wager. A galaxy with billions of stars gives technological intelligence many chances to arise. How often it actually does remains unknown. That uncertainty is part of Sagan’s case for looking.
Why a long-lived civilization must build a radio
Sagan also turned Mayr’s grimmest point — the brevity of civilizations — into an argument for the opposite conclusion. Any world like ours is surrounded by asteroids and comets, and is periodically struck; the impact that ended the age of the dinosaurs is only the most famous example. A civilization that wishes to survive the long run must therefore learn to detect and deflect such objects — and the technologies that make this possible, radar and the mastery of the electromagnetic spectrum, are precisely the technologies SETI looks for. Natural selection among civilizations, in other words, favors exactly the capability the search assumes. One need not even have eyes that see in radio; the physics is universal, a matter only of wavelength. And since the threat of impacts never ends, the technology can never be safely retired.
The empirical imperative
Finally, Sagan answered Mayr’s charge that SETI was a fantasy of the physical sciences. When in 1982 he circulated a petition in Science urging that the search be taken seriously, it was signed by some of the most eminent biologists alive — among them Francis Crick, Stephen Jay Gould, Edward O. Wilson, Linus Pauling. The petition’s heart was a single methodological conviction, and it is the truest statement of Sagan’s whole position: no argument from the armchair, however clever, can settle whether we have company in the cosmos. Only observation can. The sole honest test is to look.
VI. The Exchange: The Chain of Rebuttals
Lemarchand’s format allowed the two men not merely to state their cases but to answer each other, and the back-and-forth is where the debate comes alive. The sequence ran in four turns: Mayr opened, Sagan replied, Mayr rebutted, and Sagan closed.
Mayr’s rejoinder
Mayr held his line and sharpened it. He agreed again that planets were abundant and that the origin of life was no great obstacle; the whole question, he repeated, was the probability of higher intelligence, and on that his funnel stood unmoved. Sagan’s principle that “it is better to be smart than to be stupid,” he said, is simply refuted by the history of life, in which intelligence evolved in only one of millions of lineages. He accused Sagan of smuggling in the deterministic thinking of physics — of drawing smooth curves through a process that is anything but smooth — and dismissed the asteroid argument as unrealistic: how, he asked, could the survivors of past impacts possibly have been selected to possess a knowledge of electronics they did not yet have?
Sagan’s reply, and a dilemma
Sagan’s response was the debate’s most elegant stroke. Mayr, he pointed out, wanted to have it both ways. If it is legitimate to extrapolate from the single example of Earth, then we must note that our one example produced abundant life and, in well under the lifetime of its star, a technological species — which argues for abundance, not rarity. But if it is illegitimate to extrapolate from a single case, then Mayr’s own “one-in-fifty-billion” figure collapses, for it too rests on that single case. Mayr could not, Sagan said, have it whichever way suited him.
He pressed several further points. The prokaryotes and protists that Mayr dismissed as never “evolving toward intelligence” are in fact our own ancestors — they evolved toward intelligence along with the rest of the tree. Chimpanzees and bonobos make and use tools; one bonobo had even learned to fashion stone flakes. That only one human civilization developed radio says little about the odds, since the same civilization that built radio telescopes also built the weapons that destroyed its rivals; the Aztec, with a calendar finer than their conquerors’, might in time have built telescopes of their own had they been left undisturbed. And when Mayr claimed that plants cannot receive “electronic” signals, Sagan noted the slip — he surely meant electromagnetic — and observed that plants depend for their very existence on receiving electromagnetic radiation from the Sun. Light is the most powerful way to sense the world at a distance; any competent technological species would attend to it, and the physics of visible light and radio is one and the same.
He closed on the note that runs through all his work. To exclude, by armchair argument, the possibility of intelligent life among the hundreds of billions of stars of the Milky Way struck him as the latest in a long line of human conceits — the same impulse that once placed us at the center of the universe and set us apart in kind from the rest of life. Since Copernicus, he observed, every such presumption has fallen. Better to admit our ignorance, set the a-priori arguments aside, and use the instruments we are fortunate enough to possess to find the answer.
Mayr’s last word, and Sagan’s
Mayr replied with four terse points: that we have no evidence of the “enormous quantities” of Earth-like planets teeming with species that Sagan imagined; that there is a world of difference between a photosynthetic plant and a civilization that builds radio instruments; that we have in truth studied the evolution of life on no planet but our own; and — most important — that he was not denying the possibility of extraterrestrial intelligence, but only calculating the probability of making contact with it by the means available, which he judged to be practically nil.
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Sagan’s final word was a model of candor. He drew his conclusion of countless inhabited worlds, he acknowledged, from the very same evidence that led Mayr to the opposite view; and he noted that Mayr had now conceded that extraterrestrial intelligence might exist, doubting only our ability to reach it. On that remaining question, Sagan said, there is no convincing way to decide by argument. There is only one way to find out, and that is to search. That is what we are doing.
VII. Anatomy of the Disagreement
Read carelessly, the debate looks like optimist against pessimist. Read closely, it is something more interesting: two rigorous thinkers who agreed on the facts and divided on how to reason from them.
What they agreed on
The common ground was wide. Both men believed the galaxy holds a vast number of planets. Both believed that the origin of life is relatively easy and that living worlds are probably common. Neither regarded the origin of life as the hard step. Their entire disagreement was compressed into the final terms of Drake’s equation — the evolution of intelligence, the rise of communicative technology, and the longevity of civilizations.
The real fault line: two ways of knowing
Beneath the specific points lay a difference of scientific temperament. Mayr reasoned as a historian of life, for whom every outcome is contingent, particular, and the product of a chain of accidents that would not recur if the tape were replayed. Sagan reasoned as a physical scientist, for whom the universe is large enough and uniform enough that even improbable things happen reliably somewhere, and for whom a question with an empirical answer should not be foreclosed by calculation. Mayr trusted the one history we can read in detail; Sagan distrusted any conclusion drawn from a single case. Each accused the other, with justice, of leaning on the sample of one when it suited him.
Contingency against convergence
The deepest scientific question the debate raises is still unsettled, and it reaches well beyond SETI. Is the evolution of intelligence a fluke or a tendency? Mayr’s view found its most eloquent ally in his fellow biologist Stephen Jay Gould, who argued that if the history of life were re-run — “replay the tape,” in his phrase — nothing like us would appear again, and that the survival of our own distant ancestors through the Cambrian was a matter of the thinnest luck. Against this stands the phenomenon of convergent evolution: the eye has evolved independently many times, as have flight, echolocation, and complex sociality. If nature discovers the same good solutions again and again, perhaps intelligence is one of them, waiting to be found by many roads — which is precisely Sagan’s claim. The philosopher Daniel Dennett framed the counter-thought memorably: our sense that complexity must keep ratcheting upward may be “just an aesthetic preference,” for most organisms hit upon a workable simplicity long ago and have done little design work since. The question of whether mind is an accident or a destination remains open, and the Sagan–Mayr debate is one of its clearest statements.
The two cases at a glance
The following table sets the two positions side by side. It compresses the argument, but it may serve the visual reader as a map of the whole terrain.
VIII. The Verdict of Time: 1995 to the Present
Thirty years have passed. Both men are gone — Sagan in 1996, Mayr in 2005. What has the evidence done to their arguments?
Sagan’s abundance, vindicated
On the early terms of Drake’s equation, Sagan has been splendidly confirmed. The golden age of planet-hunting he foresaw arrived almost on schedule. The first planet around a Sun-like star was found in 1995, within months of the debate; NASA’s Kepler mission and its successors have since catalogued thousands of worlds, and the statistical harvest suggests that something like one in five Sun-like stars hosts a roughly Earth-sized planet in the temperate zone where liquid water is possible. That implies billions of potentially habitable worlds in our galaxy alone. The cosmos is every bit as crowded with planets as Sagan believed, and far more so than anyone could prove in 1995.
Mayr’s silence, unbroken
And yet. On the terms that Mayr cared about, nothing has changed. After six decades of listening, no confirmed signal from another civilization has ever been detected. The candidate blips that occasionally excite the field have all dissolved into terrestrial interference or natural sources. The abundance of worlds has only sharpened the old puzzle that Enrico Fermi posed over lunch in 1950: if they are out there, where is everybody? Mayr’s emphasis on the brevity of civilizations is today one of the leading candidate answers to Fermi’s question. The silence is real, and it is exactly what Mayr predicted.
The search itself, meanwhile, has had the precarious history Mayr’s skepticism might have forecast. NASA’s funding died in 1993; the privately built Allen Telescope Array has more than once run short of money. But in 2015 the search was revived on a scale Sagan would have relished, when the Breakthrough Listen initiative committed a hundred million dollars of private money — the very sum NASA had abandoned — to a decade of listening, and attention has broadened from radio signals alone to “technosignatures” of every kind: industrial pollutants, waste heat, the glint of great structures. The instruments are better than Sagan ever had; the sky is still silent.
A metaphor, and a third voice
One science writer, surveying the debate’s long aftermath, likened the emergence of intelligence to winning a cosmic lottery — and Mayr’s position to the claim that the jackpot had been won only once, against odds so steep that no amount of ticket-buying elsewhere could be expected to repeat it. Sagan’s rejoinder, in the same metaphor, was that we have so far bought only a handful of tickets: we have scanned a tiny fraction of the sky, at a few frequencies, for a few decades, and a search that has barely begun cannot honestly report that the prize is unclaimed.
A third voice, more recent, suggests that both men may have been too quick to equate “intelligence” with the human, technological kind. On this biocentric view — indebted to the biologist Lynn Margulis and her vision of a living planet — the truly dominant and enduring form of life on Earth is microbial, a planet-spanning network of bacteria that has regulated the biosphere for billions of years and will outlast us all. If intelligence means integrated problem-solving, bacteria have it in abundance; if it must mean radio telescopes, we may be defining the cosmos in our own narrow image — the very parochialism Sagan spent his life warning against, now turned against the terms of the search itself. The debate, in other words, is still generating new arguments.
So who was right?
The fair verdict is that each man was right about his own half of the equation and neither could prove his half of the other’s. Sagan was right that worlds are abundant and that the question is empirical; the planets are there, and the search goes on because he and a few others insisted it should. Mayr was right that the biological and social terms are the decisive ones, that they remain entirely unmeasured, and that confidence in contact outruns the evidence. The single data point that both men reasoned from — the Earth, with its one technological species — is still, maddeningly, the only one we have. Until a second example is found, or the sky is searched far more thoroughly than it yet has been, the debate cannot be closed. What can be said is that the terms in which we still argue the question are, almost exactly, the terms these two men set down in 1995.
IX. Coda: Sagan’s Wager
If the argument cannot yet be won, why did Sagan care so much that it be waged? Because for him the search was never only about the odds. It was about what kind of creature asks the question at all. To look is to admit that we do not know; to admit that we do not know is the beginning of every discovery humans have ever made. Mayr’s instinct was to compute the probability and, finding it small, to spend the money elsewhere. Sagan’s instinct was that a question of this magnitude, once it has become answerable, demands to be asked — and that the generations able to ask it for the first time have no right to look away.
In Cosmos he gave the thought its most beautiful form. Measured against the lifetime of a star, a human life is the flicker of a mayfly; we are ephemeral creatures on a small, cold, exotically rare sphere of silicate and iron, offering scarcely a hint that we were ever here. And yet this mayfly has learned to weigh the stars, to read the chemistry of worlds it will never touch, and to send its instruments and its questions out past the edge of its own solar system. Whether or not anyone is listening, the act of listening is itself a kind of answer — a declaration that we have outgrown the ancient certainty that the universe was made for us, and taken up instead the harder and nobler work of finding out.
That was Carl Sagan’s wager, and it is the fitting thing to remember on his birthday. He did not claim to know that we have company among the stars. He claimed only that the question was real, that it was ours to ask, and that the asking — patient, humble, evidence-bound — is one of the finest things our species does. The sky is still silent. The search goes on. Somewhere, something incredible may be waiting to be known.
Appendix A. The Cast
Carl Sagan (1934–1996). Astronomer, planetary scientist, exobiology pioneer, and the pre-eminent science communicator of his age. Architect of the Pioneer plaques and the Voyager golden record; author of Cosmos, The Dragons of Eden, Pale Blue Dot, and the novel Contact; co-founder and first president of The Planetary Society. The abundance of worlds he predicted began to be confirmed within months of his death. Sagan lived to see the discovery of 51 Pegasi b, the first confirmed planet orbiting a Sun-like star, announced in October 1995. He died in December 1996.
Ernst Mayr (1904–2005). One of the twentieth century’s foremost evolutionary biologists and a principal architect of the Modern Synthesis. Originator of the biological species concept and a lifelong champion of the contingency of evolutionary history. His 1993 letter to Science opened the public argument that this treatise examines.
Guillermo A. Lemarchand. Argentine physicist, researcher at the Institute for Radio Astronomy, and editor of Bioastronomy News, who organized and introduced the 1995 exchange and later saw it reprinted and translated for new audiences.
Frank Drake (1930–2022). Founder of observational SETI, author in 1961 of the Drake Equation, and the first to conduct a radio search for other civilizations. He opened the earlier 1993 round of the argument in reply to Mayr.
Appendix B. A Timeline
1934 Carl Sagan born, 9 November, Brooklyn, New York.
1959 Cocconi and Morrison propose searching for interstellar radio signals (Nature).
1960 Frank Drake conducts Project Ozma, the first SETI search.
1961 Drake formulates his equation at the Green Bank conference.
1977 Voyager spacecraft launched bearing the golden record.
1978 Sagan wins the Pulitzer Prize.
1980 Cosmos airs, reaching hundreds of millions of viewers.
1992 NASA begins the High Resolution Microwave Survey, a funded SETI program.
1993 The U.S. Senate cancels NASA SETI funding. Mayr and Drake exchange letters in Science.
1995 The Sagan–Mayr debate is published in Bioastronomy News. 51 Pegasi b, the first planet around a Sun-like star, is discovered.
1996 Carl Sagan dies, 20 December.
2005 Ernst Mayr dies, aged 100.
2009 NASA’s Kepler mission launches, soon revealing thousands of exoplanets.
2015 The Breakthrough Listen initiative commits $100 million to a decade of searching.
Today Billions of worlds known; the sky still silent; the question still open.
Appendix C. The Drake Equation
N = R* × fₚ × nₑ × fₗ × fᵢ × f_c × L
N the number of civilizations in our galaxy with which communication might be possible
R* the average rate of formation of suitable stars
fₚ the fraction of those stars that have planets
nₑ the average number of potentially habitable planets per such star
fₗ the fraction of habitable planets on which life actually appears
fᵢ the fraction of life-bearing planets on which intelligence evolves
f_c the fraction of intelligent species that develop a communicative technology
L the average length of time such civilizations release detectable signals
Sagan and Mayr agreed, in effect, on R* through fₗ and divided on fᵢ, f_c, and L.
Appendix D. Sources and Further Reading
This treatise draws on the primary debate and on a set of commentaries and tributes surrounding it.
Mayr, E., and Sagan, C. “The Debate on the Probability of Extraterrestrial Intelligence,” Bioastronomy News, vol. 7, nos. 3–4 (1995), organized and edited by G. A. Lemarchand; reprinted in The Planetary Report (1996) and in Lemarchand & Tancredi, eds., Astrobiology: From the Big Bang to Civilizations (UNESCO, 2010).
Mayr, E. Letter on SETI, Science, vol. 259 (1993); and “Does It Pay to Acquire High Intelligence?” Perspectives in Biology and Medicine, vol. 37 (1994).
Bioastronomy News, vol. 9, no. 1 (1997) — memorial issue, with tributes to Sagan by G. A. Lemarchand, Philip Morrison, Frank Drake, and Louis Friedman.
Laine, P. E. “The Sagan–Mayr Debate Revisited.”
Johnson, G. “The Intelligent-Life Lottery,” The New York Times (19 August 2014).
Slijepcevic, P. “The Astrobiological Cat,” Philosophy Now, issue 135.
Gould, S. J. Wonderful Life (1989); Dennett, D. Darwin’s Dangerous Idea (1995); Sagan, C. Cosmos (1980) and Pale Blue Dot (1994).
“For small creatures such as we the vastness is bearable only through love.”
— Carl Sagan, Contact
