We can make some very rough estimates of the number of sites where life possibly exists beyond our Solar System. These estimates take account of many factors that affect the origin and evolution of life. To appreciate the method most often used, consider first a mundane example that has no bearing on extraterrestrial life.

An Exercise in Probability Suppose that we wish to determine the number of Swedes who are now living in Boston and who have, uncharacteristically, both brown eyes and red hair. We could of course go door-to-door, searching directly for the each of them. Or we could approximate the number statistically in the following way. First, consult a recent population census, which lists ~10,000 Swedes now living in Boston, most of them surely having the more common combination (for them) of blue eyes and blond hair. Then, noting demographic data from Sweden generally, namely that the fraction of all Swedes having brown eyes is 1 in ~50 and the fraction having red hair is 1 in ~100, we can make an estimate of the number of Swedish Bostonians having both these rare traits.

The problem can be formulated in terms of the following 3 simple multiplications—assuming that there’s no natural linkage between hair and eye color (including, for example, no tendency for brown-eyed Swedes to dye their hair red):

number of Swedish Bostonians having both brown eyes and red hair =

total number of Swedish Bostonians

X fraction of Swedes having brown eyes

X fraction of Swedes having red hair.

Evaluating this equation, we find 10,000 x ¹/₅₀ x ¹/₁₀₀, or 2 such Swedes who have both brown eyes and red hair. We can't be sure that exactly 2 such Swedes live in Boston, for we didn't take an actual head count. Maybe there are 3 or 4, or perhaps only 1. Here we used statistical reasoning, limited data, and probability theory to help us make an estimate.

In a similar way, we can attempt to estimate the number of sites where life currently resides in the Universe. In formulating this problem, let's make two restrictions. First, we confine our analysis to our own Milky Way Galaxy, and second, we stipulate these life forms to be not only intelligent but also technologically competent. These are the advanced civilizations with whom we might have a chance of communicating—if they exist.

These restrictions are reasonable considering the time scales involved. Given the ages of the oldest globular cluster stars, we concluded in the GALACTIC EPOCH that our Galaxy is ~12 billion years old. Even if several billion years are needed for the most massive stars to populate interstellar space with enough heavy elements to construct rocky planets, this still means that there could exist planets as old as 8 billion years, or nearly twice the age of Earth. And if evolution proceeds at roughly the same rate throughout the Universe (an admittedly unverifiable assumption), such planets could have housed some form of life long ago, even well before our Solar System originated. Provided that such life has survived, their civilizations could be vastly more advanced than ours.

Estimating the number of galactic civilizations isn’t a trivial exercise. It requires us to summarize a great deal of knowledge covered in all of the previous 7 epochs of this Web site. A large amount of this knowledge (or ignorance!) can be compressed into a small space by formulating this problem much as for the above case of the Swedish Bostonians. Key aspects of this extraterrestrial-life problem are then cast into the following relationship, popularly known as the Drake equation, after the American astronomer who first devised an early version of it in the 1960s:

number of technologically intelligent civilizations now present in the Galaxy =

rate of star formation averaged over the lifetime of the Galaxy

X fraction of stars having planetary systems

X average number of planets within those planetary systems that are suitable for life

X fraction of habitable planets on which life actually arises

X fraction of life-bearing planets on which intelligence evolves

X fraction of intelligent-life planets that develop a technological society

X average lifetime of a technologically competent civilization.

Figure 8.24 translates this equation into pictorial form. This figure illustrates how only a small fraction of all star systems in the Milky Way are likely to generate ultimately the advanced qualities specified by all the right-hand factors in this equation.

FIGURE 8.24 — Of all the star systems in our Milky Way (represented here by the largest blue box), progressively fewer and fewer have each of the qualities typical of an enduring technological society (represented by the smallest box at the lower right corner).

To evaluate each of the factors in the Drake equation requires familiarity with many fields of knowledge. For example, we need a good deal of physics and astronomy to estimate the rate of star formation and the fraction of stars having planets. More astronomy, along with some geology and biology, are needed to specify the properties of an ecologically suitable planet on which life might arise. Insights from biology, chemistry, anthropology, and neurology are needed to estimate the chances of life, and then intelligence, developing on any given planet. A great deal more anthropology, as well as archaeology and history, are needed to know what fraction of the intelligent-life-bearing planets might actually create a cultured, technological society. And finally, the lifetime of such an advanced civilization depends on numerous additional factors, including history, politics, sociology, psychology, and many other daily influences.

Note that the lifetime specified here is really the technological lifetime—that is, the longevity of an advanced civilization, starting from the time it gains the capability to explore its planetary system or to communicate across interstellar distances. In Earth's case, such technological expertise has thus far been only a few decades, and what our full technological lifetime might be depends upon many of those uncertain issues addressed earlier in this FUTURE EPOCH.

The Drake equation amounts to a lengthy analysis in probability. The many factors in the equation are multiplied rather than added because each one is assumed independent of any other. And several of those factors could be less than 1—which means that, when multiplied together, their product can quickly become very much less than 1. For example, if each of any three fractions on the right side of the equation has a value of 0.1, their product is 0.001. So, even though the individual chances might be 1 out of 10 for each origin of life, of intelligence, and of technology, the combined chance of finding a planet with all three of those traits would be only 1 in 1000.

Probability analyses of this sort, then, can quickly lead to very small numbers. Many researchers use this kind of reasoning to argue that advanced civilizations aren’t likely to exist anywhere else in the Galaxy. Taken to its logical extreme, this type of pessimistic argument could also be used to claim that none of us should exist. After all, the chance that your parents, grandparents, great-grandparents, and so on, would have produced precisely you is extremely small. But in fact you do exist, however small the probability! Equally important, if the current generation of humans didn’t exist, another statistically indistinguishable generation surely would. Apparently, many different routes connect all states of matter and life. And it’s the multiplicity of these different paths that makes possible highly unlikely events. Key questions are: How common are those rare events? How often do they occur in the Galaxy? How widespread is intelligent life beyond Earth?

To estimate the number of civilizations now present in our Galaxy, we must provide a number for each factor on the right side of the Drake equation. Unfortunately, much of our knowledge affecting these factors is often uncertain, especially those on the far right of the equation. In particular, biologists generally argue that the probability for life or intelligence to emerge on any given planet is nearly impossible to assess with any degree of certainty. It's not that they necessarily regard the chances to be small for the origin of life or intelligence (although some of them do). Rather, most biologists judge that the science of chemical and biological evolution hasn’t yet progressed enough to make meaningful estimates of its likelihood operating on other planets.

Some researchers even contend that we have no information on which to make any of the required estimates. Others disagree, arguing that science and technology can provide some information. At any rate, virtually everyone agrees that rigorous values are currently unknown for most of the factors in the Drake equation. Values usually chosen are based on a good deal of personal insight (or prejudice) among groups of scientists.

Generally, the reliability of the estimate for each factor in the Drake equation declines markedly from left to right. Our knowledge of astronomy enables us to make a reasonably good stab at the first factor, namely the rate of star formation in our Galaxy. But it's much harder to evaluate some of the interior factors, such as the fraction of life-bearing planets that eventually develop intelligence. And as for the factor on the far right side of the equation, the longevity of technological civilizations is totally unknown. Only one known example of such a civilization exists—that's us on Earth—and how long we will last before a natural or man-made catastrophe terminates intelligent life on our planet is impossible to know.

Although accuracy is limited while judging values for most of the factors, it's still remarkable that we can formulate such a single equation to address this very general problem. Estimating the chances for advanced extraterrestrial life requires insights ranging from physics, astronomy, chemistry and geology, through biology, anthropology and politics. There's probably no other problem like it. Trying to solve this equation exemplifies the true virtue of thinking about extraterrestrial life. It forces us to stretch our imaginations, to expand our minds. It reminds us of all the beautifully woven interrelationships—interdisciplinarity at its best—among virtually all the many disciplines of human knowledge. And it provides an effective summary of much of the material presented in the previous 7 epochs of this Web site.

In what follows, we examine each of the factors, in turn, of the Drake equation, after which we shall return to the whole equation in order to compute an estimate.

Rate of Star Formation To estimate the average number of stars forming each year in the Galaxy, note that at least 100 billion stars have shone during the course of the Milky Way's lifetime of ~12 billion years. Accordingly and rounding off, we find a star-formation rate of ~10 stars/year.

Some researchers regard this value as an overestimate, especially since much galactic matter is already contained in stars. They argue that fewer stars are forming now than must have formed at earlier epochs of the Galaxy when more interstellar gas was still available. And there is some truth to that since, as noted in the GALACTIC EPOCH, the rate of stellar birth did seem to peak earlier in the Universe when galaxies were experiencing greater mergers and acquisitions. However, we cannot very easily reduce this estimate too much, since radio and infrared astronomers have recently discovered numerous sites where galactic clouds are now contracting to form stars, as noted in detail in the STELLAR EPOCH. Furthermore, our estimate includes only stars now shining; those that formed long ago, and have since exploded as supernovae, would tend to increase the star-formation rate.

All things considered, a value of ~10 stars/year seems a reasonable number when averaged over the lifetime of the Galaxy. It's probably accurate to within an order of magnitude, meaning it could be as small as 1 or as large as 100.

Fraction of Stars Having Planetary Systems This factor is a measure of the rarity or commonality of planetary systems. Inputs pertinent to this factor arose at the end of the PLANETARY EPOCH, which noted that one of the hot fields in astronomy today concerns the measured gravitational tugs by invisible planets on some nearby, low-mass stars. Only recently have we begun to obtain clear, yet indirect evidence that planet-sized objects do, in fact, exist beyond our Solar System.

Recall that the condensation model is the currently best idea for the origin of our Solar System. As such, planets are imagined to form as natural by-products of the star-formation process. Optical and infrared observations of disks rich in dust around some young stars tend to support this idea, as also noted in the PLANETARY EPOCH. And recent discoveries of extrasolar planets orbiting nearby stars further bolster this idea, suggesting that planets are common around many Sun-like stars, even if the sizes, scales, and overall makeup of those alien planetary systems don’t much resemble our own.

Recall also that the condensation model is beset by the Sun's unaccountably slow spin. Even so and ironically, the fact that many other stars experience the same problem may well be the best evidence for the widespread existence of planetary systems elsewhere. Figure 8.25 shows that the low-mass F-, G-, K-, and M-type stars usually have less angular momentum than expected. As for our own Solar System, much of their angular momentum might have been transferred to surrounding planets. In other words, many low-mass stars might spin sluggishly because they have planets orbiting about them. After all, planets of the Solar System account for most of our system's angular momentum, prompting speculation that most low-mass stars also have planets.

FIGURE 8.25 — Low-mass stars are observed to spin more slowly than high-mass stars. What's more, the low-mass stars (solid line) show a good deal less rotation than expected (dashed line), prompting speculation that F-, G-, K-, and M-type stars have planetary systems. The sharp discontinuity in spin angular momentum occurs at ~2 solar masses.

If this reasoning is correct, then O-, B-, and A-type star systems are poor sites for planets. Eliminating these three types of stars doesn’t rule out large numbers of objects, however, because high-mass stars are scarce compared to low-mass stars in our Galaxy. Provided the condensation model holds, then it would seem reasonable that most stars have planets. Accordingly, a value of 1 is a rational numerical result for this term in the Drake equation. The critical assumption here is the validity of the condensation model; if it's wrong, this term could be much less than 1.

Number of Habitable Planets per Planetary System This factor mainly concerns the range of temperature throughout a planetary system. Probably more than any other single quantity, temperature determines the feasibility of life on a given planet. Surface temperature of a planet depends mostly on two things: the planet's distance from its parent star and the thickness of the planet's atmosphere. Planets having a nearby parent star (though not too close) or some atmosphere (though not too thick) are expected to be reasonably warm, much like Earth or Mars. Objects having neither, such as many of the moons of our Solar System, will surely be cold by our standards. And planets having both, such as Venus, will be hot.These are often called "Goldilocks criteria": not too hot but not too cold, not too far and not too close, . . ., habitable planets have thermal properties that are "just right."

As drawn in Figure 8.26, a 3-dimensional zone of "comfortable" temperature surrounds every star. The extent of this region, often termed a "habitable zone," depends on the spectral type of the star, the atmosphere of the planet, and the type of biology likely present. Here we restrict ourselves to life as we know it, namely that which functions provided water remains in liquid form (273 - 373 K). An F-type star, for instance, with a hot surface temperature, has a rather large zone in which temperatures are likely to be comfortable on a planet with a moderate atmosphere. Cooler stars, namely those of G, K, and M spectral types, have progressively smaller habitable zones. (O-, B-, and A-type stars aren’t considered here because, although at face value they would have vast habitable zones, analysis of the previous term makes it unlikely that they have any planets. Even if they do, these massive stars aren’t expected to last long enough for life to develop on their planets.)

FIGURE 8.26 — The extent of a habitable zone (shading) is much larger around hot stars than cool ones. For a star like our G-type Sun, the zone extends from ~0.8 to ~2.0 A.U. Note that these zones are 3-dimensional, completely surrounding the central stars in all directions. (Pearson)

The concept of a habitable zone can be appreciated by imagining regions surrounding various sources of heat on Earth. For example, skaters on a frozen lake know well the range of distance surrounding a bonfire (analogous to a star) where the warmth is comfortable. This range depends somewhat on the amount of clothing worn (analogous to a planet's atmosphere). Not much heat is felt far from a bonfire, while just the opposite problem occurs when too close. Also, the larger the bonfire, the larger the zone of comfort, and the more people who benefit from its released heat.

To evaluate this factor, we need to estimate the average number of planets expected within an ecologically comfortable zone. At the extremes, hardly any planets likely reside in the small habitable zones around M-type stars, whereas several planets might coexist in the zone about an F-type star. Another guideline is that we know for sure that one planet orbits within the habitable zone of a certain G-type star—namely, Earth about the Sun. Actually, three planets—Venus, Earth, and Mars—reside within (or close to) the habitable zone surrounding our Sun. Venus is hotter than we like because of its thick atmosphere and proximity to the Sun; Mars is a little colder than we like for just the opposite reasons. It's interesting to note that if Venus had Mars' thin atmosphere, and Mars had Venus' thick atmosphere, both these neighboring planets would have had surface conditions resembling those on Earth.

This factor of the equation probably equals more than 1 for F-type stars but less than 1 for M-type stars. We might then ordinarily use 1 as a reasonable estimate for the average number of habitable planets orbiting any F-, G-, K-, or M-type star. But two other issues affect our assessment of this factor, the first concerning the M-type stars. Besides their having very small habitable zones, the majority of existing M-type stars undoubtedly formed in the Galaxy long ago when few, if any, heavy elements were available to make rocky planets. We can thus eliminate M-type star systems as sites for alien life: Those that are young probably have no habitable planets, while those that are old probably have no planets at all. Since M-type stars amount to about 80% of all known stars, our estimate of this factor drops to 20%, or ¹/₅.

One more issue rules out still more star systems as potential sites of extraterrestrial life—namely, the observation that many stars in our Galaxy are grouped into close-knit clusters. Life probably couldn’t emerge in such multiple-star systems, even if the stars themselves have planets. Figures 8.27(a) and (b) show how a planet within the simplest type of star cluster—a binary-star system—could maintain only certain stable orbits, during which time the planet would likely be either too hot or too cold. Even in the case of improbable, though stable "figure-8" orbits like that in Figure 8.27(c), uniform planetary heating over long periods of time—an apparent necessity for life—would be unlikely. These thermal problems worsen for star clusters having many stars.

FIGURE 8.27— In binary-star systems, planets are restricted to only a few kinds of orbits that are gravitationally stable. Planets (dashed curves) are shown in (a) closely orbiting one of the two stars, which overheat it when in the middle, (b) circling both stars in an elliptical orbit which takes it periodically in and out of a habitable zone, and (c), interweaving between the two stars in a "figure-8" pattern, which again usually overheats it in the middle. (Pearson)

Since approximately half of all known stars are members of multiple systems, we can further reduce this factor of the Drake equation. Multiplying ¹/₅ by ¹/₂, we obtain ¹/₁₀ as a rough estimate for the average number of habitable planets per star. It doesn't mean, of course, that one-tenth of a planet orbits all stars; instead, our estimate suggests that 1 out of every 10 stars has a habitable planet. Clearly, single F-, G-, and K-type stars are the best candidates for stellar systems with habitable planets having life as we know it.

Before moving on to the next factor in the equation, take note of a few cautionary remarks. Perhaps the analysis above is much too restrictive. Maybe cool M-type stars are more suitable candidates than the above argument implies. Or perhaps some planets could survive the rigors of a double-star system. Furthermore, other objects (like some of Saturn's and Jupiter's big moons) might have thick enough atmospheres to remain comfortable outside the theoretically computed habitable zones. Should any of these statements be true, then our estimate of ¹/₁₀ is surely a conservative one; it could be increased.

Yet another reason suggests that the above analysis may well underestimate the number of extraterrestrial sites suitable for life. It's not impossible that life could arise and survive in places much different from conventional planets. Given the prevalence of organic molecules in interstellar space, we can speculate about life inside galactic clouds in much the same way that this FUTURE EPOCH noted the slim (but real) possibility for life in the atmospheres of Venus or Jupiter. Only the densest interstellar regions—the dark molecular clouds—have physical conditions resembling those of planetary atmospheres. In all fairness, however, the lack of energy to provide warmth and the scarcity of matter to provide nourishment tend to make interstellar clouds unlikely abodes for life. If we're wrong, though, the prospects for extraterrestrial life would increase greatly.

Fraction of Habitable Planets on which Life Actually Arises This is the province of chemical evolution, the critical question being: Is life the only type of material complexity expected in other habitable zones, or is life only one example of many types of (non-living) complexity? In other words, is or is not life an inevitable consequence of the evolution of matter? Given the proper conditions and enough time, is life a sure bet or is it quite rare?

Current research seeks to understand how complexity arises from simplicity, as discussed in the CHEMICAL EPOCH. Much progress has been made in the past few decades, but a good appreciation for some of the most important chemical steps that led to life still eludes us. That’s because life itself is extraordinarily complex, much more so than galaxies, stars, or planets.

Consider, for a moment, the simplest known protein on Earth. This is insulin, which has 51 amino acids linked in a specific order along a chain. As noted earlier in the CHEMICAL EPOCH, probability theory can be used to estimate the chances of assembling the correct number and order of amino acids for such a protein molecule. Since there are 20 different types of amino acids that normally participate in life, the answer is 1/20⁵¹, which equals ~1/10⁶⁶. This means that the 20 amino acids must be randomly assembled 10⁶⁶, or a million trillion trillion trillion trillion trillion, times before getting insulin. This is obviously a great many combinations, so many in fact that we could assemble and reassemble the 20 amino acids trillions of times per second for the entire history of the Universe and still not achieve the correct ordering of this protein. Larger proteins and nucleic acids would be even less probable if chemical evolution operates at random. And to assemble a human being would be vastly less probable, if it happened by chance starting only with atoms or simple molecules.

This is the type of reasoning used by some researchers—especially biochemists—to argue that we must be alone, or nearly so, in the Universe. They suggest that biology of any kind is a highly unlikely phenomenon. They argue that meaningful molecular complexity can be expected at only a very, very few locations in the Universe, and that Earth is one of those special places. And since, in their view, the fraction of habitable planets on which life arises is extremely small, the number of advanced civilizations now in the Galaxy must be even smaller. Of all the myriad galaxies, stars, planets, and other wonderful aspects of the Universe, this viewpoint maintains that we are among few creatures anywhere to appreciate the grandeur of it all. If their arguments are correct, we could even be alone in the Universe.

But does chemical evolution operate at random, that is, by chance and chance alone? Alas, there’s another point of view—one often preferred by astrophysicists. Several reasons suggest that the change from simplicity to complexity may not proceed randomly. The first reason is this: Of the billions upon billions of basic organic groupings that could possibly occur on Earth from the random combinations of all sorts of simple atoms and molecules, only ~1500 actually do occur. Furthermore, these 1500 organic groups of terrestrial biology are made from only ~50 simple organic molecules, including the known amino acids and nucleotide bases. This implies that molecules critical to life aren’t assembled entirely by random chance; rather, some determinism operates as well. The electromagnetic forces at work at the microscopic level evidently remove some of the randomness by guiding the molecules into certain, specific linkages.

Direct laboratory experiments support this second view—one whereby chance mixes with necessity as throughout much of cosmic evolution. Simulations that resemble conditions on primordial Earth are now routinely performed with a variety of energies and initial reactants. These experiments demonstrate that unique (or even rare) conditions are unnecessary to produce the precursors of life. Complex acids, bases, and proteinoid compounds are formed under a rather wide variety of physical conditions (provided there’s no free oxygen present). And it doesn't take long for these reasonably complex molecules to form—not nearly as long as probability theory predicts by randomly assembling atoms.

Most tellingly, every time this type of experiment is done, the results are much the same. The oily organic matter trapped in the test tube always yields the same proportion of acids, bases, and rich proteinoids. If chemical evolution proceeded only randomly, a different result would be expected each time the experiment is run. Apparently, electromagnetic forces do govern the complex interactions of the many atoms and molecules in the soupy sea, substituting organization for randomness.

Of course, precursors of proteins and nucleic acids are a long way from life itself. But the beginnings of life as we know it seem to be the product of less-than-random interactions among atoms and molecules. That's important to know. Just how non-random—that is, how common—life itself might be is unknown.

An important caveat deserves mention here. Even if life everywhere in the Universe is based on carbon chemistry and obeys the basic laws of biology familiar to us, we shouldn’t be foolish enough to think that organisms elsewhere would evolve to look like us anatomically. Life forms on other planets—even carbonaceous organisms operating in a watery medium—would likely experience a wholly different set of environmental and genetic changes, some of which are indeed based on chance. The mechanism of biological evolution, with its mutations, natural selection, and adaptations functioning over long durations of time, would likely yield little outward resemblance to life on Earth.

So what do we choose as a numerical estimate for the fraction of habitable planets on which life actually arises? Either the number is much smaller than 1 if chance has a big influence. Or the number is close to or equal to 1 if chance plays no appreciable role in the long run. The former view suggests that life arises naturally, though rarely, whereas the latter view maintains that life is virtually inevitable given the proper ingredients, suitable environments, and long enough periods of time. No easy experiment can distinguish between these extremes.

What we really need is a laboratory where organic chemistry has been left alone for a few billion years. What transpires there could help us decide the degree of randomness inherent in the molecular reactions prerequisite for life. Fortunately, some of the nearby planets and their moons provide us with such a laboratory, and a most interesting period of exploration is now underway as our spacecraft begin to probe them for signs of life. In the minds of some researchers, the discovery of life—any kind of life, even microbial—on Mars, Europa, Titan, or some other object in our Solar System would convert the origin of life from an unlikely miracle to an ordinary statistic—to a value equal to or near 1 for this term of the equation.

In addition to "randomness" not being fully operational in chemical evolution (as in all aspects of cosmic evolution), other grounds tend to bolster the prospects for extraterrestrial life. One of these is that aliens could be based on something other than the carbon atom. The preceding section of this FUTURE EPOCH stressed that life "as we know it" is carbon-based life, operating in a water-based medium, with higher forms metabolizing oxygen. Yet once again, are we being chauvinistic by thinking that other types of biology are impossible? Perhaps so, but we've also noted several reasons why carbon-based life has more strength, diversity, and adaptability than any other.

We naturally wonder: Can we make an objective judgment of this factor in the Drake equation independent of our own prejudices? After all, chemists study Earth chemistry, not general chemistry. And biologists study the only kind of biology they know. Perhaps the alternatives haven’t yet been sufficiently investigated. At any rate, should biochemistries exist other than the carbon-in-water type studied throughout this Web site, then the prospects for extraterrestrial life increase greatly.

Fraction of Life-Bearing Planets on Which Intelligence Arises This is the province of biological evolution, a central concern being the difficulty or ease with which multicellular organisms emerge. In particular, what is the probability that advanced organisms eventually develop central nervous systems or brains?

As noted in the BIOLOGICAL EPOCH, the fossil record demonstrates that biological evolution does indeed occur. Sporadic mutations in the genes of organisms allow them to adapt to gradually changing environments, often filling the best available niches for the surrounding circumstances. Some types of organisms are naturally selected to die; others are just as naturally selected to thrive. Mixing chance with necessity yet again, natural selection is a mechanism that creates highly improbable results.

Like other aspects of life, intelligence results from a series of adaptations. Organisms that profitably use those adaptations can develop more complex behavior, which in turn enables them to completely dominate a given niche. Such complex behavior favors organisms with a variety of choices needed for their advanced development. Those that change in ways that allow them to function better in a changing environment are often considered "smarter."

What, then, are some of the factors that contributed to our complex behavior? Very basic traits, such as locomotion and food gathering, were probably not so important. These traits don’t produce superior intelligence, rather they’re needed by all animals just to ensure survival. Nature will not tamper with life's most essential needs. Primitive tools were probably more important along the road to intelligence, allowing our ancestors to inhabit whole new ecological niches. As noted in the CULTURAL EPOCH, the fossil record of several million years ago implies a clear correlation between tool use and brain size—the two seemingly got bigger together.

The invention of hunting, requiring much social cooperation and planned movement, may have been another key factor that allowed our ancestors to leave the dense forests and venture into the open savannah. The development of language was surely important, enhancing social ties and basic intelligence. By communicating, individuals could signal each other and coordinate their efforts while gathering food or seeking protection. In the opinion of many anthropologists, language was a key advance in the evolution of our brain—so important that when reduced to its essentials, human intelligence may be synonymous with human language.

So again we are faced with a decision. What numerical estimate do we choose for the fraction of life-bearing planets on which intelligence of some sort eventually develops? One school of thought maintains that given enough time, intelligence is inevitable. This viewpoint argues that every life-bearing planet has a niche labeled "intelligent life," and given that natural selection is a universal phenomenon, some organism would find it advantageous to eventually fill it. In other words, provided that organisms could afford the extra weight and energy-driven metabolism, intelligence would seem to be an obvious advantage for survival. If this view is correct, then the fifth factor in the Drake equation equals or nearly equals 1.

Other researchers object. They argue that there is nothing at all inevitable about intelligence; rather, on Earth its onset was long and arduous. For ~2.5 billion years—from the start of life ~3.5 billion years ago to the first appearance of multicellular organisms ~1 billion years ago—life didn’t advance beyond the unicellular stage. Life remained simple, and dumb, but it survived. And thus far, such microbial life on Earth has survived a lot longer than any form of intelligent life. This alternative view suggests that while life may be widespread throughout the Universe—including microbes, plants, bugs, and many less complex species—it hasn't necessarily evolved to the point of becoming intelligent. Should this extreme view be correct, the equation’s fifth factor could be very small. If so, we might be —hubris aside—the smartest life forms anywhere in the Galaxy.

Fraction of Intelligent-life Planets that Develop a Technological Society Intelligence would seem to be a useful attribute in the development of any higher species. In our case, we inherited several advantages from our reasonably smart ancestors of a few million years ago: A pollution-free environment, a sophisticated society, a good family life, a robust physique, and a taste for steak. Intelligence led to a whole new way of life—a rather comfortable state of affairs. This is the realm of cultural evolution, which was treated at length in the CULTURAL EPOCH.

But, now, modern men and women are threatened with numerous global crises, as noted earlier in this FUTURE EPOCH. The number of humans on Earth is increasing rapidly, and neither food nor energy can be distributed well enough to keep everyone content on a daily basis. As if these problems weren't enough, we also face the possibility of human-made disaster brought on by weapons of mass destruction. Other planet-wide problems loom on the horizon as well, threatening our civilization with repeated global issues, the like and scope of which Earth's societies have never before experienced.

We then ask: How did intelligent life change from the rather pleasant daily routine left to us by our ancestors a million years ago to the current predicaments we now face in the 21^st century? In other words, How did we mess it up so badly? The answer, apparently, dates back ~10,000 years—to the time when our recent ancestors began inventing agriculture, cities, states, nations, empires. Above all, they selected the road to technology—which in and of itself is a good thing, for it’s the rise of precisely our technological civilization that has given us the tools to unlock secrets of the Universe, as well as to search for extraterrestrial life. But technology also has its drawbacks, the chief one being that technology is a major source of many of our current global problems, not least the threat of destroying ourselves.

To evaluate the sixth factor of our equation, we seek to estimate the probability that intelligent life eventually develops technological competence. Should the rise of technology be inevitable, given long enough durations of time, this factor is again close to 1. If so, then at least one species on all life-bearing planets would eventually develop a technological society. By contrast, if it's not inevitable—if intelligent life can somehow avoid developing technology—then this term could be much less than 1. This latter view envisions a Universe teeming with intelligent life, yet very few among them ever becoming technologically competent. Perhaps only one managed it—us.

It’s nearly impossible to distinguish between these two extreme views. We don't even know how many prehistoric Earth cultures failed to develop technology. We do know that the roots of our present civilization arose independently in several different places on Earth, including Mesopotamia, India, China, Egypt, Mexico, and Peru. Since so many of these ancient cultures originated at about the same time, we might judge the chances to be good that some sort of culture will inevitably develop, given some basic intelligence and enough time.

But literary culture is one thing and technological culture quite another. Archaeologists argue that some of these ancient peoples never did develop technology. The Mayan civilization of Interamerica, for example, had sophisticated social and political organizations. They built primitive observatories, enabling them to study the motions of stars and planets with their naked eyes. In fact, the Mayan calendar was more accurate than that of the Spaniards who conquered them several centuries ago. Despite these accomplishments, however, archaeological records show that the Mayans used neither wheels nor metal. They built small toys with wheels, but not large carts or wheelbarrows useful in farming or herding. And they apparently had no use for metal other than in jewelry or ornaments. Either they never thought to use these as technological aids, or they realized them and rejected them.

Regardless of how many ancient earthlings accepted or rejected technology, only humans developed it and now use it. This is a sticky point for some researchers. If technology is an inevitable development, they ask, why haven't other forms of Earth life also found it useful? The probable reason is that any given niche is usually filled by only one species. And the niche labeled "technological intelligence" is currently filled by Homo sapiens.

In an evolving society, we should expect only one species per biological (or cultural) niche. As an example, recall that the recent fossil record implies the coexistence of several hominids angling for the same niche several million years ago. The apparent result was competition and the demise of all but one type of those ancestral hominids. Competition between the various australopithecines likely provided great impetus in the survivor's drive toward superior intelligence.

So the fact that only one technological society now exists on Earth doesn’t imply that the sixth factor in the Drake equation must be very much less than 1. On the contrary, it’s precisely because some species will probably always fill the niche of technological intelligence that this term is likely close to 1.

One further point is worth noting. This sixth factor could be decreased somewhat if most planets are completely covered by water. Technological intelligence is likely to develop only on the solid parts of a planet. Aquatic life may be intelligent, but it's hard to imagine how it could ever become technically advanced. To discover the laws of applied physics, something resembling hands must be able to manipulate gears, pulleys, inclined planes, and the other rudiments of elementary technology. This isn’t a criticism of the dolphins, about whose intelligence there is no question. They probably admire the stars while flipping their heads above water, perhaps even wondering if dolphins reside on other worlds. But unless they leave the water, they will probably remain technologically incompetent. Will they leave the water (again) to try to develop that technology? Probably not now, because we fill the niche of land-based technological intelligence. If they tried to evolve onto the land, they would soon enter into direct competition with us, and as the currently dominant species we would not likely tolerate it.

Lifetime of Technological Civilizations As if each of the factors discussed above were not uncertain enough, the last factor in our equation is the most uncertain of all. How can the lifetime of a technological civilization be reliably determined? After all, we’re the only known example and the duration of human society is an issue of heated debate.

One thing is certain: If the correct value for any one factor in the Drake equation is very small, then not many technological civilizations are now present in the Galaxy. Even if the average lifetime of a civilization is a million (10⁶) years, only one factor with a value as small as 1 in a million (1/10⁶) implies that we are alone in the Galaxy. For example, suppose that all the factors have optimistic values, with the exception of the evolution of intelligence. Assume further that, for some reason, there’s only 1 chance in a million that life on a suitable extraterrestrial planet will attain intelligence. Substituting this pessimistic value into the equation, along with the other optimistic values, we find

number of galactic civilizations =

10 stars per year

X 1 planetary system per star

X 0.1 habitable planet per planetary system

X 1 planet with life per habitable planet

X 0.000001 planet with intelligence per planets with life

X 1 planet with technology per planets with intelligence

X lifetime (in years) per planets with technology.

This long equation yields a simple answer, after we cancel all the units. That answer is 0.000001 x lifetime (in years). Accordingly, even if the lifetime of a typically advanced civilization is a million years, the number of galactic civilizations equals 1. That's us! For more than a single civilization to reside now in the Galaxy, assuming the above numerical factors, the average lifetime of all civilizations must exceed 1 million years.

That said, note that even with this pessimistic estimate for the eventual onset of intelligence, thousands of other civilizations might be spread across the Galaxy, provided that their average lifetime is billions of years. Thus the average longevity of technological civilizations is a critically important factor regarding the prevalence of smart extraterrestrial beings.

Further, if any two factors in the equation are very small, then chances are slim indeed that the Galaxy is teeming with intelligent life. For example, if many fewer planets exist than suggested by the condensation model, and if the development of technology is highly improbable, then the multiplication of these factors implies that many civilizations cannot possibly now reside in the Galaxy.

By contrast, the most optimistic estimate uses values close to or equal to 1 for each factor in the Drake equation. In this case, a most curious outcome occurs. If habitable planets are plentiful, and the evolution of life, intelligence, and technology inevitable, then our equation takes on the form:

number of galactic civilizations = 10 x 1 x 0.1 x 1 x 1 x 1 x average lifetime (in years).

Notice that all the interior factors cancel, since 10 x 1 x 0.1 x 1 x 1 x 1 equals 1. Thus the number of technological civilizations now present in the Galaxy equals the average lifetime of those civilizations. Consequently, this optimistic solution places even greater emphasis on the many issues affecting a civilization's lifetime.

How long do civilizations typically endure after they invent technology? What are some of the issues that could truly harm such advanced civilizations? Speaking of inevitabilities, do all technological civilizations eventually self-destruct? Answers to these questions are by no means trivial; they might be impossible. To estimate the longevity of any civilization requires us to know how intelligent beings are likely to use, or abuse, technology—in short, how cultures behave in the presence of life-prolonging yet life-threatening technology. Perhaps some insight can be gained by examining what the future holds for us on planet Earth. That’s why the whole first part of this eighth, FUTURE EPOCH raised concerns about our future path along the arrow of time.

Where Are They? If we’re not alone, then a straightforward, yet deeply puzzling, query comes naturally to mind, first posed formally by Enrico Fermi, one of the 20^th century’s greatest physicists—namely, if intelligent extraterrestrials now populate our Galaxy, then "Where are they?" Why don't we have some evidence of their advanced status—for example, interstellar radio communications, settlements on nearby planets or moons, unworldly stellar engineering projects, and the like?

An obvious possible answer to Fermi's paradox is that we are the only technological intelligent creatures in the Universe today. Advanced life forms might consciously decide to limit or even to terminate their technical growth, either out of economic necessity or ethical scruples. Or they might simply lose interest in pursuing grander objectives, in effect reaching a stage of mental stagnation wherein curiosity is minimized. Yet another possible solution to the paradox claims that the Milky Way is ruled by a superintelligent civilization, potentially one governed so masterfully that its superior inhabitants have placed aside our part of the Galaxy, as we do on Earth with wildlife preserves, so that they can learn more about cosmic evolution by studying us in our natural habitat—an idea implying that we effectively live in a galactic zoo!

An even more sobering possibility is that technological civilizations do fail to survive—a proposition not entirely unreasonable, for no civilization may be able to check its tendency to self-destruct over long time scales. This possible solution to the paradox amounts to a timing issue: At any given moment in cosmic history, the Universe might well be populated by only a few (and maybe only one) cosmic intelligences.

An analogy is useful here: Imagine an ornate chandelier having a huge number of light bulbs. The chandelier is meant to represent the Galaxy, while the bulbs denote planets ecologically suited for the emergence of technological intelligence. Each bulb illuminates only when technology on a given planet surpasses some crucial threshold—such as radio communicative ability. Two factors generally determine whether the chandelier is blazing or dim: One concerns the vitality of the evolutionary process leading to technically competent life—the extent to which evolution's hand twists far enough to screw in and light the bulb. The other concerns the length of time each bulb stays lit—the longevity of a technological civilization. At the two extremes, the chandelier could be brightly lit with many glowing bulbs, indicating much intelligent activity on many planets, or it could be completely unlit, designating a technological void. Considering the many varied and speculative time scales affecting the evolution of intelligence and of culture—especially those for technological emergence and longevity—all the bulbs in the chandelier might eventually glow, yet without any two bulbs ever being lit simultaneously.

Perhaps the most reasonable answer to Fermi's question is that we might be among the earliest of technologically oriented species to emerge in the Galaxy—the result of evolutionary fortune rather than any anthropocentrism. The development of organized structures depends ultimately on the expansion of the Universe, for as noted in the CHEMICAL EPOCH it’s this grandest of all changes that establishes the thermodynamic conditions needed for the emergence of those structures. And since the onset of life and intelligence requires certain minimal (although broadly specified) times for their origin, many technological intelligences might arise and evolve more or less in parallel throughout the Universe. If true, then our Galaxy is potentially populated with myriad civilizations resembling our evolutionary status, yet only now are technological civilizations "coming on line" in the cosmos—and we on Earth are among the vanguard of this whole new Life Era.