This chapter examines the prehistory of the search for extraterrestrial intelligence (SETI) prior to 1961. It reviews the first attempts to contact other planets at the scale of the solar system – that is, interplanetary communication (premodern SETI era). We emphasize the latter half of the nineteenth century because many efforts were made at that time to contact our neighboring planets through interplanetary telegraphy. Such a technique became conceivable in the 1860s thanks to many advances in the field of terrestrial communication (electrical telegraphy, telephone). Generally, the pioneers in interplanetary telegraphy proposed to send flashes using powerful lamps and reflectors to reach our neighboring planets, Mars and Venus. Considering their methodology, the early proposals using light flashes could be compared to modern Active SETI, or METI (messaging to extraterrestrial intelligence). The intellectual approach is similar to that of CETI (communication with extraterrestrial intelligence), since the first attempts were expected to be a two-way exchange of information. Even though they remained only theoretical, these attempts demonstrated that basic thought about a universal language had begun as early as the 1860s. At the turn of the twentieth century, new possibilities emerged with the birth of wireless telegraphy and the development of radio techniques used for telecommunications on Earth. Listening to Mars by means of radio waves was sporadically attempted in the 1920s. By the mid-twentieth century, developments in radio astronomy had a decisive influence on the birth of SETI because they allowed astronomers to contemplate the possibility of contact with extraterrestrials at a much larger scale, that of interstellar distances.

The first term in the Drake Equation is R*, the number of newly formed stars in the galaxy per year. The estimate given in 1961 was ten stars per year. Over the past fifty years, new instruments and methods have allowed us to better understand how stars begin their lives and how efficiently gas can create new ones.

Powerful instruments specifically adapted to the study of star formation include both space facilities – the Galaxy Evolution Explorer (GALEX), the Spitzer Space Telescope (SST), the Herschel Space Observatory (HSO), and the Hubble Space Telescope (HST) – and a host of ground-based optical, infrared, submillimeter, and radio telescopes. These instruments have described in unprecedented detail the key phases and physical processes that lead to the formation of individual stars.

In-depth case studies of individual star-forming regions have yielded an understanding of the central physical processes that determine how molecular clouds contract and fragment into clumps and cores and, finally, clusters and individual stars. The determination of the global star formation rate (SFR) for the Milky Way is rigorously based on measurements of the global parameters of several local star-forming regions. In general, any total flux measure that is related to the SFR of a galaxy (including the Milky Way) is completely dominated by high-mass stars, since these are responsible for virtually all of the luminosity of a galaxy.

The detailed picture of how gas is transformed into stars requires not only knowledge of the SFR but also the distribution of mass of stars at their birth, a function called initial mass function (IMF). Theoretical simulations have explored how large molecular clouds fragment into stars under very different physical conditions. These works have permitted us to identify the most important physical parameters and have led to analytical formulations of the SFR and the IMF. In particular, they give estimates of a factor that is particularly important for the Drake Equation: the fraction of stars that are binary.

Most estimates of the SFR of the Milky Way have relied on global observables. Such studies generally rely on indirect tracers of massive (O- and early-B-type) stars to determine a massive SFR. This value is then extrapolated to lower masses to derive a global SFR for our galaxy. For example, an analysis from the late 1970s led to a value of five solar masses per year by making use of the fact that the integrated flux density from an HII region is a direct measure of the number of ionizing photons required to maintain that HII region, and is therefore an indirect measure of the number of O- and early B-type stars. In 2006, an estimate of four solar masses per year was derived from observations using the European Space Agency's International Gamma-Ray Astrophysics Laboratory (INTEGRAL) mission, which measured the gamma rays emitted by radioactive aluminum as a proxy for the massive star population of the Milky Way. Another study from 2006 gave a value of 2.7 solar masses per year, by using the total 100-micron flux of our galaxy. Along the line of these examples, this chapter will review in detail the evolution of estimates of R*, which is now closer to five solar masses per year than the ten assumed in 1961.

The question of how intelligence evolves on different planets is a central factor in the Drake Equation and informs the fields of bioastronomy, astrobiology, and SETI (the search for extraterrestrial intelligence). In this chapter, I trace the history of our conceptions of intelligence through changes and growth in our understanding of brain evolution, genetics, and animal behavior, and present a modern view of intelligence that places human intelligence in an evolutionary context and linked to the multiple intelligences inhabiting this planet. Much of our current understanding of intelligence as an astrobiological question and, specifically, the nature and much-vaunted uniqueness of human intelligence, should be updated by modern knowledge and divested of outdated ideas such as the scala naturae, progressive evolution and teleology, and the anthropic principle. These notions continue to fuel a fundamental misconception of intelligence as a uniquely human phenomenon with little or no evolutionary or comparative context and, therefore, no way to understand its true biological nature. In this chapter, I will discuss these issues in detail and replace these outmoded notions with new information and insights about how and why intelligence evolves and the levels and distribution of intelligence across species on this planet. Modern understanding of intelligence shows that it is continuous across all animal life on Earth and that the human brain is embedded in the evolutionary web of primate brain evolution and contains the hallmarks of nervous-system evolution traced back to the first life forms on this planet. These updated ideas provide a biological context for understanding the mechanisms and range of intelligence on this planet and should therefore serve the critical purpose of revising notions of fi, leading to more productive outcomes for the study of the evolution of intelligence on this and other planets.

The Drake Equation factor ne, the number of planets per planetary system with an environment suitable for life, has recently received increased attention as astronomers discover many planetary systems within the Milky Way. This factor is closely associated with the notion of planetary habitability, which is not new. During the second part of the nineteenth century, astronomers such as Richard Proctor (1837–1888) and Camille Flammarion (1842–1925) studied planetary habitability in our solar system a century before this concept was updated by the systematic exploration of the solar system and the detection of exoplanets. The question was tackled in scientific terms while studies of the Martian surface were intensifying. The term “habitability” was used mainly for Mars, which was regarded as a sister planet of Earth and looked like a possible abode for life. More generally, studies during this period attempted to compare Earth with other planets in the solar system, in spite of the lack of accurate data on their environments. Analogy soon became the basic way in which to calculate the level of habitability of each planet. In the meantime, spectroscopy was developing quickly and began to provide more data about planetary atmospheres.

At the dawn of the space age, Hubertus Strughold (1898–1987) suggested that we examine the planets of the solar system using the concept of “planetary ecology.” This notion was very similar to the concept of habitability employed earlier by nineteenth-century pioneers. Strughold also coined the term “ecosphere” to describe the region around a star that has conditions suitable for life-bearing planets, a concept equivalent to the habitable zone. A decade later, Stephen Dole redefined the concept of habitability as the planetary conditions suitable for human life. He used probability methods to estimate the number of planets in the galaxy where human life might be possible. This chapter reviews the main historical contributions up to the 1960s regarding the concept of planetary habitability and its different meanings over time.

An understanding of the fl factor of the Drake Equation rests on the notion of a planet suitable for the development of life. The goal of this chapter is to present the transformation of this notion as it has evolved over the last two centuries. Three periods can be distinguished. The first period, which begins toward the end of the nineteenth century, is characterized by the conviction that life is present throughout the entire universe. Some scientists imagine other worlds, often comparable to our earth. Others develop the theory of panspermia and the dispersion of germs of life to all of the new planets. The second period, from the 1920s to the 1950s, is marked by the formulation of complete interdisciplinary hypotheses about the origins of life on earth. These hypotheses were notably focused on understanding the conditions in which life appeared on earth, the only known example of the development of life. The third period, from the 1950s to the present, has studied the possible probiotic chemical pathways that may have pertained on earth. There has been broad scientific acceptance of the idea that life may exist elsewhere in the universe, though the idea has been contested. However, such opposition has been based more on philosophical conception than on scientific argument.

The Drake Equation, a method for estimating the number of communicative civilizations in the Milky Way galaxy, was a product of its time in several important ways. After a period of several decades during which the idea of life on other planets had reached a low point due to rise of the “rare collision” hypothesis for planet formation, by the 1950s the nebular hypothesis was once again in favor, whereby planets would form as a common byproduct of stellar evolution. The Miller–Urey experiments in the early 1950s produced complex organic molecules under simulated primitive-Earth conditions, indicating life might easily originate given the proper conditions. And while little was known about the gap between primitive life and intelligent life, and a sophisticated understanding of intelligence was lacking, the Lowellian Mars still lingered in the cultural background and, along with contemporary astronomical advancements, stimulated the scientific imagination to consider aliens. The original emphasis on “radio communicative” reflected the new era of radio astronomy, exemplified by the radio telescopes under construction at the newly founded National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia, where Frank Drake was working at the time when the equation originated as the meeting agenda for an informal conference there in 1961. Drake's ability to undertake such a controversial subject, including the first radio search for extraterrestrial intelligence in 1960, was aided by senior scientists Lloyd Berkner and Otto Struve. Assessments of the probabilities of extraterrestrial life and intelligence had been sporadically undertaken in the course of the twentieth century, but most particularly by former Harvard Observatory Director Harlow Shapley in his book Of Stars and Men (1958); Drake had recently graduated from the Harvard astronomy program, and had cited the book. Here we look at the origins and development of the equation over time, including significant variations in the equation; examine positive and negative views of its epistemological status and utility ranging from scientists to popular authors such as Michael Crichton; and attempt to tease out the scientific and metaphysical assumptions behind the equation. We conclude by discussing the future of the equation, and the cultural hopes and fears it embodies.

The final variable in Drake's equation for estimating the number of such civilizations in the Milky Way is L, the lifetime of communicating civilizations. Drake's initial estimate for L was 10,000 years, but others have suggested different values or even defined the variable differently. Unfortunately, for empirical information on how to estimate L, we have a sample of only one, ourselves. But even those data are incomplete, since our own technology that permits interstellar communication is less than 100 years old and there is no obvious end to our civilization in sight. So, we have not only no idea of what the value of L should be, we also have little idea of how to go about estimating it. Additionally, like each of the other variables in the equation, L is almost certainly the product of many other variables. Therefore, the first goal of this chapter is to examine efforts at estimating values for L since 1961, including how those estimates were developed. Second, I will consider the concept of civilization in order to determine if the historical study of earthly examples can inform us about L. Third, I will examine variables that might influence the value of L by analyzing lists of existential risks proposed by a sample of individuals and organizations. Finally, I summarize these lines of thought in order to determine if they can inform us regarding possible values of L.

One of the Drake Equation factors that has changed the most since 1961 is ne, the average number of planets per star that can potentially support life. This factor is still evolving.

The definition of conditions for developing life is related to the definition of a circumstellar habitable zone. It is generally defined as the zone in which physical conditions make presence of liquid water possible. As a first approximation, it implies a temperature between 0 and 100 degrees Celsius, the so-called Goldilocks condition. This preliminary estimate is based on the temperature of the star: in other words, its spectral type and the distance between the star and its planet. Changes in the ne term are principally due to unexpected characteristics on one hand of many exoplanets and on the other hand of some objects in the solar system.

We first discuss the basic condition for life: liquid water. Stellar spectral types where life is most likely to emerge and exist are reviewed, and less favorable conditions of hot and cold stars are discussed. We also look at planets orbiting one member of a binary star, or around both stars (“circumbinary planets”). We point out the physical conditions necessary for planets to shelter life, including mass and other physical parameters. Exomoons are interesting objects and are discussed as well. Relations between the star and its planetary system are reviewed. No longer is distance the only parameter to be considered. In the case of terrestrial exoplanets with large eccentricity that cross the habitable zone, life with some phases of hibernation may be possible. Some terrestrial exoplanets orbit so close to their star that they are co-rotating, keeping one face to the star at all times, which implies that a temperate annular zone may exist between the very hot face in front of the star and the very cold face on the opposite side. Characteristics of some satellites in the solar system discovered since the space age show that some tidal effects are liable to extend the habitable zone, as can be seen by the detection of oceans flowing below the icy surface of Europa or internal water springing from the geysers of Enceladus. It would be interesting to search for and study as a source of possible life moons of giant exoplanets located in the habitable zone of their star. As a conclusion, the continuously increasing number of small rocky planets provides great encouragement to search for extraterrestrial life. Indeed, they show a high rate per star and satisfy the conditions necessary for producing life.

In the five decades since Frank Drake formulated his eponymous equation, our understanding of astrophysics and planetary science has advanced enormously. The first three terms of the equation refer to factors that are now known with reasonable precision, due in no small part to the discovery of enough extrasolar planets for meaningful statistics to be developed. Unfortunately, this progress has not been matched by a similar leap in understanding of the remaining factors – the biological ones. In particular, the probability of life emerging on an Earth-like planet, fl, remains completely unknown. In the 1960s and 1970s, most scientists assumed that the origin of life was a freak event, an accident of chemistry of such low probability that it would be unlikely to have occurred twice within the observable universe. Today, however, many distinguished scientists express a belief that life will be almost inevitable on a rocky planet with liquid water – a “cosmic imperative,” to use the evocative term of Christian de Duve. But this sentiment is based on little more than fashion. One may assign a probability to a process only after the mechanism that brings about that process is known. As we have little knowledge of how a nonliving mix of chemicals is transformed into a living thing, we can say almost nothing about its likelihood. Indeed, it is easy to imagine plausible constraints on the chemical pathway to life that would make its successful passage infinitesimally small. In the case of the fifth term in the Drake Equation – the probability that intelligence will evolve if life gets going – at least we have a well-understood mechanism (Darwinian evolution) on which to base a probability estimate (though it still remains deeply problematic). The same is true of the remaining terms. Thus, the uncertainty in the number of communicating civilizations in the galaxy, N, is overwhelmingly dominated by fl. The uncertainty would be immediately removed, however, if a second, independent sample of life was discovered. The best current hope for establishing a value of fl close to one would be the discovery of a “shadow biosphere” on Earth or a breakthrough in understanding the rules governing the organization of information in complex chemical networks.

The rate of star formation, past and present, in the galaxy is an important astronomical question that lies at the heart of our modern understanding of the nature of stars, their distribution in space and time, and the history of the galaxy itself. Here we look at history, to 1961, concentrating on the question of how stars form and what is the rate of formation of stars in the galaxy. It is related to what astronomers call the luminosity function for stars, or the frequency distribution of luminosities for all stars in the galaxy. Our recounting will encompass the correlation era of stellar statistics, when astronomers began to conceptualize evolutionary models for the galaxy: that the luminosity function changes with time, and that the rate of change is determined not only by the rate of star formation but also by the nature and rate of the evolution of stars once they have been formed. We will show that, throughout much of the first half of the twentieth century, these rates were debated and revised many times, first by the recognition that there exists a main sequence of stars, which was interpreted as the distribution of stars existing in some form of quasi-stability. Later came the recognition of giant stars and instability, then the mass–luminosity law for stars, then an awareness of the energy sources of the stars and the subsequent revision of the evolutionary place and nature of giant stars, and finally, the fact that populations of stars existed within the galaxy that were generationally distinct. Based on this review, we will then argue that Drake estimated R* (regarded by most as the number of stars forming per year in the galaxy) in a manner consistent with the state of knowledge at the time.

Of the Drake Equation's seven factors, fp, the fraction of stars with planets, is the one on which we have made the most progress. Since 1961, we have gone from ignorance to a fairly reliable determination of what this number is. Our progress has occurred in three phases. In the first, which lasted for more than thirty years after 1961, astronomers were limited to speculation and probabilities, based on the Copernican principle and their growing understanding of processes that led to the formation of our solar system. The second phase began in 1995, when the first exoplanet was discovered using the Doppler method, which kicked a new research field into high gear. Excitement mingled with confusion as hot Jupiter-like planets with highly eccentric orbits were found, planets with unprecedented properties. The next decade and a half saw a steady march of the detection limit from Jupiter mass to Neptune mass and more recently to super-Earths, exoplanets with two-to-five times Earth's mass. The census grew from a few to over five hundred. The third phase of exoplanet discovery came with the 2009 launch of the Kepler satellite. Astronomers began to use the transit or eclipse method to detect planets smaller than Earth and, as of 2014, they have found more than 3,000 planetary candidates, about 300 of which are similar to or smaller than Earth. Both the Doppler and the transit methods provide information on potential habitability, and taken together they yield a mean exoplanet density. There is remarkable agreement in the exoplanet count derived from three different detection methods: Doppler, transits, and microlensing. With an uncertainty factor of two, there is an exoplanet for every sequence star, and most of them are small, terrestrial, and orbit red dwarfs. The projected number of terrestrial planets around main sequence stars in the Milky Way is about 200 billion. These recent results put the Drake Equation on firmer epistemological footing than ever before, a trend that will continue as exoplanet research moves toward the detection of biomarkers and the imprint on exoplanet atmospheres of microbial life.

This chapter is an overview of the prehistory of L, how people across the globe from our earliest sources to 1961 have tried to understand the beginning and end of history, and the rise and fall of civilizations. Factor L is put into a longer historical context of human conceptions about history, time, and civilization. In focus is the question of how it was possible to formulate L in its modern version, as embodied in the Drake Equation. This was not possible, I argue, until the end of the nineteenth century. L required a number of philosophical, scientific, and technical discoveries and inventions before it became possible to discuss the longevity of extraterrestrial technical civilizations. Of special significance was the “discovery of time,” the emergence of a set of ideas for understanding human temporality: first, linear time, time that has a beginning and an end, and in which nothing is forever; second, long time lines, in which there was a time before humans and human civilization, and that the history of our civilization is only a fraction of the history of universe; and third, that time has a direction, that humans are historical beings – that is, knowledge, culture, and society are not something preexisting but something created by humans, evolving, that rests on the experiences and actions of previous generations in a cumulative process leading to the development of knowledge, behavior, and life conditions, or what is sometimes called the “idea of progress.”

The first section concerns the beginning of time: notions of the dawn and age of the world, thoughts about the history of the Earth and humankind, when humans entered the history of the universe, and the emergence of the notion that human civilization has existed for only a fraction of the total age of the universe. The second section concerns the direction of time: where we are heading, ideas about how societies emerge, the rise of civilizations, and the notion of advancement, the thought that civilization is not a given but something created by humans. The third section puts forward notions of the end of time: doom, cataclysms, the meaning of history, and how and why civilizations and empires – or the whole world – fall. Finally, I conclude that L is a measure of the civilizing or socialization process, and the variables that underlie it: biocultural coevolution and the interaction between the evolution of cognition and socialization.

An assessment of fl depends heavily upon perspectives gained from the single known example of our own biosphere. Extrapolating this single data point to a quantitative estimate of fl amounts to sheer speculation. But recent discoveries have identified perspectives about our biosphere and early planetary environments that are quite relevant to fl and that also benefit our search for a second example of life elsewhere. This chapter addresses these topics from the following perspectives: concepts of life, life's environmental requirements, early conditions on rocky planets, and the origins of life. The habitability of a planetary environment is defined by the intrinsic environmental requirements of life, which, in turn, arise from the most universal attributes of life itself. Key environmental requirements include a suitable solvent, the chemical building blocks of life, biologically useful sources of energy, and environmental conditions that favor the survival of key complex molecules and structures. The earliest evidence of our biosphere now extends back to more than 3.7 billion years ago, essentially as old as the oldest rocks that could have preserved recognizable evidence. The most ancestral characteristics of microbial metabolism are broadly consistent with the resources that were likely available as early as 4.4 billion years ago. Organic compounds relevant to prebiotic chemistry have been discovered in primitive bodies (e.g., meteorites, comets) and in interstellar space, and these might have been delivered intact to planetary surfaces. Thus, life might have begun very soon after habitable conditions were established. Regarding the origins of life, biochemical research is narrowing the knowledge gap between prebiotic chemicals and the first living systems. RNA molecules could have served as both self-replicators and enzymes. The earliest functional protein enzymes might have been much smaller in size and thus perhaps easier to develop than previously imagined. The prebiotic environment could have provided molecules that formed vesicles as precursors of cellular envelopes. Although the evidence certainly cannot prove the notion that another young Earth-like planet probably also sustained an origin of life, the evidence is at least quite consistent with the notion that life can arise early on Earth-like planets. Also, the evolution of stars and planets follows trajectories that allow reasonable estimations to be made of long-term changes in planetary climates and habitability. Thus, although recent discoveries have not yet reduced the enormous uncertainty in estimating fl, they have substantially improved our strategies for seeking a second example of life that would, in turn, substantially reduce that uncertainty.