In the search for extraterrestrial intelligence (SETI), it is often assumed that intelligent life on an Earth-like exoplanet would inevitably develop the technological means for interstellar communication. This assumption ignores the critical role that fossil fuels played in driving the Industrial Revolution on Earth, which ultimately gave rise to our own advanced technological civilization (ATC) and the possibility of interstellar communication. We therefore propose that any habitable exoplanet that could potentially generate an ATC must contain sizable fossil fuel deposits, especially coal, which supplied most of the energy used in the Industrial Revolution during the 19th century. Coal is critical because, based on an Earth-like geology, it is more accessible than the much deeper deposits of oil and gas. Without coal, it would have been impossible to tap into the vast underground deposits of oil and gas during the 20th century. This raises the question of the inevitability of coal formation on an Earth-like exoplanet. Here we present arguments that coal formation may be unlikely, even on an Earth-like planet, because of the many contingent factors that have been recorded in the rock and biological record of our own planet, including the evolution of oxygenic photosynthesis itself, which generated the oxygen-rich atmosphere required for complex life to develop. Central to our argument is the host of highly contingent taphonomic factors, involving plate tectonics and climate, that were required to convert the tropical lycopsid swamp forests of the Pangean supercontinent to the massive coal deposits of the Carboniferous period. Finally, we discuss the need for synchronicity of the appearance of intelligent life forms and the maturation of vast deposits of coal. We conclude that the large number of contingencies involved in coal production justifies adding a term for coal to the Drake Equation for the number of ATCs in the galaxy.

Of all species on Earth, only one – Homo sapiens – has developed a technological civilization. As a consequence, estimates of the number of similar civilizations beyond Earth often treat the emergence of human-like intelligence or ‘sophonce’ as an evolutionary unicum: a contingent event unlikely to repeat itself even in biospheres harbouring complex brains, tool use, socially transmitted behaviours and high general intelligence. Here, attention is drawn to the unexpected recency and temporal clustering of these evolutionary preconditions to sophonce, which are shown to be confined to the last ≤102 million years. I argue that this pattern can be explained by the exponential biotic diversification dynamics suggested by the fossil record, which translated into a nonlinearly expanding range of cognitive and behavioural outcomes over the course of Earth's history. As a result, the probability of sophonce arising out of a buildup of its enabling preconditions has been escalating throughout the Phanerozoic. The implications for the Silurian hypothesis and the search for extraterrestrial intelligence (SETI) are discussed. I conclude that the transition from animal-grade multicellularity to sophonce is likely not a rate-limiting step in the evolution of extraterrestrial technological intelligences, and that while H. sapiens is probably the first sophont to evolve on Earth, on macroevolutionary grounds it is unlikely to be the last.

The Drake equation has proven fertile ground for speculation about the abundance, or lack thereof, of communicating extraterrestrial intelligences (CETIs) for decades. It has been augmented by subsequent authors to include random variables in order to understand its probabilistic behaviour. However, in most cases, the emergence and lifetime of CETIs are assumed to be independent of each other. In this paper, we will derive several expressions that can demonstrate how CETIs may relate to each other in technological age as well as how the dynamics of the concurrent CETI population change under basic models of interaction, such as the Allee effect. By defining interaction as the change in the expected communication lifetime with respect to the density of CETI in a region of space, we can use models and simulation to understand how the CETI density can promote or inhibit the longevity and overall population of interstellar technological civilizations.

The Fermi paradox has given rise to various attempts to explain why no evidence of extraterrestrial civilizations was found so far on Earth and in our Solar System. Here, we present a dynamical model for the development of such civilizations, which accounts for self-destruction, colonization and astrophysical destruction mechanisms of civilizations including gamma-ray bursts, type Ia and type II supernovae as well as radiation from the supermassive black hole. We adopt conservative estimates regarding the efficiency of such processes and find that astrophysical effects can influence the development of intelligent civilizations and change the number of systems with such civilizations by roughly a factor of $2$; potentially more if the feedback is enhanced. Our results show that non-equilibrium evolution allows for solutions in-between extreme cases such as ‘rare Earth’ or extreme colonization, including scenarios with civilization fractions between $10^{-2}$ and $10^{-7}$. These would imply still potentially large distances to the next such civilizations, particularly when persistence phenomena are being considered. As previous studies, we confirm that the main uncertainties are due to the lifetime of civilizations as well as the assumed rate of colonization. For SETI-like studies, we believe that unbiased searches are needed considering both the possibilities that the next civilizations are nearby or potentially very far away.

The Drake equation has been used many times to estimate the number of observable civilizations in the galaxy. However, the uncertainty of the outcome is so great that any individual result is of limited use, as predictions can range from a handful of observable civilizations in the observable universe to tens of millions per Milky Way-sized galaxy. A statistical investigation shows that the Drake equation, despite its uncertainties, delivers robust predictions of the likelihood that the prevalent form of intelligence in the universe is artificial rather than biological. The likelihood of artificial intelligence far exceeds the likelihood of biological intelligence in all cases investigated. This conclusion is contingent upon a limited number of plausible assumptions. The significance of this outcome for the Fermi paradox is discussed.

We use recent results from astrobiology, particularly the A-form of the Drake equation and combine it with data on the evolution of life on Earth to obtain a new assessment of the prevalence of technological species in our Universe. A species is technological if it is, in theory, capable of interstellar communication. We find that between seven and 300 technological species have likely arisen in the Milky Way until today, the current state of which however unknown. Assuming that we are currently alone in our Galaxy, we estimate that we would need to wait for roughly 26 million years for a 50% chance of another technological species to arise. By relating our results to the much-debated Fermi–Hart paradox, we discuss if and to what extent our results may help quantify the chances of humanity to manage the transition to a long-term sustainable path of existence.

The number of people able to end Earth's technical civilization has heretofore been small. Emerging dual-use technologies, such as biotechnology, may give similar power to thousands or millions of individuals. To quantitatively investigate the ramifications of such a marked shift on the survival of both terrestrial and extraterrestrial technical civilizations, this paper presents a two-parameter model for civilizational lifespans, i.e. the quantity L in Drake's equation for the number of communicating extraterrestrial civilizations. One parameter characterizes the population lethality of a civilization's biotechnology and the other characterizes the civilization's psychosociology. L is demonstrated to be less than the inverse of the product of these two parameters. Using empiric data from PubMed to inform the biotechnology parameter, the model predicts human civilization's median survival time as decades to centuries, even with optimistic psychosociological parameter values, thereby positioning biotechnology as a proximate threat to human civilization. For an ensemble of civilizations having some median calculated survival time, the model predicts that, after 80 times that duration, only one in 1024 civilizations will survive – a tempo and degree of winnowing compatible with Hanson's ‘Great Filter.’ Thus, assuming that civilizations universally develop advanced biotechnology, before they become vigorous interstellar colonizers, the model provides a resolution to the Fermi paradox.

If an industrial civilization had existed on Earth many millions of years prior to our own era, what traces would it have left and would they be detectable today? We summarize the likely geological fingerprint of the Anthropocene, and demonstrate that while clear, it will not differ greatly in many respects from other known events in the geological record. We then propose tests that could plausibly distinguish an industrial cause from an otherwise naturally occurring climate event.

In this paper, we determine the habitability of Sun-like stars in the Galaxy using Monte Carlo simulations, which are guided by the factors of the Drake Equation for the considerations on the astrophysical and biological parameters needed to generate and maintain life on a planet's surface. We used a simple star distribution, initial mass function and star formation history to reproduce the properties and distribution of stars within the Galaxy. Using updated exoplanet data from the Kepler mission, we assign planets to some of the stars, and then follow the evolution of life on the planets that met the habitability criteria using two different civilization hypotheses. We predict that around 51% of Earth-like planets in the habitable zone (HZ) are inhabited by primitive life and 4% by technological life. We apply the results to the Kepler field of view, and predicted that there should be at least six Earth-like planets in the HZ, three of them inhabited by primitive life. According to our model, non-technological life is very common if there are the right conditions, but communicative civilizations are less likely to exist and detect. Nonetheless, we predict a considerable number of detectable civilizations within our Galaxy, making it worthwhile to keep searching.

At this point in time, there is very little empirical evidence on the likelihood of a space-faring species originating in the biosphere of a habitable world. However, there is a tension between the expectation that such a probability is relatively high (given our own origins on Earth), and the lack of any basis for believing the Solar System has ever been visited by an extraterrestrial colonization effort. From the latter observational fact, this paper seeks to place upper limits on the probability of an interstellar civilization arising on a habitable planet in its stellar system, using a percolation model to simulate the progress of such a hypothetical civilization's colonization efforts in the local Solar neighbourhood. To be as realistic as possible, the actual physical positions and characteristics of all stars within 40 parsecs of the Solar System are used as possible colony sites in the percolation process. If an interstellar civilization is very likely to have such colonization programmes, and they can travel over large distances, then the upper bound on the likelihood of such a species arising per habitable world is of the order of 10−3; on the other hand, if civilizations are not prone to colonize their neighbours, or do not travel very far, then the upper limiting probability is much larger, even of order one.

This paper discusses ideas that impact the fc factor as defined by Frank Drake in 1961, i.e. the fraction of planets with intelligent creatures capable of interstellar communication. This factor remains one of the most speculative terms of the equation. We suggest that the ability of sharing information is an important parameter to take into account in evaluating the tendency of a civilization to make contact (or share data) with other civilizations. Thus, we give special consideration to the fraction of planets with intelligent creatures capable of producing and sharing large amount of data. First, we determine the level of our own civilization in the framework of Sagan's energy- and information-based classification, by taking into account the recent improvements in computing and networking technologies. Second, we distinguish two types of organization, hierarchical and heterarchical, with respect to information sharing. We illustrate this distinction in the case of SETI and we show that the probability to detect a civilization would be greater if it is heterarchical than if it is hierarchical and if we utilize heterarchical principles for SETI.

I compare three methods for transmitting signals over interstellar distances: radio transmitters, lasers and artificial transits. The quantitative comparison is based on physical quantities depending on energy cost and transmitting time L, the last parameter in the Drake equation. With our assumptions, radio transmitters are the most energy-effective, while macro-engineered planetary-sized objects producing artificial transits seem effective on the long term to transmit an attention-getting signal for a time that might be much longer than the lifetime of the civilization that produced the artefact.

I propose a unified framework for a joint analysis of the Drake equation and the Fermi paradox, which enables a simultaneous, quantitative study of both of them. The analysis is based on a simplified form of the Drake equation and on a fairly simple scheme for the colonization of the Milky Way. It appears that for sufficiently long-lived civilizations, colonization of the Galaxy is the only reasonable option to gain knowledge about other life forms. This argument allows one to define a region in the parameter space of the Drake equation, where the Fermi paradox definitely holds (‘Strong Fermi paradox’).

This paper outlines hypotheses relevant to the evolution of intelligent life and encephalization in the Phanerozoic. If general principles are inferable from patterns of Earth life, implications could be drawn for astrobiology. Many of the outlined hypotheses, relevant data, and associated evolutionary and ecological theory are not frequently cited in astrobiological journals. Thus opportunity exists to evaluate reviewed hypotheses with an astrobiological perspective. A quantitative method is presented for testing one of the reviewed hypotheses (hypothesis i; the diffusion hypothesis). Questions are presented throughout, which illustrate that the question of intelligent life's likelihood can be expressed as multiple, broadly ranging, more tractable questions.

The search for extraterrestrial intelligence (SETI) has been heavily influenced by solutions to the Drake Equation, which returns an integer value for the number of communicating civilizations resident in the Milky Way, and by the Fermi Paradox, glibly stated as: ‘If they are there, where are they?’. Both rely on using average values of key parameters, such as the mean signal lifetime of a communicating civilization. A more accurate answer must take into account the distribution of stellar, planetary and biological attributes in the galaxy, as well as the stochastic nature of evolution itself. This paper outlines a method of Monte Carlo realization that does this, and hence allows an estimation of the distribution of key parameters in SETI, as well as allowing a quantification of their errors (and the level of ignorance therein). Furthermore, it provides a means for competing theories of life and intelligence to be compared quantitatively.

For over 40 years the formalism known as the Drake equation has helped guide speculation about the likelihood of intelligent extraterrestrial life contacting us. Since the equation was formulated there have been significant advances in astronomy and astrophysics, sufficient to merit a review of the significance of the Drake equation. The equation itself is as a series of terms which, when combined, allow an informed discussion of the likelihood of contact with an alien intelligence. However, whilst it has a mathematical form (i.e. a series of terms multiplied together to give an overall probability) it is best understood not as an equation in the strictly mathematical sense. Some of the terms have a physically quantifiable, numerically based meaning (e.g. obtainable from astronomy) and some are more social in content in that they describe the behaviour and evolution of societies and thus are more social science in nature and not truly estimable without observation of a set of societies. Initially, almost all the terms had to be estimated based on informed guesswork or belief. However, in the intervening period since the early 1960s, many of the a priori scientific terms which were themselves initially so uncertain as to require estimation by guess work or belief are now, or will soon be, directly measurable from current or planned astronomical projects. This leaves the non-scientific terms as a distinct class of their own, still subject to analysis only by discussion. Thus observational astronomy has nearly caught up with parts of the Drake equation and will soon quantify the purely physical science parts of the equation. The social parts (concerning intelligent societies, etc.) are still a priori unknowable. In addition, the growth of the subject called astrobiology (i.e. the study of life in the Universe) has developed so fast that communicating with intelligent life is now increasingly seen as just one small part of a much larger discipline. The knowledge as to whether there is life per se (apart from on Earth) in our galactic neighbourhood may be obtainable in the near future directly from observation. Such knowledge will have a profound impact on mankind and will be obtained without the form of communication envisaged by the Drake equation.