Could We Detect an Ancient Industrial Civilization in the Geological Record?

As a species, we humans tend to take it for granted that we are the only ones that live in sedentary communities, use tools, and alter our landscape to meet our needs. It is also a foregone conclusion that in the history of planet Earth, humans are the only species to develop machinery, automation, electricity, and mass communications – the hallmarks of industrial civilization.

But what if another industrial civilization existed on Earth millions of years ago? Would we be able to find evidence of it within the geological record today? By examining the impact human industrial civilization has had on Earth, a pair of researchers conducted a study that considers how such a civilization could be found and how this could have implications in the search for extra-terrestrial life.

The study, which recently appeared online under the title “The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record“, was conducted by Gavin A. Schmidt and Adam Frank – a climatologist with the NASA Goddard Institute for Space Studies (NASA GISS) and an astronomer from the University of Rochester, respectively.

Carbon dioxide in Earth’s atmosphere if half of global-warming emissions are not absorbed. Credit: NASA/JPL/GSFC

As they indicate in their study, the search for life on other planets has often involved looking to Earth-analogues to see what kind conditions life could exist under. However, this pursuit also entails the search for extra-terrestrial intelligence (SETI) that would be capable of communicating with us. Naturally, it is assumed that any such civilization would need to develop and industrial base first.

This, in turn, raises the question of how often an industrial civilization might develop – what Schmidt and Frank refer to as the “Silurian Hypothesis”. Naturally, this raises some complications since humanity is the only example of an industrialized species that we know of. In addition, humanity has only been an industrial civilization for the past few centuries – a mere fraction of its existence as a species and a tiny fraction of the time that complex life has existed on Earth.

For the sake of their study, the team first noted the importance of this question to the Drake Equation. To recap, this theory states that the number of civilizations (N) in our galaxy that we might be able to communicate is equal to the average rate of star formation (R*), the fraction of those stars that have planets (fp), the number of planets that can support life (ne), the number of planets that will develop life ( fl), the number of planets that will develop intelligent life (fi), the number civilizations that would develop transmission technologies (fc), and the length of time these civilizations will have to transmit signals into space (L).

This can be expressed mathematically as: N = R* x fp x ne x fl x fi x fc x L

The Drake Equation, a mathematical formula for the probability of finding life or advanced civilizations in the universe. Credit: University of Rochester

As they indicate in their study, the parameters of this equation may change thanks to the addition of the Silurian Hypothesis, as well as recent exoplanets surveys:

“If over the course of a planet’s existence, multiple industrial civilizations can arise over the span of time that life exists at all, the value of fc may in fact be greater than one. This is a particularly cogent issue in light of recent developments in astrobiology in which the first three terms, which all involve purely astronomical observations, have now been fully determined. It is now apparent that most stars harbor families of planets. Indeed, many of those planets will be in the star’s habitable zones.”

In short, thanks to improvements in instrumentation and methodology, scientists have been able to determine the rate at which stars form in our galaxy. Furthermore, recent surveys for extra-solar planets have led some astronomers to estimate that our galaxy could contains as many as 100 billion potentially-habitable planets. If evidence could be found of another civilization in Earth’s history, it would further constrain the Drake Equation.

They then address the likely geologic consequences of human industrial civilization and then compare that fingerprint to potentially similar events in the geologic record. These include the release of isotope anomalies of carbon, oxygen, hydrogen and nitrogen, which are a result of greenhouse gas emissions and nitrogen fertilizers. As they indicate in their study:

“Since the mid-18th Century, humans have released over 0.5 trillion tons of fossil carbon via the burning of coal, oil and natural gas, at a rate orders of magnitude faster than natural long-term sources or sinks. In addition, there has been widespread deforestation and addition of carbon dioxide into the air via biomass burning.”
Based on fossil records, 250 million years ago over 90% of all species on Earth died out, effectively resetting evolution. Credit: Lunar and Planetary Institute

They also consider increased rates of sediment flow in rivers and its deposition in coastal environments, as a result of agricultural processes, deforestation, and the digging of canals. The spread of domesticated animals, rodents and other small animals are also considered – as are the extinction of certain species of animals – as a direct result of industrialization and the growth of cities.

The presence of synthetic materials, plastics, and radioactive elements (caused by nuclear power or nuclear testing) will also leave a mark on the geological record – in the case of radioactive isotopes, sometimes for millions of years. Finally, they compare past extinction level events to determine how they would compare to a hypothetical event where human civilization collapsed. As they state:

“The clearest class of event with such similarities are the hyperthermals, most notably the Paleocene-Eocene Thermal Maximum (56 Ma), but this also includes smaller hyperthermal events, ocean anoxic events in the Cretaceous and Jurassic, and significant (if less well characterized) events of the Paleozoic.”

These events were specifically considered because they coincided with rises in temperatures, increases in carbon and oxygen isotopes, increased sediment, and depletions of oceanic oxygen. Events that had a very clear and distinct cause, such as the Cretaceous-Paleogene extinction event (caused by an asteroid impact and massive volcanism) or the Eocene-Oligocene boundary (the onset of Antarctic glaciation) were not considered.

Artistic rendition of the Chicxulub impactor striking ancient Earth, with Pterosaur observing. Credit: NASA

According to the team, the events they did consider (known as “hyperthermals”) show similarities to the Anthropocene fingerprint that they identified. In particular, according to research cited by the authors, the Paleocene-Eocene Thermal Maximum (PETM) shows signs that could be consistent with anthorpogenic climate change. These include:

 “[A] fascinating sequence of events lasting 100–200 kyr and involving a rapid input (in perhaps less than 5 kyr) of exogenous carbon into the system, possibly related to the intrusion of the North American Igneous Province into organic sediments. Temperatures rose 5–7?C (derived from multiple proxies), and there was a negative spike in carbon isotopes (>3%), and decreased ocean carbonate preservation in the upper ocean.”

Finally, the team addressed some possible research directions that might improve the constraints on this question. This, they claim, could consist of a “deeper exploration of elemental and compositional anomalies in extant sediments spanning previous events be performed”. In other words, the geological record for these extinction events should be examined more closely for anomalies that could be associated with industrial civilization.

If any anomalies are found, they further recommend that the fossil record could be examined for candidate species, which would raise questions about their ultimate fate. Of course, they also acknowledge that more evidence is necessary before the Silurian Hypothesis can be considered viable. For instance, many past events where abrupt Climate Change took place have been linked to changes in volcanic/tectonic activity.

Scientists were able to gauge the rate of water loss on Mars by measuring the ratio of water and HDO from today and 4.3 billion years ago. Credit: Kevin Gill

Second, there is the fact that current changes in our climate are happening faster than in any other geological period. However, this is difficult to say for certain since there are limits when it comes to the chronology of the geological record. In the end, more research will be necessary to determine how long previous extinction events (those that were not due to impacts) took as well.

Beyond Earth, this study may also have implications for the study of past life on planets like Mars and Venus. Here too, the authors suggest how explorations of both could reveal the existence of past civilizations, and maybe even bolster the possibility of finding evidence of past civilizations on Earth.

“We note here that abundant evidence exists of surface water in ancient Martian climates (3.8 Ga), and speculation that early Venus (2 Ga to 0.7 Ga) was habitable (due to a dimmer sun and lower CO2 atmosphere) has been supported by recent modeling studies,” they state. “Conceivably, deep drilling operations could be carried out on either planet in future to assess their geological history. This would constrain consideration of what the fingerprint might be of life, and even organized civilization.”
Two key aspects of the Drake Equation, which addresses the probability of finding life elsewhere in the galaxy, are the sheer number of stars and planets out there and the amount of time life has had to evolve. Until now, it has been assumed that one planet would give rise to one intelligent species capable of advanced technology and communications.
But if this number should prove to be more, we may a find a galaxy filled with civilizations, both past and present. And who knows? The remains of a once advanced and great non-human civilization may very well be right beneath us!

Further Reading: arXiv

If We Do Hear Signals From Aliens, They’re Probably Long Gone

In 1961, famed astrophysics Frank Drake proposed a formula that came to be known as the Drake Equation. Based on a series of factors, this equation sought to estimate the number of extraterrestrial intelligences (ETIs) that would exist within our galaxy at any given time. Since that time, multiple efforts have been launched to find evidence of alien civilizations, which are collectively known as the search for extraterrestrial intelligence (SETI).

The most well-known of these is the SETI Institute, which has spent the past few decades searching the cosmos for signs of extraterrestrial radio communications. But according to a new study that seeks to update the Drake Equation, a team of international astronomers indicates that even if we did find signals of alien origin, those who sent them would be long dead.

Continue reading “If We Do Hear Signals From Aliens, They’re Probably Long Gone”

What is the Drake Equation?

Is there life out there in the Universe? That is a question that has plagued humanity long before we knew just how vast the Universe was – i.e. before the advent of modern astronomy. Within the 20th century – thanks to the development of modern telescopes, radio astronomy, and space observatories – multiple efforts have been made in the hopes of finding extraterrestrial intelligence (ETI).

Continue reading “What is the Drake Equation?”

Maybe the Aliens Aren’t Hiding, they’re Sleeping, Waiting for the Universe to Get Better

When you consider that age of the Universe – 13.8 billion years by our most recent counts –  and that which is “observable” to us measures about 93 billion light-years in diameter, you begin to wonder why we haven’t found signs of extra-terrestrial intelligence (ETI) beyond our Solar System. To paraphrase Enrico Fermi, the 20th-century physicists who advanced the famous Fermi Paradox – “where the heck are all the aliens?”

Naturally, Fermi’s Paradox has attracted a lot of theoretical explanations over the years – which include ETI being very rare, humanity being early to the Universe, and the aliens being extinct! But a new study by a team of scientists from the Future of Humanity Institute (FHI) offers a different take on this age-old paradox. According to their study, the key to answering this question is to consider the possibility that the aliens are engaged in “aestivation”.

Continue reading “Maybe the Aliens Aren’t Hiding, they’re Sleeping, Waiting for the Universe to Get Better”

Where Are The Aliens? How The ‘Great Filter’ Could Affect Tech Advances In Space

“One of the main things we’re focused on is the notion of existential risk, getting a sense of what the probability of human extinction is,” said Andrew Snyder-Beattie, who recently wrote a piece on the “Great Filter” for Ars Technica.

As Snyder-Beattie explained in the article, the “Great Filter” is a response to the question of why we can’t see any alien civilizations. The “Great Filter” deals with similar issues as the Drake Equation, which talks about the probability of communicating civilizations outside of Earth, and the Fermi Paradox, which asks where the civilizations are.

Simply speaking, the idea is that if a civilization continues to expand (especially at the technological pace we humans have experienced), it wouldn’t take all that long in the lifespan of the universe for artificial processes to be visible with our own telescopes. Yes, this is even taking into account a presumed speed limit of no more than the speed of light. So something could be preventing these civilizations from showing up. That’s an important part of the Great Filter, but more details about it are below.

Here are a few possibilities for why the filter exists, both from Snyder-Beattie and from the person who first named the Great Filter, Robin Hanson, in 1996.

‘Rare Earth’ hypothesis

Maybe Earth is alone in the universe. While some might assume life must be relatively common since it arose here, Snyder-Beattie points to observation selection effects as complicating this analysis.  With a sample size of one (only ourselves as the observers), it is hard to determine the probability of life arising – we could very well be alone.  By one token, that’s a “comforting” thought, he added, because it could mean there is no single catastrophic event that befalls all civilizations.

Artists impression of an asteroid flying by Earth. Credit: NASA
Artists impression of an asteroid flying by Earth. Credit: NASA

Advanced civilizations are hard to get

Hanson doesn’t believe that one. One step would be going from modestly intelligent mammals to human-like abilities, and another would be the step from human-like abilities to advanced civilizations.  It only took a few million years to go from modestly intelligent animals to humans. “If you killed all humans on Earth, but you left life on Earth — and the animals have big brains — it wouldn’t necessarily be that long before it came back again.” Some of the filter steps leading up to that would have taken longer, though, including the emergence of multicellular animals and the emergence of brains, roughly on the timespan of a billion years each per stage.

‘The Berserker Scenario’

In this scenario, powerful aliens sit hiding in wait to destroy any visible intelligence that appears.  Hanson doesn’t believe that would work because if there were multiple berserker species, there would be opposing parties. “As an equilibrium, you’d have these competing teams of these berserkers all trying to smash each other.”

Maybe natural activities are masking the extraterrestrials

Maybe the big natural activities of those beyond Earth just happens to look exactly as if they are not there. Hanson said it seems rather unlikely, as it would be a “remarkable coincidence” if advanced artificial processes were actually responsible for all the astronomical phenomena we do explain from natural occurrences,- from pulsars to dark matter

Artist's conception of a gamma-ray pulsar. Gamma rays are shown in purple, and radio radiation in green. Credit: NASA/Fermi/Cruz de Wilde
Artist’s conception of a gamma-ray pulsar. Gamma rays are shown in purple, and radio radiation in green. Credit: NASA/Fermi/Cruz de Wilde

A natural disaster

There certainly is an inherent risk to just being an Earthling. One asteroid strike, a stream of radiation from a nearby supernova, or a large enough volcano could end civilization as we know it — and possibly much of life itself. “But the consensus is we have a track record of surviving these things. But it’s unlikely that all life would be destroyed forever. “If those humans who were left, it took them 10,000 years to come back to civilization, that’s hardly a blink of an eye, that doesn’t do it,” Hanson said. The next is that statistically speaking, although these events happen, they don’t happen often. “It is unlikely one of these very rare events would happen in the next century or 300 years,” Snyder-Beattie said.

A ‘fundamental technology’ that ends civilization

This is open to complete speculation. For example, climate change could be the catalyst, although it would seem extraordinary for all civilizations to encounter such similar political failures, Snyder-Beattie said. More generalized possibilities could be the rise of machine intelligence or distributed biotechnology, a force that is self-replicated. Hanson, however, points out that even that has its limitations — presumably then it would be the robots that head out through the cosmos and leave traces of civilization themselves.

The solution

For the fate of our own civilization, the key is to focus on what we can control, Hanson says. This means drawing up a list of the things that could kill us — however theoretical — and then work on ways of addressing those.

The question of why other civilizations are not visible still persists, however. What are your thoughts about the Great Filter? Let us know in the comments.

Inside the Drake Equation: A Chat with Frank Drake

This interview with Frank Drake — sometimes called the Father of the Search for Extraterrestrial Intelligence – was recorded in 2012 but not released until now to celebrate the beginning of the 30th year of the SETI Institute. As interviewer Andrew Fraknoi says, “I don’t think anyone had a conversation like this that was recorded with Galileo or William Herschel or Edwin Hubble, but I get to do it with Frank Drake!”

This is a great conversation that alternates between Drake’s current work with SETI and the history of his work that led to the famous Drake Equation. Fraknoi and Drake have an interesting exchange about the value of N, which is the number of civilizations in The Milky Way Galaxy whose electromagnetic emissions would be detectable.

It was recorded in June 2012 at an event called SETICon, which featured a series of talks, panels, and events featuring scientists, authors, futurists, and film-makers.

Fraknoi is a professor of astronomy and also works with the Astronomical Society of the Pacific and they have made available a written history of Frank Drake and his equation.

Astronomy Without A Telescope – How To Impress An Alien (Or Not)

[/caption]

It’s about fifty years since Frank Drake sent out our first chat request to the wider universe. I say about as I think the official date is 11 April 1960 – but I notice a lot of fifty year anniversary blogs and interviews are already being published, so what the heck, I’m not waiting either.

While no-one is really concerned that we haven’t had an answer back yet, it is a little despondent to have scanned the skies for someone else’s chat request all this time and found nothing.

In a recent New Scientist interview (actually January 2010 – they were really getting in early), Drake refers to his equation delivering an answer in the order of one in 10 million stars having an advanced civilization – and he uses that statistic to indicate it’s too early to think we have done a statistically adequate scan yet.

Nonetheless, the chances of there being advanced civilizations near enough to enable a future United Federation of Planets already looks doubtful.

Drake’s initial communication efforts in Project Ozma were small scale, but his clever and carefully constructed Arecibo message out to Messier 13 (a globular cluster of approximately 300,000 stars) in 1974 aroused some criticism that telling the aliens where we are might result in an invasion.

This is a little implausible, since Messier 13 is 25,000 light years away. By the time the invasion fleet arrives we will either be long gone or have spent the intervening period developing the technology to blast them out of the sky if they don’t turn back immediately.

Actually, that’s probably an important consideration if we ever decide to invade someone. We will need to take a couple of universities along to keep our technology advancing ahead of theirs. However, if we are travelling near the speed of light, the time differential means that they will get ahead anyway. Hmm…

The Arecibo message composed of 1679 bits, being the product of two prime numbers 73 and 23 (i.e. the number of rows and columns). Impressive, huh?

Anyway, here in the 21st century, I want to suggest that more attention should be given to us just not looking stupid. There’s already all the bad TV out there. We can fairly claim that all that was never meant for alien consumption, but recently we advanced humans have quite deliberately transmitted a Beatles song to Polaris and sent a bunch of text messages to Gliese 581. I mean, huh?

Polaris, being a Cepheid variable – and in any case a short-lived and already dying supergiant – was probably never stable enough to support planets, so we probably got away with that one. However, there’s no getting around us sending text messages to Gliese 581c in 2008 (from Ukraine) and subsequently following that up with another set blasted at 581d in 2009 (from Australia, sorry…).

This was because when we recalculated, it was apparent that the exoplanet 581d was more likely to be in the habitable zone of its star than 581c. Hopefully those 20 light year distant aliens will appreciate that the inconsequential shift in the main focus of those two transmissions is an indication of our extreme cleverness.

See, it’s a bit like reading Shakespeare to a dolphin. With no comprehension of the language, you will just look like someone who is content to sit for hours making funny noises while dangling your feet in a pool. But with a bit of comprehension, the dolphin can be reasonably expected to reply – hey Brainiac, I’m a dolphin, what’s forsooth mean?

There are aliens among us who already think we’re a bit daft. How about we first check in with Frank Drake next time we feel like shouting out the window?

A New “Drake” Equation for Potential of Life

[/caption]
The famed Drake equation estimates the number of technologically advanced civilizations that might exist in our Galaxy. But is there a way to mathematically quantify a habitat’s potential for hosting life?
“At present, there is no easy way of directly comparing the suitability of different environments as a habitat for life” said Dr. Axel Hagermann, who is proposing a method to find a “habitability index” at the European Planetary Science Congress.

“The classical definition of a habitable environment,” said Hagermann, “is one that has the presence of a solvent, for example water, availability of the raw materials for life, clement conditions and some kind of energy source, so we tend to define a place as ‘habitable’ if it falls into the area where these criteria overlap on a Venn diagram. This is fine for specific instances, but it gives us no quantifiable way of comparing exactly how habitable one environment is in comparison with another, which I think is very important.”
Drake Equation
Hagermann and colleague Charles Cockell have the ambitious aim of developing a single, normalized indicator of habitability, mathematically describing all the variables of each of the four habitability criteria. Initially, they are focusing on describing all the qualities of an energy source that may help or hinder the development of life.

“Electromagnetic radiation may seem simple to quantify in terms of wavelengths and joules, but there are many things to consider in terms of habitability,” Hagermann said. “For instance, while visible and infrared wavelengths are important for life and processes such as photosynthesis, ultraviolet and X-rays are harmful. If you can imagine a planet with a thin atmosphere that lets through some of this harmful radiation, there must be a certain depth in the soil where the ‘bad’ radiation has been absorbed but the ‘good’ radiation can penetrate. We are looking to be able to define this optimal habitable region in a way that we can say that it is ‘as habitable’ or ‘less habitable’ than a desert in Morocco, for example.”

The pair will be presenting their initial study and asking for feedback from colleagues at the European Planetary Science Congress. “There may be good reasons why such a habitability index is not going to work and, with so many variables to consider, it is not going to be an easy task to develop. However, this kind of index has the potential to be an invaluable tool as we begin to understand more about the conditions needed for life to evolve and we find more locations in our Solar System and beyond that might be habitable.”

Source: Europlanet

The Milky Way Could have Billions of Earths

With the upcoming launch in March of the Kepler mission to find extrasolar planets, there is quite a lot of buzz about the possibility of finding habitable planets outside of our Solar System. Kepler will be the first satellite telescope with the capability to find Earth-size and smaller planets. At the most recent meeting of the American Association for the Advancement of Science (AAAS) in Chicago, Dr. Alan Boss is quoted by numerous media outlets as saying that there could be billions of Earth-like planets in the Milky Way alone, and that we may find an Earth-like planet orbiting a large proportion of the stars in the Universe.

“There are something like a few dozen solar-type stars within something like 30 light years of the sun, and I would think that a good number of those — perhaps half of them would have Earth-like planets. So, I think there’s a very good chance that we’ll find some Earth-like planets within 10, 20, or 30 light years of the Sun,” Dr. Boss said in an AAAS podcast interview.

Dr. Boss is an astronomer at the Carnegie Institution of Washington Department of Terrestrial Magnetism, and is the author of The Crowded Universe, a book on the likelihood of finding life and habitable planets outside of our Solar System.

“Not only are they probably habitable but they probably are also going to be inhabited. But I think that most likely the nearby ‘Earths’ are going to be inhabited with things which are perhaps more common to what Earth was like three or four billion years ago,” Dr. Boss told the BBC. In other words, it’s more likely that bacteria-like lifeforms abound, rather than more advanced alien life.

This sort of postulation about the existence of extraterrestrial life (and intelligence) falls under the paradigm of the Drake Equation, named after the astronomer Frank Drake. The Drake Equation incorporates all of the variables one should take into account when trying to calculate the number of technologically advanced civilizations elsewhere in the Universe. Depending on what numbers you put into the equation, the answer ranges from zero to trillions. There is wide speculation about the existence of life elsewhere in the Universe.

To date, the closest thing to an Earth-sized planet discovered outside of our Solar System is CoRoT-Exo-7b, with a diameter of less than twice that of the Earth.

The speculation by Dr. Boss and others will be put to the test later this year when the Kepler satellite gets up and running. Set to launch on March 9th, 2009, the Kepler mission will utilize a 0.95 meter telescope to view one section of the sky containing over 100,000 stars for the entirety of the mission, which will last at least 3.5 years.

The prospect of life existing elsewhere is exciting, to be sure, and we’ll be keeping you posted here on Universe Today when any of the potentially billions of Earth-like planets are discovered!

Source: BBC, EurekAlert

The Odds of Intelligent Life in the Universe

When it comes to contemplating the state of our universe, the question likely most prevalent on people’s minds is, “Is anyone else like us out there?” The famous Drake Equation, even when worked out with fairly moderate numbers, seemingly suggests the probable amount of intelligent, communicating civilizations could be quite numerous. But a new paper published by a scientist from the University of East Anglia suggests the odds of finding new life on other Earth-like planets are low, given the time it has taken for beings such as humans to evolve combined with the remaining life span of Earth.

Professor Andrew Watson says that structurally complex and intelligent life evolved relatively late on Earth, and in looking at the probability of the difficult and critical evolutionary steps that occurred in relation to the life span of Earth, provides an improved mathematical model for the evolution of intelligent life.

According to Watson, a limit to evolution is the habitability of Earth, and any other Earth-like planets, which will end as the sun brightens. Solar models predict that the brightness of the sun is increasing, while temperature models suggest that because of this the future life span of Earth will be “only” about another billion years, a short time compared to the four billion years since life first appeared on the planet.

“The Earth’s biosphere is now in its old age and this has implications for our understanding of the likelihood of complex life and intelligence arising on any given planet,” said Watson.

Some scientists believe the extreme age of the universe and its vast number of stars suggests that if the Earth is typical, extraterrestrial life should be common. Watson, however, believes the age of the universe is working against the odds.

“At present, Earth is the only example we have of a planet with life,” he said. “If we learned the planet would be habitable for a set period and that we had evolved early in this period, then even with a sample of one, we’d suspect that evolution from simple to complex and intelligent life was quite likely to occur. By contrast, we now believe that we evolved late in the habitable period, and this suggests that our evolution is rather unlikely. In fact, the timing of events is consistent with it being very rare indeed.”

Watson, it seems, takes the Fermi Paradox to heart in his considerations. The Fermi Paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for, or contact with, such civilizations.

Watson suggests the number of evolutionary steps needed to create intelligent life, in the case of humans, is four. These include the emergence of single-celled bacteria, complex cells, specialized cells allowing complex life forms, and intelligent life with an established language.

“Complex life is separated from the simplest life forms by several very unlikely steps and therefore will be much less common. Intelligence is one step further, so it is much less common still,” said Prof Watson.

Watson’s model suggests an upper limit for the probability of each step occurring is 10 per cent or less, so the chances of intelligent life emerging is low — less than 0.01 per cent over four billion years.

Each step is independent of the other and can only take place after the previous steps in the sequence have occurred. They tend to be evenly spaced through Earth’s history and this is consistent with some of the major transitions identified in the evolution of life on Earth.

Here is more about the Drake Equation.

Here is more information about the Fermi Paradox.

Original News Source: University of East Anglia Press Release