Rare Earth – why we may be the only intelligent species in the universe

Preface. I think that Ward & Brownlee’s 2000 book “Rare Earth : why Complex Life is Uncommon in the Universe” is one of the most profound books I’ve ever read.  What if we are the only intelligent species in the galaxy, or universe?  There are dozens of reasons to think so.  Bacteria on the other hand, are probably a dime a dozen, splattered over planets within a reasonable Goldilocks zone from their star. 

But even the Goldilocks zone doesn’t guarantee life can exist. Mars is within this zone and almost certainly impossible to live on, yet so many hope we can escape there if we mess this planet up. Think again: “Escape to Mars after we’ve trashed the Earth?”  And in the last fiction story from the New Yorker, we can’t count on the Space Aliens to rescue us if they show up.

As I see “Rare Earth” stories in the news, I add them below. The book “Rare Earth” has many additional reasons why Earth may be the only complex life in the galaxy, and so much has to go right for 5 billion years, it wouldn’t surprise me to find that humans were one of the few intelligent lifeforms in the universe. Let’s not blow it!

Alice Friedemann   www.energyskeptic.com  author of “Life After Fossil Fuels: A Reality Check on Alternative Energy”, 2021, Springer, “When Trucks Stop Running: Energy and the Future of Transportation”, 2015, Springer and “Crunch! Whole Grain Artisan Chips and Crackers”. Podcasts: Derrick Jensen, Practical Prepping, KunstlerCast 253, KunstlerCast278, Peak Prosperity , XX2 report


Tyrell T (2021) How has earth stayed habitable for billions of years? Inverse.

It took evolution 4 billion years to produce Homo sapiens. If the climate had completely failed just once evolution would have come to a crashing halt, and we would not be here now. This is not a trivial problem. Current global warming shows us that the climate can change considerably over the course of even a few centuries. Over geological timescales, it is even easier to change the climate. Calculations show that there is the potential for Earth’s climate to deteriorate to temperatures below freezing or above boiling in just a few million years. We also know that the Sun has become 30% more luminous since life first evolved. In theory, this should have caused the oceans to boil away by now, given that they were not generally frozen on the early Earth.

Theory #1: The Earth has something like a thermostat – feedback mechanisms preventing the climate from ever wandering to fatal temperatures.

Theory #2: Luck. The most likely explanation (read the article for why).

Heller, J. 2020. Habitability of the early Earth: Liquid water under a faint young Sun facilitated by strong tidal heating due to a nearby Moon. Earth and Planetary Astrophysics.

The sun is thought to have been 70% dimmer its first 100 million years, so faint the Earth should have been a frozen snowball for up to 2 billion years. That it wasn’t has long plagued astronomers, but now we might have an answer: the moon helped keep Earth warm.

When the moon and Earth formed about 4.4 billion years ago, the moon was about 20,000 kilometres away versus 380,000 km now. Earth was also rotating much faster, as quickly as once every 3 hours. These two factors mean the gravitational interaction between the two bodies would have been much stronger – enough to produce tidal heating from the gravitational squeeze. This would have slightly warmed Earth and could have triggered the eruption of volcanoes, giving our planet a thicker atmosphere that could trap more heat.

But this may not be the right explanation. Other theories include that Earth had a thicker carbon dioxide atmosphere as a result of the planet being molten following the giant impact that formed the moon, trapping more heat. Another is that the planet’s orbit brought it closer to the sun at times, warming it up, or that the sun had more mass at the time and was brighter than we think.

Longrich, N. 2019. Evolution tells us we might be the only intelligent life in the universe. Phys.org

Complex animals evolved once in life’s history, suggesting they’re improbable. Surprisingly, many critical events in our evolutionary history are unique and, probably, improbable. One is the bony skeleton of vertebrates, which let large animals move onto land. The complex, eukaryotic cells that all animals and plants are built from, containing nuclei and mitochondria, evolved only once. Sex evolved just once. Photosynthesis, which increased the energy available to life and produced oxygen, is a one-off. For that matter, so is human-level intelligence.

As far as we can tell, life only happened once.

Curiously, all this takes a surprisingly long time. Photosynthesis evolved 1.5 billion years after the Earth’s formation, complex cells after 2.7 billion years, complex animals after 4 billion years, and human intelligence 4.5 billion years after the Earth formed. That these innovations are so useful but took so long to evolve implies that they’re exceedingly improbable.

These one-off innovations, critical flukes, may create a chain of evolutionary bottlenecks or filters. If so, our evolution wasn’t like winning the lottery. It was like winning the lottery again, and again, and again. On other worlds, these critical adaptations might have evolved too late for intelligence to emerge before their suns went nova, or not at all.

Imagine that intelligence depends on a chain of seven unlikely innovations—the origin of life, photosynthesis, complex cells, sex, complex animals, skeletons and intelligence itself—each with a 10% chance of evolving. The odds of evolving intelligence become one in 10 million.

But complex adaptations might be even less likely. Photosynthesis required a series of adaptations in proteins, pigments and membranes. Eumetazoan animals required multiple anatomical innovations (nerves, muscles, mouths and so on). So maybe each of these seven key innovations evolve just 1% of the time. If so, intelligence will evolve on just 1 in 100 trillion habitable worlds. If habitable worlds are rare, then we might be the only intelligent life in the galaxy, or even the visible universe.

2019. New study dramatically narrows the search for advanced life in the universe. Phys.org.

Scientists may need to rethink their estimates for how many planets outside our solar system could host a rich diversity of life. In a new study, a UC Riverside–led team discovered that a buildup of toxic gases in the atmospheres of most planets makes them unfit for complex life as we know it. Accounting for predicted levels of certain toxic gases narrows the safe zone for complex life by at least half—and in some instances eliminates it altogether.

Using computer models to study atmospheric climate and photochemistry on a variety of planets, the team first considered carbon dioxide. Any scuba diver knows that too much of this gas in the body can be deadly. But planets too far from their host star require carbon dioxide—a potent greenhouse gas—to maintain temperatures above freezing. Earth included.

“To sustain liquid water at the outer edge of the conventional habitable zone, a planet would need tens of thousands of times more carbon dioxide than Earth has today,” said Edward Schwieterman, the study’s lead author and a NASA Postdoctoral Program fellow working with Lyons. “That’s far beyond the levels known to be toxic to human and animal life on Earth.”

Carbon dioxide toxicity alone restricts simple animal life to no more than half of the traditional habitable zone. For humans and other higher order animals, which are more sensitive, the safe zone shrinks to less than one third of that area.

What is more, no safe zone at all exists for certain stars, including two of the sun’s nearest neighbors, Proxima Centauri and TRAPPIST-1. The type and intensity of ultraviolet radiation that these cooler, dimmer stars emit can lead to high concentrations of carbon monoxide, another deadly gas. Carbon monoxide cannot accumulate on Earth because our hotter, brighter sun drives chemical reactions in the atmosphere that destroy it quickly.

If life exists elsewhere in the solar system, Schwieterman explained, it is deep below a rocky or icy surface. So, exoplanets may be our best hope for finding habitable worlds more like our own.

“I think showing how rare and special our planet is only enhances the case for protecting it,” Schwieterman said. “As far as we know, Earth is the only planet in the universe that can sustain human life.”

Source: Schwieterman, E. W., et al. 2019. A limited habitable zone for complex life. The Astrophysical Journal.

Wikipedia. 2019. Rare Earth Hypothesis. https://en.wikipedia.org/wiki/Rare_Earth_hypothesis

Requirements for complex life

The Rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a galactic habitable zone, a central star and planetary system having the requisite character, the circumstellar habitable zone, a right-sized terrestrial planet, the advantage of a gas giant guardian like Jupiter and a large natural satellite, conditions needed to ensure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans, the role of “evolutionary pumps” such as massive glaciation and rare bolide impacts, and whatever led to the appearance of the eukaryote cell, sexual reproduction and the Cambrian explosion of animal, plant, and fungi phyla. The evolution of human intelligence may have required yet further events, which are extremely unlikely to have happened were it not for the Cretaceous–Paleogene extinction event 66 million years ago removing dinosaurs as the dominant terrestrial vertebrates.

In order for a small rocky planet to support complex life, Ward and Brownlee argue, the values of several variables must fall within narrow ranges. The universe is so vast that it could contain many Earth-like planets. But if such planets exist, they are likely to be separated from each other by many thousands of light years. Such distances may preclude communication among any intelligent species evolving on such planets, which would solve the Fermi paradox: “If extraterrestrial aliens are common, why aren’t they obvious?”

The right location in the right kind of galaxy

Rare Earth suggests that much of the known universe, including large parts of our galaxy, are “dead zones” unable to support complex life. Those parts of a galaxy where complex life is possible make up the galactic habitable zone, primarily characterized by distance from the Galactic Center. As that distance increases:

  1. Star metallicity declines. Metals (which in astronomy means all elements other than hydrogen and helium) are necessary to the formation of terrestrial planets.
  2. The X-ray and gamma ray radiation from the black hole at the galactic center, and from nearby neutron stars, becomes less intense. Thus the early universe, and present-day galactic regions where stellar density is high and supernovae are common, will be dead zones.
  3. Gravitational perturbation of planets and planetesimals by nearby stars becomes less likely as the density of stars decreases. Hence the further a planet lies from the Galactic Center or a spiral arm, the less likely it is to be struck by a large bolide which could extinguish all complex life on a planet.
  4. Item #1 rules out the outer reaches of a galaxy; #2 and #3 rule out galactic inner regions. Hence a galaxy’s habitable zone may be a ring sandwiched between its uninhabitable center and outer reaches.
  5. Also, a habitable planetary system must maintain its favorable location long enough for complex life to evolve. A star with an eccentric (elliptic or hyperbolic) galactic orbit will pass through some spiral arms, unfavorable regions of high star density; thus a life-bearing star must have a galactic orbit that is nearly circular, with a close synchronization between the orbital velocity of the star and of the spiral arms. This further restricts the galactic habitable zone within a fairly narrow range of distances from the Galactic Center. Lineweaver et al. calculate this zone to be a ring 7 to 9 kiloparsecs in radius, including no more than 10% of the stars in the Milky Way, about 20 to 40 billion stars. Gonzalez, et al. would halve these numbers; they estimate that at most 5% of stars in the Milky Way fall in the galactic habitable zone.
  6. Approximately 77% of observed galaxies are spiral, two-thirds of all spiral galaxies are barred, and more than half, like the Milky Way, exhibit multiple arms. According to Rare Earth, our own galaxy is unusually quiet and dim (see below), representing just 7% of its kind. Even so, this would still represent more than 200 billion galaxies in the known universe.
  7. Our galaxy also appears unusually favorable in suffering fewer collisions with other galaxies over the last 10 billion years, which can cause more supernovae and other disturbances. Also, the Milky Way’s central black hole seems to have neither too much nor too little activity (Scharf 2012).
  8. The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with a period of 226 Ma (million years), closely matching the rotational period of the galaxy. However, the majority of stars in barred spiral galaxies populate the spiral arms rather than the halo and tend to move in gravitationally aligned orbits, so there is little that is unusual about the Sun’s orbit. While the Rare Earth hypothesis predicts that the Sun should rarely, if ever, have passed through a spiral arm since its formation, astronomer Karen Masters has calculated that the orbit of the Sun takes it through a major spiral arm approximately every 100 million years. Some researchers have suggested that several mass extinctions do correspond with previous crossings of the spiral arms.

Orbiting at the right distance from the right type of star

According to the hypothesis, Earth has an improbable orbit in the very narrow habitable zone (dark green) around the Sun.

The terrestrial example suggests that complex life requires liquid water, requiring an orbital distance neither too close nor too far from the central star, another scale of habitable zone or Goldilocks Principle: The habitable zone varies with the star’s type and age.

For advanced life, the star must also be highly stable, which is typical of middle star life, about 4.6 billion years old. Proper metallicity and size are also important to stability. The Sun has a low 0.1% luminosity variation. To date no solar twin star, with an exact match of the sun’s luminosity variation, has been found, though some come close. The star must have no stellar companions, as in binary systems, which would disrupt the orbits of planets. Estimates suggest 50% or more of all star systems are binary. The habitable zone for a main sequence star very gradually moves out over its lifespan until it becomes a white dwarf and the habitable zone vanishes.

The liquid water and other gases available in the habitable zone bring the benefit of greenhouse warming. Even though the Earth’s atmosphere contains a water vapor concentration from 0% (in arid regions) to 4% (in rain forest and ocean regions) and – as of February 2018 – only 408.05 parts per million of CO2, these small amounts suffice to raise the average surface temperature by about 40 °C, with the dominant contribution being due to water vapor, which together with clouds makes up between 66% and 85% of Earth’s greenhouse effect, with CO2 contributing between 9% and 26% of the effect.

Rocky planets must orbit within the habitable zone for life to form. Although the habitable zone of such hot stars as Sirius or Vega is wide, hot stars also emit much more ultraviolet radiation that ionizes any planetary atmosphere. They may become red giants before advanced life evolves on their planets. These considerations rule out the massive and powerful stars of type F6 to O (see stellar classification) as homes to evolved metazoan life.

Small red dwarf stars, one of the most common kinds of stars in our galaxy, have small habitable zones wherein planets are in tidal lock, with one very hot side always facing the star and another very cold side; and they are also at increased risk of solar flares (see Aurelia), coronal mass ejections, sterilization from ionizing radiation, and atmospheric erosion since their habitable zone is so close to the star (Hunt 2020). Life therefore cannot arise in such systems. Rare Earth proponents claim that only stars from F7 to K1 types are hospitable. Such stars are rare: G type stars such as the Sun (between the hotter F and cooler K) comprise only 9% of the hydrogen-burning stars in the Milky Way.

Such aged stars as red giants and white dwarfs are also unlikely to support life. Red giants are common in globular clusters and elliptical galaxies. White dwarfs are mostly dying stars that have already completed their red giant phase. Stars that become red giants expand into or overheat the habitable zones of their youth and middle age (though theoretically planets at a much greater distance may become habitable).

An energy output that varies with the lifetime of the star will likely prevent life (e.g., as Cepheid variables). A sudden decrease, even if brief, may freeze the water of orbiting planets, and a significant increase may evaporate it and cause a greenhouse effect that prevents the oceans from reforming.

All known life requires the complex chemistry of metallic elements. The absorption spectrum of a star reveals the presence of metals within, and studies of stellar spectra reveal that many, perhaps most, stars are poor in metals. Because heavy metals originate in supernova explosions, metallicity increases in the universe over time. Low metallicity characterizes the early universe: globular clusters and other stars that formed when the universe was young, stars in most galaxies other than large spirals, and stars in the outer regions of all galaxies. Metal-rich central stars capable of supporting complex life are therefore believed to be most common in the quiet suburbs of the larger spiral galaxies—where radiation also happens to be weak.

With the right arrangement of planets

Rare Earth proponents argue that a planetary system capable of sustaining complex life must be structured more or less like the Solar System, with small and rocky inner planets and outer gas giants. Without the protection of ‘celestial vacuum cleaner’ planets with strong gravitational pull, a planet would be subject to more catastrophic asteroid collisions.

Observations of exo-planets have shown that arrangements of planets similar to our Solar System are rare. Most planetary systems have super Earths, several times larger than Earth, close to their star, whereas our Solar System’s inner region has only a few small rocky planets and none inside Mercury’s orbit. Only 10% of stars have giant planets similar to Jupiter and Saturn, and those few rarely have stable nearly circular orbits distant from their star. Konstantin Batygin and colleagues argue that these features can be explained if, early in the history of the Solar System, Jupiter and Saturn drifted towards the Sun, sending showers of planetesimals towards the super-Earths which sent them spiralling into the Sun, and ferrying icy building blocks into the terrestrial region of the Solar System which provided the building blocks for the rocky planets. The two giant planets then drifted out again to their present position. However, in the view of Batygin and his colleagues: “The concatenation of chance events required for this delicate choreography suggest that small, Earth-like rocky planets – and perhaps life itself – could be rare throughout the cosmos.”

A continuously stable orbit

Rare Earth argues that a gas giant must not be too close to a body where life is developing. Close placement of gas giant(s) could disrupt the orbit of a potential life-bearing planet, either directly or by drifting into the habitable zone.

Newtonian dynamics can produce chaotic planetary orbits, especially in a system having large planets at high orbital eccentricity.

The need for stable orbits rules out stars with systems of planets that contain large planets with orbits close to the host star (called “hot Jupiters“). It is believed that hot Jupiters have migrated inwards to their current orbits. In the process, they would have catastrophically disrupted the orbits of any planets in the habitable zone. To exacerbate matters, hot Jupiters are much more common orbiting F and G class stars.

A terrestrial planet of the right size

It is argued that life requires terrestrial planets like Earth and as gas giants lack such a surface, that complex life cannot arise there.

A planet that is too small cannot hold much atmosphere, making surface temperature low and variable and oceans impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and plate tectonics may be brief or entirely absent. A planet that is too large will retain too dense an atmosphere like Venus. Although Venus is similar in size and mass to Earth, its surface atmospheric pressure is 92 times that of Earth, and surface temperature of 735 K (462 °C; 863 °F). Earth had a similar early atmosphere to Venus, but may have lost it in the giant impact event.

With plate tectonics

Rare Earth proponents argue that plate tectonics and a strong magnetic field are essential for biodiversity, global temperature regulation, and the carbon cycle. The lack of mountain chains elsewhere in the Solar System is direct evidence that Earth is the only body with plate tectonics, and thus the only nearby body capable of supporting life.

Plate tectonics depend on the right chemical composition and a long-lasting source of heat from radioactive decay. Continents must be made of less dense felsic rocks that “float” on underlying denser mafic rock. Taylor emphasizes that tectonic subduction zones require the lubrication of oceans of water. Plate tectonics also provides a means of biochemical cycling.

Plate tectonics and as a result continental drift and the creation of separate land masses would create diversified ecosystems and biodiversity, one of the strongest defences against extinction. An example of species diversification and later competition on Earth’s continents is the Great American Interchange. North and Middle America drifted into South America at around 3.5 to 3 Ma. The fauna of South America evolved separately for about 30 million years, since Antarctica separated. Many species were subsequently wiped out in mainly South America by competing Northern American animals.

Diamonds: bad for life. The planets circling some stars may be too diamond-rich, as much as 50% pure diamond. Their mantle might consist of a hard, brittle diamond that is incapable of flowing. Whereas iron and silicon trap heat inside our planet, resulting in geothermal energy, diamonds transfer heat so readily that the planet’s interior would quickly freeze. Without geothermal energy, there couldn’t be any plate tectonics, magnetic field, or atmosphere. Panero describes these diamond super-earths as “very cold, dark” worlds (Wilkins 2011).

A large moon

The Moon is unusual because the other rocky planets in the Solar System either have no satellites (Mercury and Venus), or only tiny satellites which are probably captured asteroids (Mars).

The Giant-impact theory hypothesizes that the Moon resulted from the impact of a Mars-sized body, dubbed Theia, with the young Earth. This giant impact also gave the Earth its axial tilt (inclination) and velocity of rotation. Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable. The Rare Earth hypothesis further argues that the axial tilt cannot be too large or too small (relative to the orbital plane). A planet with a large tilt will experience extreme seasonal variations in climate. A planet with little or no tilt will lack the stimulus to evolution that climate variation provides. In this view, the Earth’s tilt is “just right”. The gravity of a large satellite also stabilizes the planet’s tilt; without this effect the variation in tilt would be chaotic, probably making complex life forms on land impossible.

If the Earth had no Moon, the ocean tides resulting solely from the Sun’s gravity would be only half that of the lunar tides. A large satellite gives rise to tidal pools, which may be essential for the formation of complex life, though this is far from certain.

A large satellite also increases the likelihood of plate tectonics through the effect of tidal forces on the planet’s crust. The impact that formed the Moon may also have initiated plate tectonics, without which the continental crust would cover the entire planet, leaving no room for oceanic crust. It is possible that the large scale mantle convection needed to drive plate tectonics could not have emerged in the absence of crustal inhomogeneity. A further theory indicates that such a large moon may also contribute to maintaining a planet’s magnetic shield by continually acting upon a metallic planetary core as dynamo, thus protecting the surface of the planet from charged particles and cosmic rays, and helping to ensure the atmosphere is not stripped over time by solar winds.


A terrestrial planet of the right size is needed to retain an atmosphere, like Earth and Venus. On Earth, once the giant impact of Theia thinned Earth’s atmosphere, other events were needed to make the atmosphere capable of sustaining life. The Late Heavy Bombardment reseeded Earth with water lost after the impact of Theia. The development of an ozone layer formed protection from ultraviolet (UV) sunlight. Nitrogen and carbon dioxide are needed in a correct ratio for life to form. Lightning is needed for nitrogen fixation. The carbon dioxide gas needed for life comes from sources such as volcanoes and geysers. Carbon dioxide is only needed at low levels] (currently at 400 ppm); at high levels it is poisonous. Precipitation is needed to have a stable water cycle. A proper atmosphere must reduce diurnal temperature variation.

One or more evolutionary triggers for complex life

Regardless of whether planets with similar physical attributes to the Earth are rare or not, some argue that life usually remains simple bacteria. Biochemist Nick Lane argues that simple cells (prokaryotes) emerged soon after Earth’s formation, but since almost half the planet’s life had passed before they evolved into complex ones (eukaryotes) all of whom share a common ancestor, this event can only have happened once. In some views, prokaryotes lack the cellular architecture to evolve into eukaryotes because a bacterium expanded up to eukaryotic proportions would have tens of thousands of times less energy available; two billion years ago, one simple cell incorporated itself into another, multiplied, and evolved into mitochondria that supplied the vast increase in available energy that enabled the evolution of complex life. If this incorporation occurred only once in four billion years or is otherwise unlikely, then life on most planets remains simple. An alternative view is that mitochondria evolution was environmentally triggered, and that mitochondria-containing organisms appeared soon after the first traces of atmospheric oxygen. Oxygen was needed for powering the process of aerobic respiration for both plants and animals.

The evolution and persistence of sexual reproduction is another mystery in biology. The purpose of sexual reproduction is unclear, as in many organisms it has a 50% cost (fitness disadvantage) in relation to asexual reproduction. Mating types (types of gametes, according to their compatibility) may have arisen as a result of anisogamy (gamete dimorphism), or the male and female genders may have evolved before anisogamy. It is also unknown why most sexual organisms use a binary mating system, and why some organisms have gamete dimorphism. Charles Darwin was the first to suggest that sexual selection drives speciation; without it, complex life would probably not have evolved.

The right time in evolution

While life on Earth is regarded to have spawned relatively early in the planet’s history, the evolution from multicellular to intelligent organisms took around 800 million years. Civilizations on Earth have existed for about 12,000 years and radio communication reaching space has existed for less than 100 years. Relative to the age of the Solar System (~4.57 Ga) this is a short time, in which extreme climatic variations, super volcanoes, and large meteorite impacts were absent. These events would severely harm intelligent life, as well as life in general. For example, the Permian-Triassic mass extinction, caused by widespread and continuous volcanic eruptions in an area the size of Western Europe, led to the extinction of 95% of known species around 251.2 Ma ago. About 65 million years ago, the Chicxulub impact at the Cretaceous–Paleogene boundary (~65.5 Ma) on the Yucatán peninsula in Mexico led to a mass extinction of the most advanced species at that time.

If there were intelligent extraterrestrial civilizations able to make contact with distant Earth, they would have to live in the same 12Ka period of the 800Ma evolution of life.

Chain of Improbable coincidences (Gribbin 2018)

Many things had to go right for us to exist. Serendipity in the timing and location of our home star and planet as well as lucky conditions on earth and fortuitous developments in the evolution of life, resulted in human beings.

Timing. If the sun and earth had been born any earlier in galactic history, our planet would likely have had too few metals to form life. These elements are created during stellar deaths, and it took billions of years for enough stars to form and die to enrich the materials that built our solar system.

Location. The sun lies in a goldilocks zone within the milky way – not too close to the galactic center, where stars are more crowded and dangerous events such as supernovae and gamma-ray bursts are common, and not too far, where stars are too sparse for enough metals to build up to form rocky planets.

Technological Civilization. Once multicellular life arose, the development of an intelligent species was far from assured, and our species may have come close to extinction several times. Evolution doesn’t have a goal of creating intelligence, and if you asked an elephant what the goal of evolution was, she would probably tell you to evolve here extraordinary trunk with its thousands of muscles and consequent exquisite flexibility.  And without fossil fuels, we would have the civilization of what existed in the 14th century.  To become who we are today required language, an opposable thumb, the invention of fire, and much more, all very unlikely to have happened, yet here we are.

Snowball Earth (Ward & Brownlee)

It is possible that the extreme conditions of snowball earth were required to force multicellular life to evolve 650 million years ago when the Earth’s surface became entirely or nearly frozen at least once. 

Complex life evolved just once. All complex life is descended from a single common ancestor. Why? Nick Lane says that natural selection normally favors fast replication, keeping simple cells simple. Then a freak event occurred: an archaeon engulfed a bacterium and the 2 cells formed a symbiotic relationship. That transformed the dynamics of evolution, leading to a period of rapid change that produced innovations such as sex. The incorporated bacterium eventually evolved into mitochondria, the energy generators of complex cells.  So there was nothing inevitable about the rise of the sophisticated organisms from which we evolved. “The unavoidable conclusion is that the universe should be full of bacteria, but more complex life will be rare” (NS 2010).

If an alien civilization does arise, it will wipe itself out (Williams 2016)

‘Stargazing Live’ presenter Brian Cox believes the search for celestial life will ultimately prove futile. Cox believes that any alien civilization is destined to wipe itself out shortly after it evolves.

“One solution to the Fermi paradox is that it is not possible to run a world that has the power to destroy itself and that needs global collaborative solutions to prevent that,” Cox said.

The physicist explained that advances in science and technology would rapidly outstrip the development of institutions capable of keeping them under control, leading to the civilizations self-destruction: “It may be that the growth of science and engineering inevitably outstrips the development of political expertise, leading to disaster. We could be approaching that position.”

Tyrell T (2021) Earth’s Habitability Today Is Basically Due to Luck, Millions of Simulations Show

It took evolution 3 or 4 billion years to produce Homo sapiens. If the climate had completely failed just once in that time then evolution would have come to a crashing halt and we would not be here now. So to understand how we came to exist on planet Earth, we’ll need to know how Earth managed to stay fit for life for billions of years. Using a model that took into account factors like asteroid impacts, supervolcanoes and more, there was just one time out of 100,000 that a planet had such strong stabilizing feedbacks that it stayed habitable all 100 times, irrespective of the random climate events. On nearly every occasion in the simulation when a planet remained habitable for 3 billion years, it was partly down to luck. But luck alone was never sufficient, and planets that had no feedbacks at all never stayed habitable. By implication, Earth must therefore possess some climate-stabilizing feedbacks but at the same time good fortune must also have been involved in it staying habitable.

More articles on why we may be the only intelligent life in our galaxy:

Longrich NR (2019) Evolution tells us we might be the only intelligent life in the universe. The conversation


Gribbin, J. 2018. Why we are probably the only intelligent life in the galaxy. Scientific American.

Hunt K (2020) Observations of our closest neighboring star dampen hopes of a potentially habitable planet. CNN.

NS. 2010. An unlikely story. New Scientist.

Scharf, C. 2012. The benevolence of black holes. Scientific American.

Wilkins, A. 2011. The galaxy could be full of diamond planets. Gizmodo.

Williams, O. 2016. Brian Cox Explains Why He Thinks We’ll Never Find Aliens. His answer doesn’t bode well for the future of humanity. Huffingtonpost

Bonus article

This website explains why renewables can’t replace fossil fuels, or keep trucks running.  At a science writers conference I was asked by many people, well, what can we do then, and I threw up my hands and said “The Space Aliens will have to save us”.   Let’s hope it’s not these space aliens though.

Paul Simms. 2009. Attention, people of earth. The New Yorker.

We are on our way to your planet. We will be there shortly. But in this, our first contact with you, our “headline” is: We do not want your gravel.

We are coming to Earth, first of all, just to see if we can actually do it. Second, we hope to learn about you and your culture(s). Third—if we end up having some free time—we wouldn’t mind taking a firsthand look at your almost ridiculously bountiful stores of gravel. But all we want to do is look.

You’re probably wondering if we mean you harm. Good question! So you’re going to like the answer, which is: We mean you no harm. Truth be told, there is a faction of us who want to completely annihilate you. But they’re not in power right now. And a significant majority of us find their views abhorrent and almost even barbaric.

But, thanks to the fact that our government operates on a system very similar to your Earth democracy, we have to tolerate the views of this “loyal opposition,” even while we hope that they never regain power, which they probably won’t (if the current poll tracking numbers hold up).

By the way, if we do take any of your gravel, it’s going to be such a small percentage of your massive gravel supply that you probably won’t even notice it’s gone.

You may be wondering how we know your language. We are aware that there’s a theory on your planet that we (or other alien species from the far reaches of the galaxy) have been able to learn your language from your television transmissions. This is not the case, because most of us don’t really watch TV. Most of our knowledge about your Earth TV comes from reading Zeitgeisty think pieces by our resident intellectuals, who watch it not for fun but for ideas for their print articles about how Earth TV holds a mirror up to Earth society, and so on. We mean, we’ll watch Earth TV sometimes—if it happens to be on already—but, generally, we prefer to read a good book or revive the lost art of conversation.

Sadly, Earth TV is like a vast wasteland, as the Earthling Newton Minow once said. But, for those of you who can understand things only in TV terms, just think of us as being very similar to Mork from Ork, in that he was a friendly, non-gravel-wanting alien who visited Earth just to find out what was there, and not to harvest gravel.

Speaking of a vast wasteland, you might want to start picking out and clearing off a place for our spacecraft to land. Our spacecraft, as you will see shortly, is huge. Do not be alarmed; this does not mean that each one of us is that much bigger than each one of you. It’s just that there were so many of us who wanted to come that we had to build a really huge spacecraft.

So, again, no cause for alarm.

(Full disclosure: each of us actually is much bigger than each of you, and there’s nothing we can do about it. So please don’t use any of your Earth-style discrimination against us. This is just how we are, and it’s not our fault.)

Anyway, re our spacecraft: it’s kind of gigantic. The deceleration thrusters alone are sort of, like . . . well, imagine four of your Vesuvius volcanoes (but bigger), turned upside down.

We don’t want to hurt anyone, so, if you could just clear off one continent, we think we can keep unintended fatalities to a minimum. Australia would probably work. (But don’t say Antarctica. Because we’d just melt it, and then you’d all end up underwater. Which would make it virtually impossible for us to learn about your hopes and your dreams, and your culture, and to harvest relatively small, sample-size amounts of your gravel, just for scientific study.)

A little bit about us: our males have two penises, while our females have only one. So, gender-wise, if you use simple math, we’re pretty much identical to you.

And, as far as protocol goes, we’re a pretty informal species. If you want to put together a welcoming ceremony with all your kings and queens and Presidents and Prime Ministers and leading gravel-owners, that’s fine. But please don’t feel like you have to.

Technically, it would be possible for us to share our space-travel technology with you, so that you could build a spacecraft and travel to our planet also. But, for right now, it just feels like it would be better if we came to your place.

Speaking of gravel, one thing we can’t tell from our monitoring of Earth is how your gravel tastes. It’s just something we’re curious about, for no real reason. Is it salty? It looks salty.

Maybe you could form a commission of scientists/gravel-tasters to look into this and let us know. Just have them collect all the gravel you have and put it in one big pile. (There are some pretty big empty parts of Utah, New Mexico, and Russia that might be good spots for such a large gravel pile, but that’s just an F.Y.I.)

Then, if you could have your top scientists/gravel-tasters go through this gravel pile, tasting each and every piece, that would be great. Also, if it’s not too much of a hassle, have them put all the saltier-tasting pieces in a separate pile.

Anyway, that about wraps up this transmission! Looking forward to seeing you very soon. (Sorry we couldn’t have given you more notice, but we didn’t want you Earth people going crazy and looting stuff and having sex in the streets out of panic about losing all your delicious gravel, which is something that is definitely not going to happen, because, when it comes down to it, what is gravel really but just a bunch of baby rocks?)

Our E.T.A. on Earth is sometime in the next 450 to 500 years, which we know is a blink of an eye in your Earth time, so start getting ready! Let’s have fun with this.


A Species from a Galaxy You Haven’t Even Noticed Yet

P.S.—We saw that you sent some people to your moon recently. Good job! But, just to let you know, don’t waste your time with the moon. There’s no gravel there. We already checked.

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11 Responses to Rare Earth – why we may be the only intelligent species in the universe

  1. NJF says:

    On a little bit of a tangent, I am wondering how you arrive at this 14th century figure in terms of life without fossil fuels? Isn’t 1650 “pre-fossil fuels”?

    Yes, people were mining a small amount of coal for niche applications, but the coal wasn’t mining itself for huge amounts of energy at that point.

    Also, with regards to advancement, I bet you anything that bioengineered viruses, bacteria etc. will be the straw that breaks the camel’s back and causes widespread self-implosion. I am sure national “Deep States” have engineered viruses ready to go right now to wipe out the surplus 6+ billion when we run out of fuel. If they haven’t then they’re fools, the tech is going to be available very soon and you can bet China is going to do it.

    • energyskeptic says:

      You’re right, I use that because that’s when England began using coal and started the industrial revolution. Maybe engineered diseases aren’t needed, people are quite vulnerable to ordinary flus when they’re starving.

  2. Olaf Stapledon’s Brilliant 1932 novel “Last and First Men” specifically describes a world that burns through its fossil fuel, crashes for tens of thousands of years. Fascinating in its assumption that America and China are the “last superpowers” before the creation of the First World State, that eras of mechanical and genetic engineering alternate, etc. Worthy of a review – https://www.amazon.com/Last-First-Men-Star-Maker/dp/0486219623/ref=sr_1_4

    Get this one, a more recent edition rewrote the first chapters.

    Then, after a many millennia, a new civilization masters solar, then nuclear energy, destroys the earth in atomic war – crashes for 10 million years, due to climate change. Relic human population evolves to new human species. Story goes on and on, until the final 18th Men terraform Neptune, but are destroyed by the Sun going nova. A must-read for future history.

  3. A few things that have changed since “Rare Earth” was published.

    1. It is possible to have planets with stable orbits around binary stars. Apparently, even close binaries can have stable planets. So, better than “Rare Earth” thought.

    2. However, Planetary formation theories in 2000 assumed that most systems would have rocky, earthlike planets in the habitable zone. Instead, exoplanet discoveries have revealed that most systems have the big, Jupiter-style planets “spiral in” and the earthlike planets are destroyed by their star. Our solar system is a very rare exception. Typical orbits for rock planets give them temperatures of thousands of degrees, up to 4,000 degrees in one instance. So, the number of rocky earthlike planets is lower than “Rare Earth” predicted.

    3. Other planetary systems revealed by exoplanets are highly eccentric – with gas giants on egg-shaped orbits, disrupting the orbits of small planets.

    3. Of the remaining rocky planets, no “Earth” size planets exist. Instead, we have “super-earths” – 2-4 times larger than the Earth. There seems to be some mechanism preventing Earth or Mars-sized planets from forming around stars like our sun. At least some of the “super earths” are actually Neptune-sized gas giants which are so close to their star that their huge atmospheres burned off – completely unearthlike.

    4. The few cases where Earthlike planets are seen is around “red dwarf” stars much smaller and cooler than the Sun. The Trappist-1 system is a good example. However, red dwarfs have very strong solar flares and solar wind. The flares with blast the nearby planets with lethal radiation, and the winds would tend to strip atmospheres, like Mars in our system. This is likely because a habitable planet would be “tidally locked” and not rotate, and therefore could not have an Earth-style magnetic field to shield the atmosphere from radiation. Again, “habitable planet” likelihood is lower than thought in 2000.

    6. Smaller super-earths are likely “water worlds” with extremely deep oceans, possibly hundreds of miles deep. The problem is that the enormous pressure would prevent both plate tectonics and volcanic eruptions or geothermal events, so the oceans would be nutrient-poor, worse than the deep oceans on earth. There would also be a problem with nutrients rising in such a deep ocean.

    5. The assumption that “bacteria are easy” can also be called into question. Earth life appeared very early, but to date (despite numerous false media reports) nobody has ever demonstrated a plausible jump from organic molecules to life. It is as if we find the bricks of a building everywhere, and one example of a building – but nothing in between. It may be that life is extremely uncommon. Chance collisions to make something the size of a protein molecule would be around 1 in 10 to the 70th power. There are chemical bootstrapping methods that could lower this – but we just don’t know if they make life probable or rare.

    6. However panspermia is deemed more likely now than in 2000. Life cold survive in rocks blasted off one planet and frozen (deep in the rock, away from radiation) for millions of years.

    Everything adds up to “another Earth” being even more rare than we thought in 2000.

    • Richard Benton says:

      That’s not enough everything Wikipedia lists many facts have changed since rare earth that go against the hypothesis any conclusions one way or the other are premature It’s hard to believe now but at one time the idea that there were billions if not trillions of planets was in doubt

  4. Richard Benton says:

    Wikipedia has extensive criticisms of rare earth It is alleged there are intelligent design ideas behind it in that they believe earth is unique Plate tectonics are not rare .The idea Jupiter is necessary for impact protection is wrong It has actually increased impacts.The Truth is we know far too little at this point to conclude anything.There are many variables in life creation

    • Richard, your critique comes from the original publication of “rare earth” and the data on Wikipedia is old. We only have one true example of plate tectonics in the solar system (Earth), so any assertion that “plate tectonics is not rare” is speculation based on computer models, not observation.

      But that’s not a big issue since we don’t even know if the origin of life required plate tectonics.

      Actually, people assumed planetary systems were common beginning in the 1600s. The Kant-Laplace model of star system formation (1790s) also implied planets were extremely common. In the early 20th century, problems with the model made astronomers favor other, rare events. However, the inclusion of magnetic and electric fields in the Kant-Laplace model once again has most stars having planets.

      The “Jupiter protection” idea is indeed obsolete. The bigger problem in many exoplanet systems is that the “Jupiters” not broiling next to their star are in egg-shaped, disruptive orbits. The average “Jupiter” is also usually several times (10x) the size of “our” Jupiter, increasing effects.

      But that’s not the big point. “Rare Earth” is partly validated since we have now looked at THOUSANDS of planets across MILLIONS of stars. Planets that are the size of Earth are basically nonexistent – they’re all larger (“super earths”). “Super Earth” is a media name – that definitely doesn’t mean Earthlike.

      In most systems, planetary migration destroyed the smaller planets, and you have “hot Jupiters” right next to their star. Sometimes you have one or two “super earths” with a mega-Jupiter (10x Jupiter size).

      At one time this could have been sampling error, but no more – we have lots of data. Out of the thousands of star systems, there’s nothing like our solar system. It has been suggested that the normal process of “spiraling in” of Jupiter was halted in our case by a freak, ultra-close supernova as the solar system formed. No matter what it is, our particular kind of solar system (small planets close in, big ones further out) are rare.

      Here’s a more recent plot:

      You can see a couple of near-Earth planets in orbital periods of about a year. Trouble is, they orbit red dwarfs, and are as cold as Pluto.

      The best candidate to date is a group of planets around the red dwarf Trappist-1, which unfortunately has severe flares and solar winds which probably stripped atmospheres from the planets. Earlier speculation assumed a lot of water, but new consensus is that this was incorrect. However, the “stripping” problem may not be as bad as thought.


      However, there is a LOT of conflicting speculation, see SpaceRef – http://nasawatch.com/search.html?r=trappist

      In other words, speculation until James Webb get signatures for any atmospheres that might be present.

      The current belief is that many of the “super-Earths” are actually the cores of Neptune-sized planets, with their atmospheres stripped. A very small subset of these might be non-Neptune cores more like our planet. So, multiple it up, an we’re talking possibly 1/1 million star systems have a planet that might be habitable.

      So not impossible, but rare.

      Finally (I used to work as a molecular biologist and in fact did some “origin of life” experiments at the University of Chicago) the face that we don’t understand how life originated is NOT a reason to believe that it is common. Until we actually understand what it takes to create life from inorganics, it is a complete guess. It certainly doesn’t happen easily (70 years of tries). The speculation “life is common” typically comes from physicists, not biologists who actually work with and understand living things.

      • energyskeptic says:

        Pete, thanks for your wonderful and interesting comment, I haven’t kept up with the exoplanet and solar systems research.

    • energyskeptic says:

      I’m not concluding anything, Rare Earth theory isn’t including anything. It’s making a good case that intelligent life is very rare.

  5. Richard benton says:

    I’ll repeat myself you are doubling down you don’t know you can’t know definitively one way or the other the data we have is much too small of a set the jury is still out why is that such a problem for you all science is considered tentative pending new evidence although it is perverse to withhold assent once the evidence is sufficient as in evolution the evidence is not all in and even if there is life on other planets the universe is so vast and the distance so greater will never meet them let alone prove it one way or the other