Ramses Ramirez, whose work on what he calls the Complex Life Habitable Zone was the subject of a recent Alex Tolley essay (see Are Classic Habitable Zones Too Wide for Complex Life?), joins us today with a look back at Rare Earth on the occasion of the book’s 20th anniversary. Written by Peter Ward and Donald Brownlee, Rare Earth examined a wide range of factors that argued against the ubiquity of complex life in the cosmos. I remember well when it came out, as I was in the midst of writing my Centauri Dreams book for Copernicus, Ward and Brownlee’s publisher, and my editor (the brilliant Paul Farrell) and I had to wrestle with the question of whether Rare Earth rendered the search for intelligent life elsewhere irrelevant. Fortunately, we plunged ahead anyway. As Dr. Ramirez shows this morning, many of the factors put forward by Ward and Brownlee can be re-examined with new data as work on exoplanets continues. Ramses is a research scientist at the Earth-Life Science Institute (ELSI) in Tokyo specializing in habitability issues, and a member of the Martian Moons eXploration (MMX) science team. His research at ELSI is augmented by collaborations with both JAXA, the Japanese space agency, and the National Astronomical Observatory of Japan.
by Ramses M. Ramirez
One of the most profound questions that mankind has ever asked is: Are we alone in the universe? This age-old question has motivated many philosophers and scientists alike to question how common life, particularly intelligent life, is in the universe.
In 2000, Peter Ward and Donald Brownlee (University of Washington) wrote the influential book, Rare Earth: Why Complex Life Is Uncommon in the Universe, to address this very question based on our (then) understanding of various scientific fields, including astronomy, geology, atmospheric science, planetary science, and biology. Summarizing this evidence, these authors had concluded that simple unicellular lifeforms may be common throughout the universe, although complex life, including multicellular plants and animals, may be very rare.
Although this sounds like bad news for many science fiction aficionados, this is acceptable for the field of astrobiology, which is dedicated to searching for alien lifeforms of any sort, including simpler microbial life. Hence, the conclusions inferred by Ward and Brownlee most directly impact SETI, or the Search for Extraterrestrial Life, a scientific endeavor dedicated to the search for intelligent life on other planets.
Shortly after the publication of Rare Earth, my Ph.D. advisor and colleague, Evan Pugh Professor James F. Kasting (Penn State University), had written a comprehensive and overall positive review of Rare Earth (title: Peter Ward and Donald Brownlee’s “Rare Earth”). Interestingly his assessment of the same scientific evidence led to a different conclusion, however: That Ward and Brownlee may have been overly pessimistic regarding the prospects of extraterrestrial complex life. Professor Kasting instead argued that many of the points raised in Rare Earth could be used to support the premise that complex life may be common.
However, the exponential increase in data and observations over the past 20 years warrants a re-examination of the scientific arguments made in this book. These are truly exciting times and I am honored that Centauri Dreams has invited me to review Rare Earth on its 20th year anniversary. In here, I revisit these and other arguments using the most updated science.
I decided to write my critique using a similar outline that Professor Kasting implemented in his original review, which I hope proves useful both for comparing the arguments and for highlighting how the field has evolved over time. I have also added a couple of additional scientific questions that have become important since the publication of Rare Earth.
Jupiter’s Importance as a Shield Against Large Impacts
Previous work had shown that Jupiter helps deflect comets and asteroids away from the inner solar system . This is because Jupiter’s large mass (approximately 300x Earth’s) reduces the frequency of such impacts by approximately a factor of 1,000 to 10,000. From this, the authors of Rare Earth reasoned that the existence of a Jovian in the outer solar system may be a key ingredient for the emergence of life (and, eventually, complex life) on potentially habitable planets. However, during the time in which Rare Earth was published, there were very few of these gas giants discovered orbiting other stars. The few that were found were “hot Jupiters”, which are located in the inner stellar system and could potentially destabilize the orbits of neighboring terrestrial planets.
It is then reasonable to wonder how advances over the past 20 years have updated our understanding of exoplanetary distributions. By most measures, this has been a great success story. Approximately 4,000 exoplanets have been confirmed to date. Out of the nearly 1,300 discovered Jovian planets, over 800 are hot Jupiters (Figure 1). Although a seemingly impressive number, ~ ½ of 1% of M-dwarf stars host a hot Jupiter at all, whereas this number is closer to 1% for F – K stars , which is consistent with the lower inferred disk masses for M-dwarf systems . Indeed, studies calculate that impact frequencies and velocities should be higher for M-dwarf systems [4,5], which suggests that such environments may not be ideal for supporting complex life. Recent work finds that the relatively lower Jovian frequency around M-dwarfs may facilitate water delivery to terrestrial planets . Such models predict worlds with water masses many times that of Earth. However, the interior pressures of these water worlds would also be too high to support volcanism and plate tectonics, likely inhibiting the emergence of complex life [7,8]. If such models are correct, they suggest that complex life may be rare on M-dwarf planets, even if simpler life cannot be ruled out .
We have some information for larger stars too. Limited data from ground-based methods, including radial velocity surveys, indicate a much larger frequency of “cold Jovians” orbiting more massive stars . Even so, it is not clear that such Jovians would always shield terrestrial planets from cometary and meteoritic impacts. Alternatively, Jupiter could have helped trigger instabilities that delivered large influxes of cometary and asteroidal material into the inner solar system, possibly delaying the emergence of life. Thus, the importance of Jupiter-like planets to complex life remains highly uncertain.
Figure 1. Confirmed exoplanets classified as a function of stellar proximity and mass as of July 2, 2018. RE = Earth radii, ME = Earth masses. (Credit: Abel Mendez, PHL @UPR Arecibo).
The Effect of the Moon in Stabilizing Earth’s Obliquity
The importance of the Moon in maintaining the habitability of Earth was a key issue raised in Rare Earth. Earlier modeling simulations  predicted that Earth would have exhibited much larger obliquity swings (from 0 to ~85°) than it does today (~22.1 to 24.5°) in the absence of a large Moon and assuming a present day orbital spin rate. As we know, a non-zero obliquity is necessary for the existence of seasons. At an obliquity (tilt) of zero degrees, our planet would cease to have them. Nevertheless, the relatively small ~2 degree change in tilt occurs over a period of about 41,000 years, in accordance with the predicted Milankovitch Cycles.
According to Ward and Brownlee, without the stabilizing presence of the Moon, the resultant obliquity cycles and climate variations would be so drastic that conditions would prohibit the emergence of complex life. This argument is likely still valid because even though more complex models find slightly smaller obliquity variations than originally predicted  (~50 degrees ), the resultant temperature variations are still large enough to be catastrophic for the survival of complex life.
One way to diminish these tilt-dependent temperature variations is by greatly increasing the atmospheric pressure, which enhances the transport of heat and moisture across the planet. Although this may be accomplished by having a multi-bar CO2 atmosphere , such a dense CO2 atmosphere is likely unbreathable by animals or other complex life, at least as-we-know-it to exist on our planet .
Another way to minimize climatic variations on a Moon-less Earth is by reducing its spin rate . This can be achieved if, for example, the impactor event that led to the formation of the Moon never happened. However, the problem here is that much of the water delivered to the Earth was brought in during the late stages of accretion, possibly by the Moon-forming impact . Thus, without that catastrophic event, Earth may have been a much drier, possibly lifeless, planet, one without plate tectonics (more on this next).
The Importance and Uniqueness of Plate Tectonics
Ward and Brownlee argue that plate tectonics is rare because it would require a serendipitous Moon-forming impact. In their book, they stress that plate tectonics is key to continental formation, which potentially promotes biodiversity and are key habitats for animal (complex) life. It is also thought that liquid water is necessary for the initiation of plate tectonics, for at least two reasons. First, water lubricates the plates, which permits sliding motions. Secondly, water cools the seafloor as it spreads, which helps maintain plate rigidity.
If the Moon-forming impact (among any other late impactors) was what delivered much of our planet’s water, then Ward and Brownlee could be right about what initiated plate tectonics. However, we still do not know when plate tectonics on our planet actually started. If plate tectonics was triggered by ample water delivery, then we would expect plate tectonics to have initiated early on, possibly soon after the Earth’s surface became habitable and capable of supporting liquid water ~4.3 – 4.4 Gyr . The problem is that a wide range of start dates for plate tectonics has been proposed, between ~4.2 – 0.85 Gyr ago . Thus, if plate tectonics started later, the implication is that water may only be a necessary – but insufficient – condition for plate tectonics.
In recent years, there has been a debate over whether plate tectonics is possible on terrestrial planets that are larger than Earth. One school of thought says that increased gravitational resistance on super-Earths reduces subduction tendency, dampening the potential for plate tectonics [16,17]. However, others argue that increased shear stresses help overcome resistance to plate motions . Clearly, how interior processes scale to more massive planets is poorly understood.
The Difference Between the Habitable Zone and the Complex Life Habitable Zone
Ward and Brownlee define the “animal habitable zone” (AHZ) as the region around a star where a planet could support a mean surface temperature between 0 and 50 degrees Celsius. They correctly argue that that the AHZ should be significantly narrower than the classical habitable zone (HZ) (~0.95 – 1.67 AU) originally formulated by Kasting et al. . This is because the classical HZ posits that multi-bar CO2 atmospheres could exist near the outer edge, which would be deleterious to life as-we-know-it on Earth. Thus, this classical HZ definition should not be used to search for complex life.
Recent studies have shown that a HZ for complex life would be approximately half as wide as the classical HZ [12, 20]. However, such work shows that a complex life habitable zone (CLHZ) is not just dependent on temperature ranges, but on respiration limits determined by the atmospheric composition and pressure. The Meyer-Overton correlation , which relates respiration to the solubility of gases in lipids, was used to show that many land animals on Earth could acclimate to breathing in air with CO2 and N2 pressures no higher than ~0.1 bar and 2 bar, respectively . Experimental data also support these theoretical predictions . Above such limits, narcotic effects, similar to intoxication, could become potentially lethal. I have used these theoretical and experimental results to compute a solar system CLHZ outer edge at ~1.31 AU  (Figure 2).
At the inner edge, 1-D models had predicted that a moist greenhouse is triggered above a mean surface temperature of ~340 K, leading to atmospheric escape rates to space that can deplete an Earth-like surface water inventory within in ~4.5 Gyr (the age of the solar system). Other more advanced models find that a full runaway greenhouse is triggered at lower mean surface temperatures (~330 K) before a moist runaway greenhouse can even occur [22, 23]. Nevertheless, the CLHZ width for our solar system is approximately half that of the classical HZ, assuming alien life would exhibit similar respiratory limits as does life on Earth. This latter assumption may or may not be the case, however, and future experiments and studies would need to shed light on this question.
Figure 2. The complex life habitable zone , most applicable to animal life, is significantly narrower than the classical habitable zone . Adapted from work in ref:12.
The Pre-Main-Sequence Habitable Zone
The temporal evolution of the HZ is central to our ability to find potentially habitable exoplanets for follow-up atmospheric characterization. Although the classical HZ is concerned with the main-sequence evolution of the host star, it is the pre-main-sequence evolution that largely determines whether a planet can acquire and retain volatiles, especially those orbiting M-dwarf stars [24-26]. This impacts the likelihood that a planet that is later located in the main-sequence HZ is habitable or not.
The reason for this is straightforward. At the beginning of the pre-main-sequence stage, a young star is at its brightest and becomes dimmer as it slowly contracts until it achieves gravitational equilibrium at the start of the main-sequence phase. Unlike other stars, which are only slightly brighter during the pre-main-sequence phase, M-stars can be orders of magnitude brighter during their pre-main-sequence phase as compared to when they appear on the main-sequence (Figure 3). As a result, this pre-main-sequence HZ is much farther out during this time, sweeping inward as the star dims. During this stage of stellar contraction, conditions within the inner stellar system are very hot and runaway greenhouse conditions are triggered, possibly desiccating any planets before the star reaches the main sequence.
Figure 3. Pre-main-sequence stellar luminosity starts high and decreases with time until the start of the main-sequence (red point). Figure reproduced from ref:58 and based on work from ref: 25.
Binary Star Systems do have Planets
Ward and Brownlee had argued that binary star systems, which comprise about half of all stellar systems , may not have planets. Indeed, when Rare Earth was written, planets had not been detected around binary systems. Since then, however, we have certainly detected many such systems . At this point, the debate is no longer whether such planets exist (they do), but whether they can be habitable. Some of these planets have been found in the HZ of their stars , which suggests the potential to host life. However, the fluxes that planets in these systems receive can vary wildly throughout the orbital cycle, impacting their habitability. Some may be repeatedly entering and exiting their stars’ HZ. Dynamic effects also impact the orbital stability of these multiple star systems.
The Galactic Habitable Zone
The galactic habitable zone (GHZ) is the galactic equivalent of the traditional stellar HZ and is the region within the Milky Way where liquid water could be stable on a planetary surface and animal life can be supported . Subsequent work further quantified the GHZ based on stellar age, metallicity, and galocentric distance  (Figure 4). For instance, too close to the galactic center, and deadly supernovae and gamma ray bursts are much more abundant. Likewise, if the star is either too far away or too old, the system is unlikely to possess the heavier elements that are necessary to produce rocky planets.
Ward and Brownlee had applied the GHZ to argue that our solar system happens to be located in a very special region where these factors are just right for animals and beings like us to emerge. Even if this was true, however, one should note that our Milky Way galaxy spans ~100,000 light years and contains a few hundred billion stars. This would still mean that there are many millions of stars located within just a few hundred light years from our Sun, providing ample opportunity for life, even complex life, to be found within our solar neighborhood. Moreover, recent telescopes, like Kepler, are designed to observe planets that are no more than a couple thousand light years from our solar system, which would be well within GHZ regions that are most favorable to life.
Figure 4: The galactic habitable zone . Our Sun is located within the galactic region that is most suitable for life. Figure reproduced from ref: 31.
The “Anomalously” High Metallicity and Lower Activity Level of Our Sun
Ward and Brownlee cite Gonzalez , who indicated that our Sun is more metal-rich than the average star, including those of its same spectral type (G-star). On this basis, it was concluded that our Sun has an anomalously high metallicity. However, there is some confusion on this topic because Gonzalez  did not actually measure the metallicity per se, but the ratio of iron to hydrogen. To further confuse a planetary scientist (like myself), astronomers tend to refer to all elements heavier than H or He as “metals”. This Fe/H ratio is often taken as a proxy for metallicity. Nevertheless, Gonzalez  found that ~10% of the stars had an even higher Fe/H, suggesting an above average, but not anomalously high metallicity for our Sun.
However, subsequent studies have done more extensive stellar mineralogy that go well beyond the Fe/H story. These investigations find that refractory element (non-volatile) abundances are somewhat higher whereas those for volatiles are somewhat lower for solar analogues than for the Sun . The cause for this pattern is unclear, but it may be attributed either to loss of disc material during planet formation or ingestion of planetary material by the host stars themselves [34-36]. More recent work has found that stellar magnetic activity and galactic chemical evolution can also affect measured stellar abundances, making diagnostic determinations difficult [37,38].
Overall, the inferred mineralogic trends and associated mechanisms to explain them continue to be a subject of debate. However, there is no a priori reason to assume that our Sun is a unique outlier in this regard. We even have two neighboring stars – Alpha Centauri A and B – that are quite similar to our Sun in many respects. Indeed, the mean surface temperature of Alpha Centauri A is almost identical to that of our Sun and Fe/H is somewhat higher .
Even though solar metallicity may not be that unusual, the Sun exhibits lower levels of stellar activity than those measured for nearly any other Sun-like star . This could suggest that our Sun is a rare star. Alternatively, perhaps the Sun is in a temporary state of low activity and will become much more active in the future. If so, then complex life is only possible during a very narrow window of time in a Sun-like star’s history.
Most Stars don’t have Planets
Ward and Brownlee argued that only 5 – 6% of detected stars have planets, and that most of these are hot Jupiters, which are uninhabitable. Of course, planetary data were relatively scarce when this inference was made. Thanks to the Kepler mission, however, we now believe that nearly all stars have planets . What’s more is that a wide variety of planet types, including Earths, super-Earths, mini-Neptunes, Neptunes, and Jupiters (both hot and cold), populate the cosmos. Indeed, the earlier reported high frequency of hot Jupiters can now be attributed to observational biases in the transit and radial velocity methods, techniques that are particularly sensitive to larger planets traveling in smaller orbits. What’s more is that two classes of relatively common exoplanets, super-Earths and mini-Neptunes, have no equivalents in our solar system. We live in exciting times. Indeed, the apparent ubiquity of exoplanets is an argument some use to suggest that our Earth may not be so rare after all.
It even seems quite common to find terrestrial planets located within their star’s HZ. Recent work has estimated ηearth, which is the fraction of stars that host at least one terrestrial planet within their star’s HZ. This ηearth value is important because missions use it to estimate the number of habitable zone planets that can be detected by upcoming missions, directly influencing the size of the telescope that is eventually designed. This ηearth value was found to be between ~15 – 45% for M-stars . Extended to F-K stars, ηearth approaches 9% , assuming a conservative definition of the classical CO2-H2O HZ . Other studies over the years have found similar results. The point here is that rocky planets are common, including those located within the HZ. That said, ηearth has not been estimated for the CLHZ , which is more appropriate for animal life. However, given that the CLHZ is approximately half as wide as the classical HZ, the above numbers would probably not drop by much more than a factor of 2, suggesting a CLHZ ηearth approaching 5 – 10% or so.
Massive Stars cannot evolve complex life
Main-sequence stars that are heavier than our Sun are hotter and burn their stellar fuel (converting hydrogen to helium) more quickly. This is why massive stars have shorter lifetimes. Rare Earth then suggests that the main-sequence lifetimes of stars with masses exceeding ~1.5 times that of our Sun (~ a F5) are too short for animals to evolve. This comes from assuming that it would take an Earth-like 3.5 – 4 Gyr for animals to emerge on potentially habitable exoplanets. However, there is no a priori reason to believe that complex life elsewhere would take the same amount of time to arise that it took here. I think it is more reasonable to expect that animals can arise either more slowly or quickly on other planets, depending on the specific circumstances.
The authors further argue that the high UV radiation on planets orbiting more massive stars could be harmful to surface life. We know this from everyday experience, which is why we should wear sunblock on particularly sunny days. However, enhanced UV radiation from massive stars is offset by ozone absorption in an oxygen-rich atmosphere . Moreover, UV-C radiation (between 100 and 280 nm) may actually be beneficial to prebiotic chemistry .
Ward and Brownlee also argue that UV fluxes would be high enough from massive stars to drive atmospheric escape (EUV and XUV fluxes are energetic enough). Although such fluxes may be 2 or 3 times higher for massive stars than they are for the Sun , HZ planets orbiting these brighter stars are also on longer orbits, which is partially offsetting because stellar radiation decreases rapidly with distance.
Earth has Just the Right Amount of Water
Another argument made in Rare Earth is that Earth has just the right amount of water present on its surface. Too much, and there are no continents. Too little, and it may not be enough for life to exist. As Ward and Brownlee point out, water-rich impactors (e.g. carbonaceous meteorites) would have supplied the young forming Earth. During this late stage of accretion, a steam atmosphere would have likely arrived in an unsteady equilibrium with the water dissolved within the hot silicate melt that characterizes the surface and much of the interior.
There has been significant progress on this topic within the last several years. In the case of some M-stars, the interior may need to exceed ~5% wt. H2 to produce a steam atmosphere at all . This problem is further compounded by early atmospheric escape processes associated with high stellar radiation fluxes. Thus, whether a planet can eventually retain any surface water at all depends on a complex interplay between these competing escape and delivery processes.
Furthermore, decay of radioactive isotopes, like Al-26, within the interior can also desiccate planets . The half-life of Al-26 is very short (only about 700,000 years) so this mechanism can potentially desiccate small planets with very short formation timescales, like (possibly) a young Mars . Radiogenic heating can also desiccate very young planetesimals in the early stages of accretion before they combine to form planets .
However, Earth took a long time to form, approximately ~50 million years. If Earth had acquired most of its water during the late stages of accretion, possibly in the Moon-forming impact , among any others, early desiccation via radioactive decay of Al-26 would not have occurred. Thus, given all of the various ways that things could go wrong during water delivery, the possibility that Earth might be rare in this aspect deserves further attention.
Earth has just the right amount of Carbon Dioxide
The authors assert that if Earth had more CO2, it could have ended up as hot as Venus (~ 735 K mean surface temperature). However, this exact scenario has been assessed by myself and others and we find that predicted mean surface temperatures are not much higher than ~500 K, even if 100 bars of CO2 were abruptly added to the atmosphere [51,52]. This is because the atmospheric water vapor pressure is outstripped by that of CO2. The lower atmospheric water vapor concentration then produces a weakened overall CO2-H2O greenhouse effect. Nevertheless, such temperatures are still much too high for the existence of life as-we-know-it, being more than 100 K hotter than what is needed to break atomic bonds . Again, such CO2 pressures are also much too high for complex life, which requires significantly lower CO2 pressures .
In a more realistic case, CO2 levels would increase slowly and modestly above current CO2 levels. In that scenario, some of that atmospheric CO2 would be slowly drawn down through precipitation and silicate weathering, in a process called the carbonate-silicate cycle, which operates over million-year time scales. Over these relatively long timescales, Earth would likely re-equilibrate to a slightly higher temperature to compensate for the gradually brightening Sun. In any case, CO2 levels would be low enough to support complex life.
The Importance of Earth’s Magnetic Field
As Ward and Brownlee noted, various indicators suggest that Earth’s magnetic field contributed to its habitability. Earth’s magnetic field ensures that harmful solar particles don’t gradually erode the atmosphere. To make some sense of this, it is worth looking at the other classical HZ planet in our solar system, Mars. Although the Red Planet does not possess an intrinsic magnetic field today, it did have one at least 3.5 Gyr ago, which would have granted some protection to its early atmosphere, possibly keeping it thick long enough to help explain the geologic evidence indicating a once warmer and wetter planet. Unfortunately, Mars was too small to retain its interior heat for as long as Earth has, which led to a shutdown in its magnetic field. This would have rendered its atmosphere exceedingly vulnerable to sputtering and other loss processes, as MAVEN has shown are still occurring today .
It is also helpful to evaluate the other two terrestrial planets. Although Mercury does not possess an atmosphere, it does have a magnetic field, albeit weaker than Earth’s. One may use this to argue against the importance of a magnetic field except that Mercury is located too close to the Sun and far away from the HZ. Therefore, the planet would have been unable to sustain water on its surface with or without a magnetic field.
Likewise, one may be tempted to argue that Venus is able to retain a thick atmosphere without a magnetic field. On face value, this argument may make sense but there are a couple of key caveats. First, like Mercury, Venus is not located in the HZ, and its close proximity to the star ensures that water cannot be supported on its surface today. Secondly, its thick atmosphere likely results from a cessation in plate tectonics, possibly caused by an early runaway greenhouse that desiccated the planet . This would have shut down atmospheric CO2 removal processes (like rainfall), resulting in unmitigated volcanism and an excessive buildup of atmospheric CO2. Such dense CO2 atmospheres exhibit high cooling rates, making them far less susceptible to losses from solar wind stripping. This is how a planet with no magnetic field, like Venus, can still retain a thick atmosphere. However, such a planet is clearly not habitable, so this is a moot point. Magnetic fields do not necessarily make planets habitable, but they can sustain habitable conditions on planets that already are. For the two known cases of terrestrial HZ planets, one has a thick magnetic field (Earth) whereas the other doesn’t (Mars). So, Ward and Brownlee may be on the right track by suggesting that potent magnetic fields may be common among habitable planets capable of producing complex life (Figure 5).
Figure 5: Strong magnetic fields appear to help maintain the habitability of habitable planets Figure reproduced from ref: 59. Image Credit: Tulsa Voralia
The Cambrian Explosion Was Triggered by a Special Event
We still do not know what triggered the Cambrian explosion on our planet nor how it happened. This “explosion” refers to the sudden appearance of most of the major body plans for complex lifeforms, in an event that occurred at the beginning of the Cambrian period (~540 million years ago). Ward and Brownlee note 4 proposed hypotheses: 1) a rise in atmospheric O2; 2) increased availability of nutrients; 3) an increase in global temperatures following a global snowball event; or 4) rapid true polar wander and continental reconfiguration. However, in isolation, each of these explanations is problematic. For instance, the rise in O2 occurred some 50 Myr or more before the Cambrian explosion, suggesting the former may not have been the immediate trigger for the latter. Also, molecular evidence indicates that simple metazoans may have existed more than a hundred million years before the major rise of O2 . An increase in global temperatures also cannot explain why this increase in biodiversity had not occurred when temperatures were presumably higher in other times during Earth’s past. As mentioned earlier, atmospheric CO2 levels exceeding ~0.1 bar CO2 (~250 times PAL) may be deleterious to complex life as we know it on Earth. So, it would not matter much if rapid true polar wander occurred while CO2 levels remained too high for the emergence of complex life. Indeed, CO2 levels were gradually decreasing throughout Earth’s geologic history , decreasing below ~0.01 bar by the Cambrian explosion. Altogether, it is probable that there was not one single trigger, but a series of overlapping conditions that had to be met before complex life finally arose. Such a special scenario could be seen as supporting Ward and Brownlee’s core argument.
An impartial evaluation of the evidence supports the premise that complex life, including animal life, might be rare throughout the cosmos. At the same time, there remains sufficient uncertainty to question that conclusion. The Fermi Paradox, which attempts to explain the apparent contradiction between the perceived high probability of extraterrestrial intelligence with the lack of actual contact, still cannot be answered at this time. If the Rare Earth hypothesis is correct, a SETI search for complex (i.e., intelligent) life faces unbelievably low chances of success.
On the other hand, if animal and (ultimately) intelligent alien life are relatively common, we could obtain positive proof of their existence within the next few decades. As exciting as this latter prospect is, it could also be rather frightening. After all, some of these civilizations may be more advanced than we are, requiring the formulation of careful protocols to ensure safe contact. However, if intelligent life elsewhere really is common, we have to ask why nobody has visited us yet. This scenario could suggest that the technology to visit such alien systems is extremely difficult to develop, if not impossible, unless we are among the most advanced civilizations in the universe.
Nevertheless, we wish to maximize our chances of finding life elsewhere by assuming that E.T. is rare and difficult to find. Under that assumption, we will not find complex life on other planets with our current technology. This news should not discourage us, but instead embolden our leaders to invest more heavily in space science and engineering. Such initiatives would help produce improved telescopes and techniques that would enable us to search for extraterrestrial intelligence and have a reasonable chance of finding it.
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