Where Are They?

This monograph, for a general audience, will explain why highly evolved life forms like ourselves are rare in our Universe, and why finding advanced life beyond Earth will be far more difficult than we might have imagined. The conclusions presented here draw on recent research results from a range of active scientific fields, from Astronomy to Zoology. Certain recent advances within these fields bear on this question, and, when combined with others, show how difficult it is to develop advanced life forms in our Universe. There will be millions, if not billions of beginnings of single cell life, but evolving to the level of humans or beyond is such a fraught path that few will make it. Those few, randomly distributed over the immensity of the Universe, mean that we're very unlikely to find neighbors at our doorstep, or even in our neighborhood, or region. The closest may only lie somewhere in our Local Group of galaxies, or within our Local Supercluster.

"Where are they?" (The Fermi Paradox) has been a frequent question in recent times by those thinking about possible intelligent life beyond Earth. After all, there are so many stars with so many planets around them, all over the Universe. Humans have made it this far on one of them, others must have too, elsewhere. Where are they and why haven't we seen positive evidence of them? That is the question we answer here.

The particular scientific results that we marshall to reach our conclusions do not have much to say about the possible forms another intelligent life form might take. That is a question for a different discussion, and is not treated here. But we do go so far as to restrict our focus to beings that have evolved on a planet with a solid surface, possibly partially or totally covered with oceans, not on a gas giant. We consider only those possible beings that have a definite shape, a stable outer surface one can touch if you will (e.g. a skin, a membrane, a shell, or an exoskeleton for instance), and a definite system of reproduction. They may be marine beings living in an ocean, or have evolved entirely on land, or some combination of the two, as on Earth. The essential requirement is having a physical form with some degree of fragility that that implies, and a limited lifetime, like life here. Ephemeral, amorphous beings floating in the circulating cloud belts of gas giants would not be affected by many of the considerations here. They need a separate discussion, as do even more exotic proposed forms of life. We also make the assumption that, as physical life evolves to more complex forms, those higher forms will generally require a narrower range of favorable conditions than their progenitors. On Earth single cell life forms thrive in a far wider range of conditions of temperature, pressure, chemical activity and energy source than do mammals. The discussion is generally referring to this point when it makes a distinction between single cell life and advanced, intelligent forms.

Nor are we concerned here with the details of how life first came to be on Earth, which is also a different discussion. The fact that it has happened here is sufficient proof that it can happen. The processes behind life elsewhere may be identical, but more likely have some notable differences. Either case fits the discussion here.

The discussion proceeds from the largest scale to the smallest, in that order, so we start with cosmology, astrophysics, and the Universe as a whole. The first facts we need about it concern the evolution of the Universe itself, the stars and galaxies within it, and the origins of the elements, but must also draw on some newer facts from microbiology. There is much recent progress in these fields that affects our central question.

Cosmology

The age of the Universe has been quite well established at 13.4 billion Earth years, as has the fact that the original elements present in any quantity were only Hydrogen (~75%), and Helium (~25%). The same chemical elements as found on the Earth and in the Sun have been identified throughout the Universe: Chemistry is the same everywhere in the Universe. After Helium the most abundant elements available for chemistry and biology are Oxygen, Carbon, Neon, Iron and Nitrogen. Except for Iron these are among the lightest and most easily made elements after Helium. However, Helium and Neon are inert noble gases that do not interact with any other elements: They have no chemistry nor biology, so do not contribute to this discussion. Is it any wonder then that all current life on Earth is made from compounds richest in Carbon, Nitrogen, Oxygen, and Hydrogen (CNOH)? Compounds made primarily from these four are found throughout the Universe, in some stars, in the remains of supernova, in molecular clouds in interstellar space, in meteorites that land on Earth, and in asteroid samples returned to Earth. Carbon biochemistry is the only known foundation for life on Earth. Carbon, uniquely, can form very long chains with itself which also include many other elements, giving it the most varied and extensive biochemistry of any element. Its versatility makes possible the ~100,000 proteins, and DNA itself, that all mammals, including humans, depend on for life. Many tens of millions more proteins not seen in life on Earth are possible, leaving a vast range available for other possible life forms elsewhere. Consequently we will assume that any life of the type we are considering must be based on CNOH biochemistry. (We specifically do not require, however, that such life forms be based on the precise form of biochemistry found on Earth, nor on its DNA, only that it be based on CNOH.) In that case, then, such life could not have begun in the young Universe, for lack of a sufficient abundance of CNO. In addition, many elements beyond those three, especially Silicon, are also required in abundance to make a rocky planet to host that life, and were also missing in the young Universe.

Those necessary elements beyond Helium were built up gradually thru succeeding generations of stars that evolved as the Universe aged from its infancy to today. Nuclear fusion using Hydrogen in those stars steadily made Carbon and Oxygen in quantity, as well as the heavier elements needed to form rocky planets. Current thinking is that the early stars that formed in the Universe were larger, on average, than the average size we see today. Stars of such sizes are known to end their lives in spectacular explosions (supernova) that violently disperse their contents far and wide. The late stage of their lives and that final explosion produce Nitrogen, and some heavier elements also used by life on Earth, in good quatities. Gravity gradually draws those elements together again with interstellar Hydrogen and Helium into cold (molecular) clouds that condense and heat up to form new stars, surrounded by planets formed from the same materials, all now containing a growing proportion of heavier elements. (referred to as "metals" by astronomers.) In addition it has recently been determined that some elements which cannot be created in quantity by fusion processes in the stars are created instantaneously and abundantly in the life-ending explosions of these stars, or by other recently elucidated and understood stellar events. These too, then, are made available for inclusion in the next generation of stars and their planets. It is also known from some examples today that stars of the likely sizes of the early stars in the Universe live much shorter lives, generally tens or hundreds of millions rather than billions of years as for most of today's stars. The early stars were hotter and denser inside due to their greater sizes and the compressive force of gravity. Their fusion proceeded more rapidly, until the fuel ran out and the explosion occured. A greater number of generations fits in any particular span of time, building a supply of the heavier elements more rapidly in the past than can occur in today's Universe.

Clearly any planets circling the earliest stars could only have been gas giants, with primarily Hydrogen and Helium available for their formation, and only trace amounts of the heavier elements, if any at all. Consequently no life of the kind we are talking about, requiring a rocky planet, could come about at that time, for lack of the necessary elements. Lives of most of the potential host stars were far too short and their endings too violent for any significant amount of evolution to survive, even had an occasional rocky planet and beginning life managed to overcome the odds and develop. We will return to this point a bit later. The overall conclusion we draw in this section is that rocky planets with a solid surface, possibly oceans, a suitable abundance and variety of elements, minerals and rocks, and a hospitable place for single cell life to take hold only became sufficiently common many billions of years into the life of our Universe.

Recent results in Cosmology strongly suggest that the matter in the Universe (including its Dark Matter), when viewed on a scale larger than galactic superclusters, forms something somewhat like a foam of air bubbles. These imaginary bubbles are delineated by immense filaments and walls of tens of thousands of galaxies or more. The central volume of these imaginary bubbles is totally devoid of any observable matter, and likely of any matter at all. All of the Universe's visible and dark matter is concentrated in the filaments, walls and their junctions. Having no matter, not even any plasma, seems to make the empty interior of the imaginary bubbles an ideal place for evolution to flourish on any recent stellar system ejected into such a void. However, the gravity of the nearest matter, with none within the bubble, will tend to pull such a system back into a populated region. A very few may overcome such a fate and still be wandering in such voids, though likely not many. Force sufficient to eject a star strongly enough from its host galaxy to prevent recapture stands a good chance of disrupting or destroying any planetary orbits it has, with a consequent effect on any developing life. Although a place of very quiet surroundings conducive to a long fruitful evolutionary process, the voids are unfortunately not a likely place to find intelligent life. The very thing that gives rise to life, gravitating matter, is, at the same time, the source of many of the hazards to its existence and evolution. Successful intelligent life is a rare, fragile, and delicate balance between random conflicting forces.

Astronomy

The average stars in galaxies that we see near* us today are many generations beyond the first stars. They undergo well-understood life-processes that make an orderly evolutionary plot, referred to as the "main sequence" stars by astronomers. They have been confirmed to have a substantial content of elements up to Iron, and a lifetime in the range of 10 billion years or more. Many close enough to test have been found to have rocky planets, like our Sun, which lies on the main sequence. The Earth is known to be roughly 4.8 billion years old, with the Sun and its system of orbiting objects only slightly older. So the Solar system was formed at approximately the 8+ billion year age of the Universe. The Sun is an average star of its type among 10 billion or more others in our large but otherwise average galaxy, which itself is among hundreds of billions of others visible in the Universe. It seems that by approximately the 7-8 billion year age of the Universe conditions must have begun to be ripe for single cell life to be blooming on planets everywhere. Now, many billions of years later, evolution should certainly have made many advanced life forms there, including many advanced ones that we might have heard from. Where are they?

The position of the Sun in our galaxy has been an important factor in sustaining the evolution of life on Earth. Our Sun lies in the Milky Way galaxy, which can be a rough and tumble place, like any galaxy. Their centers, most often created and maintained by supermassive black holes, are a maelstrom of activity filled with intense lethal radiation from the tightly packed stars, from matter spiraling into the black hole, from the birth, death and jostling interactions of those stars, and so on. An unlikely place for fragile life forms to survive and flourish, even if their host planet does, which seems unlikely in itself. Whatever single cell life might arise there appears to have a slim-to-none chance of evolving very far, from what we know about the physical, chemical, and biological effects of cosmic rays, gamma rays, ultraviolet radiation, and intense stellar winds, not to mention frequent gravitational disturbances. However, paradoxically, and not surprisingly, the heavier elements necessary to make rocky planets are most abundant in the centers of galaxies, and in their stars. The further out a star forms in a galaxy the lower its concentration of heavy elements, and the smaller and fewer its rocky planets. So we must go a ways out from the centers of galaxies to find a balance between tranquility and enough heavy elements. The outermost fringes won't do, at least not yet. As stars age and generations succeed one another, even the fringes will acquire enough heavy elements to host many rocky planets, but that point is still far in the future.

Many galaxies, possibly most, and especially the larger ones, contain clear evidence of earlier collisions and/or mergers. Some close enough are clearly visible as being in a collision in process at the time we are viewing them. Galaxies are mostly empty space, so large objects in colliding galaxies rarely directly collide themselves. However their gas and dust interacts and heats up, often stimulating a burst of new star formation and thus higher levels of dangerous radiation and more supernovas. Changing gravitational forces during the collision can disrupt or destroy orbits, with severe or fatal consequences for fragile advanced life forms. Near misses can even have similar effects, clearly seen in photographs by the Hubble telescope and others. Luckily for us the Sun lies roughly 2/3's of the way out from the center of the Milky Way, in a relatively quiet location, at least of late. It has recently been calculated that a supernova can spew enough lethal radiation into its entire neighborhood to kill all exposed life of the type found on the Earth. We've been lucky enough to avoid that fate on the Earth, at least in the last billion years or so.

The position of our galaxy within its larger region may have also played a positive role in enabling successful evolution of life on Earth. It's beginning to appear that the Milky Way borders a large empty region of space, possibly a mini-bubble among the bubbles. If so that might account for the apparently quieter past of the Milky Way, with fewer galactic collisions, near-collisions and mergers, than for others like it. That possibility in turn suggests that such galaxies, bordering large empty spaces in the Universe, are the more probable places to find intelligent life. Such a condition is likely to be somewhat transient though, since the galaxies themselves are in constant motion relative to one another. Much is yet to be learned here.

Stars don't stay put within galaxies. Those passing close enough (or their system of orbiting bodies) can disrupt planetary orbits sufficiently to extinguish life on board, resulting in planets wandering in the galaxy freed from their original host star, and lacking its life-giving warmth. Some wanderers (sometimes called rogues) have actually been identified, and some even appear to have been captured by a new host star. The advent of supercomputers is making possible accurate calculations of the gravitationally coupled motions of multiple bodies orbiting a star that were never possible before. Many of these calculations are indicating the relatively fragile gravitational balance of a system of many planets orbiting a star. Surprisingly small changes, previously only suspected, can eject a planet entirely, or alter its orbit in a major way, sure death for life forms aboard. Yet another benefit of living in a quieter, less dense neighborhood of the galaxy. Altogether it appears that only the less populous reaches of galaxies, whether elliptical or spiral, and the small fraction of their stars and planets found there, are most likely to have sufficient long-term tranquility to allow evolution to advanced, intelligent life from single cells. Stars which get ejected from galaxies into intergalactic space (estimated to be 10% of all stars) surprisingly probably do not have as favorable an environment. Intergalactic space generally appears filled with an energetic plasma and a correspondingly strong background of X-rays. A planetary magnetic field only protects against charged particles, including some cosmic rays, but not X-rays. X-ray resistant single cells might eventually develop and even evolve to multicelluar forms, over a long period. Evolution would need to develop X-ray protection for each new type of molecular bond needed as cells and organisms increase in complexity, thus proceeding at a much slower pace. Bursts of higher X-ray intensity could set evolution back to resume from an earlier point, causing further delays in evolution..

We note that lying in the less populous section of a galaxy confers only minor protection against galactic collision disruptions. The collision trajectories of galaxies appear today to be essentially random, so any part of another galaxy can sweep thru or near our location. If it's the center of another galaxy that passes thru our region, survival of advanced life here becomes unlikely. If the low density sections of another galaxy pass thru or near, disruptions are less likely than from its center, but still more likely than without any collision or close encounter. Far better for life and evolution, of course, is no such disruption at all. The record shows that few galaxies, if any, have been so lucky. The bigger the galaxy the more collisions, mergers, and encounters appear in its record. Our galaxy, the Milky Way, does show a lower level of previous encounters than many most similar to it. Our existence here today may be due to a rare lucky combination of events and non-events that is not often repeated elsewhere in the Universe.

The last 100 years of astronomy has totally changed our view of the Universe. From a staid and static place with isolated galaxies spinning majestically in place, while their stars evolve quietly down the main sequence, to a place of incredible activity, variety, interaction, and sporadic violence. As we learn more thru more powerful telescopes we find that practically anything we can imagine happening does, and is, somewhere. Unfortunately, much of this interaction is hazardous to life, and especially to evolution. As long as natural physical processes have the upper hand, biological processes and successful evolution to intelligent life are going to be relegated, on average, to the quietest out-of-the way corners of the galaxies and the Universe. It's no accident that that's where Earth is.

Planetary Science

In our own solar system it's becoming apparent that it takes more than just a rocky planet in the habitable zone to host life long term. Only 1/3 of such planets have succeeded. Venus might have, for a short while after its formation, but, if so, lost it early. The possible cause is still being researched: (1) Too much volcanism, (2) Possibly a major collision, (3) Slightly too close to the Sun, (4) Maybe others, or combinations of all of these. The resulting run-away greenhouse effect and thickening of its atmosphere, with scorching surface temperatures, crushing pressures, and the loss of all water, effectively ended all possibility of the evolution of higher life forms, and likely even ended single-cell life, had there been any. On Mars the situation is less certain, especially since it still has some water. Conditions are the opposite from Venus: An atmosphere too thin (~0.1% of Earth's) rather than too thick, due mostly to its lower gravity. Small amounts of single-cell life might be possible, but development into robust complex life forms is almost certainly ruled out by the limitations that imposes. A second factor making any life unlikely is Mars' lack of a substantial magnetic field during most of its later existence. Such a field protects sensitive life forms from the constantly out-flowing solar wind of energetic charged particles irradiating everything in its path. Although Venus also lacks a magnetic field its dense atmosphere provides some degree of protection against the solar wind for possible life on its surface. However its closer orbit to the Sun increases the intensity of what it receives, making this hazard yet another strike against the possiblity of any advanced life there. Mars' greater distance from the Sun does reduce the intensity there, but the lack of either an atmospheric or magnetic shield against the particles strongly suggests a lack of any history of life as well. Ultra-violet light from the sun and cosmic rays constantly bombarding Mars' surface would likely eradicate any beginning life forms struggling in its thin atmosphere, and not already taken out by the solar wind.

We are rapidly gaining an improved picture of the formation and stabilization of the solar system. Simulations and data suggest that at least some of the planets were not formed where they are presently orbiting, Jupiter in particular. The fact that it has settled into its present position for a while has been fortunate for us. Its immense gravity seems to help to stabilize the current configuration of planets and thus Earth's orbit. Its gravity may help protect the Earth, Mars and Venus from objects arriving from outside Neptune's orbit. It may even have conferred some stabilizing benefit during galactic collisions, near-misses and mergers influencing the solar system gravitationally. Its gravity helps stabilize the asteroid belt, still a formidable hazard for Earth and its life. Thus far, our Solar system's particular configuration, of outer gas giants and inner rocky planets, has not been common among exoplanets detected. It may well have been a stabilizing factor helping evolution on Earth to run long enough for humans to emerge, by comparison to other planetary systems.

There are several other features of the Earth as a rocky planet that are beginning to appear uncommon, and may be important for evolution to intelligent life. Some will be needed in the discussion of Evolutionary Biology and Genetics. The first is that the Earth apparently has had a mixed surface of water and rocky land for most of its existence. Some moons in the outer solar system contain internal water and a rocky surface, or an entire surface ocean covered in deeply in ice, with the rocky inner surface beneath, but the particular makeup on Earth has not been found on any other body within the Solar system. Thus far data from the few Earth-like exoplanets are not sufficiently detailed to bear on this point. This particular configuration may be related to the second notable point, which is the large size of Earth's moon relative to the body it orbits. It's unique in the solar system, with no data yet from exoplanets either. Some have suggested that the impact with Earth that created the moon may also have created a depression on Earth's surface that has evolved into the ocean basins today. The importance of these two points lies in the existence of substantial tides and inter-tidal zones on Earth, dependent on both facts. Substantial tides exist on a planet with two features like this. This point will contribute to the section on Evolutionary Biology. A closely related feature of Earth is its tectonic plates, which are also thought to require partial surface coverage by oceans that lubricate the sliding of the plates over one another. Such plates of lighter rocky material over heavier rock are also unique to Earth within the Solar system, although the larger moons of some of the outer planet may have tectonic plates of ices of various kinds. Again exoplanet data is not yet available. Tectonic plate motion also may have a strong role to play in Evolutionary Biology as will be discussed there. And finally, Earth's rotational tilt relative to its orbit produces its seasons, which are important factors in two ways. The first is that the variations in temperature with latitude that the tilt produces causes circulation and mixing of the atmosphere between the poles and the equator. This mixing, in turn, evens temperatures and smooths the distribution of solar heating over the surface of the planet. Most of the surface then becomes habitable for higher forms of life. Without this mixing the extremes of surface temperatures between the equator and the poles are likely to confine more advanced organisms to narrow belts, or prevent their evolution altogether. This effect of horizontal wind belts that don't mix the latitudes much can be clearly seen on photographs of Jupiter, which has negligible tilt. The second factor is the possible effect that the seasons may have as an evolutionary "push" factor. We'll take up that point later, only noting here the planetary science facts. The Earth and Mars both have axial tilt in the 20-30 degree range, making for uniform seasons on both. Venus rotates slowly upside down, while Uranus rotates on its side. The other large bodies in the solar system, including the other planets (excepting Saturn and Neptune) and the large moons that do rotate, have negligible axial inclination (< 5 degrees). No exoplanet data yet. These solar system data indicate that an ideal tilt, not too little and not too much, may also be somewhat uncommon. There is no consensus yet on the cause of either Earth's or Mars' inclination. The resolution of that point could make the incidence of this feature even more uncommon as well.

Paleontology and Geology

There are two significant facts from this area of research that bear on our question. The first is that it is beginning to appear that the earliest forms of life first appeared on Earth roughly 3.5 - 4 billion years ago, based on both fossil and geochemical evidence from rocks in that age range, and from sediment cores. These life forms were presumably single cell forms, with the ability to reproduce under the conditions which prevailed on the Earth at that time. Those conditions were totally unlike those today, as were those presumed earliest life forms. The second vital fact is that there have clearly been numerous setbacks during evolution on Earth, periods when a majority of existing species became extinct, and after which new species evolved to succeed them. The causes are still being investigated, but a few of the more major ones have been identified. One, the most severe of all, called the Permian-Triassic extinction event, is closely associated with a gigantic outflow of lava over much of present-day Siberia, most likely its cause. Another, that ended the dinosaurs, was almost certainly caused by a very large meteor impact, which has become well known. A third and possibly fourth are associated with evidence that the Earth's surface, including the oceans, became at least partially frozen during those periods. Volcanism and meteor impacts have been suggested as causes of this freezing, as has a possible imbalance among life forms that removed all greenhouse gasses from the atmosphere. It may well be that at least two of these causes together were required to create a partially frozen Earth. It is also beginning to appear that regular oscillation on Earth's surface between greenhouse and icehouse conditions may have been ongoing since the beginning of life here 4 billion years ago. These oscillations are on a much, much longer time scale than that of the recent ice ages, and also appear to cause some substantial evolutionary setbacks (extinctions). It has been determined that the relatively recent ice age cycles depend on variations in solar radiation received at Earth, caused by well-understood variation in its distance from the Sun and the known slow wobble of its rotational axis. The other oscillations occur on a much longer time scale and appear to be due to instabilities in the amount of atmospheric CO2, at least partially caused by variations in species, i.e. by evolution itself, and possibly aided by volcanism and tectonic plate activity. These results further illustrate the fragility of evolution on an aqueous planet in a habitable zone. The temperature range between a frozen planet and one so hot that all surface water is lost is narrow, and depends strongly on conditions on the planet, as well as its distance from its star.

The key point here is that evolution of life on Earth has not been a simple linear process. It's been a stumbling, halting, variable path from the first microbes to humanity, with many, many disruptions, setbacks, and new directions along the way. Some of the causes seem almost certain to be mainly physical ones, i.e. due the Earth itself (e.g. volcanism), or forces originating outside it (meteor impacts, radiation bursts, gravitational deflections), while others are internal to the evolutionary process itself, due to its dead ends, detours, incompatible mixtures of species, and other such occurrences. We can only assume that evolution of life here is typical of the process in general, not a special case. This then leads to the conclusion that the length of time it takes to evolve intelligent life is likely to be highly variable, due to the random nature of the disruptions we see here. Some fortunate instances will produce intelligent life in less than the ~4 billion years it has taken here, and others longer. If our ~4 billion years represents the most likely time (the median, not the average), then the majority of cases will take longer, out to periods of 6 or 8 billion years and more, due to statistical laws that govern the accumulation of such infrequent random events. And of course some of the events may be severe enough to end life and evolution totally. It's also worth pointing out the apparent similarity of early Earth and Venus in having greater volcanism in early life. On Venus, as noted before, volcanism may have been strong enough to prevent or end all life there and create the runaway greenhouse atmosphere and loss of all surface water. Apparently it was enough less on Earth that it only disrupted and delayed evolution, but did not end it. The otherwise close similarity of the early life of the two planets suggests a rather fine line on a young rocky planet between success and failure in producing and sustaining life. Note that larger amounts of volcanism early in life, and less later, is to be expected for most rocky planets. A new planet formed close to its star has substantial interior heat that can only be reduced by transmitting it to the surface and thence into space. Volcanism is a natural part of that cooling process. Tectonic plate activity is too, when present, which may have been what helped Earth to escape Venus' fate. Large enough meteor impacts can also partially reheat a planet's interior, possibly producing later increases in volcanism that disrupt evolution again, in addition to their immediate consequences.

Evolutionary Biology and Genetics

Evolution has been extensively studied from many angles. An important one for this discussion, perhaps less well known, is what we might call EES's, Environmental Evolutionary Stresses or Environmental Evolutionary Stimulants. The question of what pushes evolution forward has been much discussed and researched with no clear winner to date. One possibility favored by many genetics researchers is called "genetic drift", a gradual change in the genome of a species simply due to the continual combining of slightly differing parent genes into each child over a very long series of generations. Mutations and natural selection may or may not be additional factors in the process. EES is an additional, not necessarily competing, possibility. By EES we mean any environmental factor that may delay or reverse evolution, or possibly stimulate it. The first one to be considered is the large ocean tides on Earth whose role may have been one of a stimulant. Without the tides complex organisms crawling out of an ocean to escape crowding or competitors would immediately encounter a hostile and unforgiving land environment and likely death. A large tidal zone is an intermediate environment that very likely allowed such early emigrants on Earth to benefit from a partially different environment without being killed by it. Eventual adaptation to tidal waters then allowed further temporary escapes to dry land for longer and longer periods, until there were the beginnings of amphibians able to function well in both environments, and eventually the totally land-adapted reptiles and others. An open question is whether the lack of tides would completely prevent the development of land animals. Lack of substantial tidal zones would likely significantly delay the evolution of land animals at the least. As noted elsewhere intelligent land animals on any planet with partially clear skies are likely to send signals space-ward far sooner than any others. Humans may well be one of the earliest intelligent beings to evolve in the Universe due to having a large moon close to Earth for a long time, among other benefits.

A second possible stimulant is tectonic plates, which have also been active on the Earth for at least a substantial part of its existence. An unanswered question in evolutionary biology is what would happen to a static ecosystem over hundreds of millions or billions of years? Would animal species continue to evolve toward greater intelligence, or simply stagnate at some level? It seems likely that species need a kick of sorts, provided by an environmental change, to increase the problem-solving ability that greater intelligence provides, and to find ways to overcome stresses due to the environment. The leading form of that on Earth has been the tectonic plates, which have isolated and combined various ecosystems repeatedly. In particular the Central America land bridge which formed between North and South America set up competition between two previously isolated ecosystems with the result that many original South American species were out-competed and eliminated in favor of new species formed from both. As with the moon it's possible that tectonic plate motion has stimulated a more rapid evolution toward intelligence on Earth than would have occurred otherwise, again helping Earthlings to be the first ones within their region of the Universe to begin looking for others. It is worth noting that no purely marine species on Earth has evolved even close to human intelligence in 4 billion years. The most intelligent forms of life in the seas are mammals that have returned there from a previous land existence. The sea is a relatively static environment compared to the land, lending support to the hypothesis that environmental prods, stimulants are needed to produce intelligent species. It's possible that they might occur in the oceans on some other planet. They haven't thus far on the Earth.

Marine life became highly complex on Earth long before any land animals or plants existed. In most places life almost certainly flourishes first in wet environments. On planets that are all ocean, without any land surface, intelligence could only first develop in marine life. In such cases interest in interstellar communication may be much slower to develop than here. The night sky and stars will not be easily seen by most of that intelligent life. Interest in and successful technical development of radio, microwave and laser technology will be slow to develop within a salt water environment. It's very likely that such beings would be sending us signals much later in their evolutionary process than land-based beings would. Intelligent beings on a planet totally and permanently wrapped in clouds would likely also be similarly slow to send signals space-ward. Both all-ocean and all-cloud planets and moons occur in our own solar system, suggesting that they may be fairly common everywhere. These facts further reduce the number of possible communication partners that we might find today.

Time and Distance

The last points we have to consider are not hazards to life at all, but just brutally simple facts that limit whom we can reach or see. The finite speed of light means that every laser or microwave signal we receive from space left its origin sometime before we received it. The further away an object is the farther into its past we are seeing it when receiving its light or microwaves. We have shown that intelligent life, on average, is unlikely to have developed in the Universe very much earlier than ourselves. Those sending signals our way would have to have reached at least our level of technology at the time the signals were sent, not when we are receiving them. Thus the farther out into the Universe we extend our search, the farther we are looking back in time, and the less and less likely we are to find signals from intelligent life. We might take our Local Supercluster of ~10,000 galaxies, approximately 100 million light years in diameter as a likely nearest search target. This means limiting our search to beings that reached our current technology level no more than 100 million years ahead of us, when dinosaurs were dominating the Earth. Our visible Universe is approximately 90 BILLION light years in diameter, so that usable search volume represents only about one billionth of the volume of our visible Universe, a very tiny part indeed. Anybody we receive from within that distance had some set of conditions more favorable than Earth. An earlier start on evolution perhaps, or a shorter run to intelligent life, and also had the good luck to avoid any of the fatal hazards mentioned here, at least up to the point that their signals were sent toward us. A second factor further complicating any search for signals comes from simple geometry. The intensity of any signal broadcast in all directions drops off as the square of the distance from the source. Its initial intensity gets spread thinner and thinner over a larger and larger area the farther and farther it goes. Only broadcast signals from a few light years away stand any chance of retaining sufficient intensity to be detected here with today's technology. Broadcasting a signal does have the advantage that it can be sent continuously and covers all directions automatically, but has this disadvantage of requiring huge amounts of power. A directed, focused beam will reach much further of course, with the same power. The problem then becomes one of looking in the direction from which it is coming at the precise time it arrives here. If beam technology reaches us from 10 or 100 light years distant our reply takes that long to reach the sender, if our focus and directional control is as good as theirs. The round trip messaging would take up to 200 years, or more, just to open the line of communication, assuming that they are still looking our way when our reply signal reaches them. Civilizations further away than that will take longer of course. Hopefully technological progress both here and there will eventually extend those distances greatly, and maybe even shorten the communication time, until some useful contact finally is made.

A further minor complication of signaling is the fact that interstellar dust and gas are clumpy rather than uniform throughout galaxies. Some places are obscured from one another by the denser clumps. The stars and their planets move within the galaxy, as does the dust and gas, making that situation change over long timescales. It's one more factor that can interfere with signals we might receive, reducing their possible number or strength.

These last three unfortunate physical facts are far and away the greatest limitation on whom we may hear from. The conclusions above limit where within that communicable distance likely sources lie and how abundant they are likely to be: These are (1) the sparser sections of certain nearest galaxies, including our own, and (2) on only a few very specially favored rocky planets orbiting stars there, yet to be identified, and (3) not likely to be very abundant at all, a very good chance of none. Nonetheless many, many instances of intelligent life substantially further out in the Universe are almost certain by now. Their great distance from us means that there hasn't been enough time for them to signal us yet, or we them. Over the long range technological progress at both ends may overcome that limitation.

It's important to note the randomness of both the hazards and stimuli that affect evolutionary success. We have deduced what these are from the sole example we have of life on Earth, but, due to their very random natures, the exact combination affecting any other evolutionary system is almost certain to be different. Any one or more of the hazards and stimuli noted above may be missing in any particular case. It's the fact that so many are possible and the cumulative effect of those that actually occur in any particular case that gives rise to the rarity of intelligent life and the variability in time until it arises. The total effect, averaged over all cases, results in only widely scattered rare occurrences available to us today.

In conclusion we can say that the ongoing research and the tremendous growth of knowledge since the Fermi Paradox orignated have resolved it. As explained herein, it is now clear where they are likely to be, how relatively rare they are likely to be, and how difficult and infrequent successful communication with another civilization will be at this time. Nevertheless it's important to keep trying, for both the likely benefits, and possibly the safety of life on Earth.

It should be apparent that this is a dynamic, active subject that can change with new results, or new insights on existing results. As those occur they will be folded into the narrative here to keep it up to date with the most current thinking.

Critique

In this section we'll consider some things that might alter the conclusions above. The first of these possibilities is the fact that fluctuations on both sides of statistical averages occur, with declining frequency the further from the average they are. Thus there is a very small chance of some intelligent life form developing somewhere in the Universe considerably sooner than it did on Earth, and thus being many tens or hundreds of millions of years more advanced than we are at this time. That advanced state might include some way to circumvent the limitation of the speed of light for communication, and possibly even for travel. They might be able contact us within days, weeks, or months from anywhere in the Universe if they knew where we are and were interested in doing so. The first problem here would simply be finding us in the vastness of the Universe. The second is that there is likely very little motivation in such a society at large to seek out and contact very primitive life forms that are way behind their own evolutionary status. They don't represent competitors nor potential adversaries, nor something from which new useful things are likely to be learned. Such places would primarily be of scientific interest to such an advanced civilization, as in E.T. Such study would likely be non-intrusive, just as we study animal life on some isolated islands today. Speculation that they would like to eliminate us in order to use Earth for themselves seems very improbable. Any civilization able to defeat the limitation of the speed of light and travel here will very likely be advanced enough to secure any resources they need in other ways. And we would not be their competitors any more than monkeys are ours.

Intelligent life forms could occur elsewhere much earlier than us if their evolution occurred in large steps rather than the much smaller steps that normally separate species on Earth. These large steps might occur via a form of gene exchange or merger between widely differing successful advanced species on that planet. Gene exchange between organisms has been found to occur to a very limited extent on Earth, mainly among single cell life forms, and might extend to advanced forms elsewhere. Its primary effect, if it happens anywhere at all, would be to create a somewhat higher number of more advanced life forms than ourselves at this point in time.

Binary star systems hold some interesting possibilities that have yet to be explored. A sizable fraction of the brighter stars in our galaxy are part of a binary system, possibly up to 50%. Having two suns instead of one suggests new possibilities that might help life to flourish, as well as some additional possible hazards. Little is known or theorized about such planetary systems. The first was discovered in early 2024 but will take years to fully understand due to the size of some of the orbits. Hopefully others will be found in the meantime and furnish additional insight.

In a similar vein moons, especially of gas giants, may be more common hosts to life than primary planets, especially since many gas giant exoplanets seem to orbit close to their star. Life forms there might evolve to have rapidly varying dormant and active periods in sync with their orbit around the gas giant. Binary star systems and life on moons might increase the total number of occurrences in the Universe by a factor of 5 or even 10, but probably not more. Not enough to provide us near neighbors in all likelihood.

Another factor to fold into the discussion is the inter-species competition characteristic of evolution. Species replace other, less well adapted species, either by direct, violent means or by out-competing the others for resources. This competitiveness extends to intra-species competition as well, as we see in many other species around us and in humans themselves. This observation has led to speculation that some highly advanced life forms elsewhere may have ended by eliminating themselves in wars, or by poisoning their planet, or through an induced climate disaster, leaving even fewer civilizations for us to communicate with. A nuclear, chemical, or biological holocaust could even put an end to all life on the planet, leaving no life to evolve again.

Since human space flight has developed some animal species in their dormant state (Tardigrades, water bears) have been found to survive the radiation and ionized particle hazards of space discussed above, up to 1000 times the levels that are fatal to other animals. This capability appears to be largly due to a much more robust DNA repair system. Under the stimulation of high ambient radiation levels evolution might take such a turn, reducing or eliminating that hazard from the discussion above, thus allowing for a greater number of successful evolutionary locations. A magnetic field and possibly ozone layer would no longer be necessary. The chemical, physical (collisions, gravity issues, etc.) and evolutionary conflict hazards still remain, but here too the time to advanced intelligent life might be shorter than it has been on Earth.

Radiation hardened evolution increases the number of successful locations slightly while competitive extinction decreases it slightly, so in total the conclusion may not change much due to these two effects. A statistical fluctuation creating a much earlier civilization anywhere with super advanced technology today likely still leaves us in the dark, and with the same conclusion as to hearing from others more like ourselves. We need to know more about binary systems and moons to make some guess as to their impact, but their numbers indicate only a small increase is possible from either or both of these.

* Galaxies we see near us are also those near ours in age, and the ones from which we can obtain the most information. The more distant a galaxy is the earlier in its history are we viewing it today. The JWST telescope, and others to come, can sense light from the earliest galaxies in our Universe, which also makes them the farthest away. The weakness of that light limits the extent of detail that can be deciphered from it, compared to those nearby.