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According to Aldiss (op. cit.) and others, belief that intelligent life is commonplace in the Universe was taken for granted by scholars and scientists until well into the nineteenth century. In the Victorian era, when the existence of extraterrestrial beings was widely accepted, the Rev. Dr. Thomas Dick encouraged the notion that intelligent life, divinely created, was common in the universe.
Space travel since the late 1950s reignited the debate, which even now attracts discussion by serious, professional scientists. Intelligent amateurs with computers can also get into the act, by participating in the online SETI project, lending otherwise unused computer cycles to the processing of radio noise from the universe.
Faithies have come down on both sides of the question from the beginning. Many of them believe that intelligent life must be common in the universe, simply because they can't believe that their "god" would have "created" the whole universe just as a backdrop to our one, small planet. Others believe that we are alone in the universe, simply because that increases our importance as the unique product of the "creator's" will. Intelligent people are also split on the existence of life elsewhere in the universe.
As always, when we want to know what's really going on in the real universe, we must turn to science for answers. Since World War II, our understanding of the universe and our place in it has undergone dramatic improvement. From wondering whether "galactic nebulae" were inside our galaxy or outside, we've learned a lot about the nature and numbers of galaxies, their distribution, ages, and evolution. From wondering what process keeps the Sun burning, we've arrived at an understanding of the types, origins, and evolution of stars and populations of stars. We've gone from a basic understanding of the laws of genetics to a detailed understanding of its biochemistry, and an amazing depth of knowledge about the origin and evolution of life on Earth. Let's look at all this in its proper order and see what it tells us about the probability of other intelligent life in our galaxy.
It is now believed that new universes are born constantly from the quantum foam of existing universes. Most of these new universes cease to exist immediately, because their physical laws don't permit them to persist. Others, which we might call broken or pocket universes, don't undergo the expansion which leads to matter, galaxies, and stars. Only universes similar to our own can have living things occupying planets under friendly stars.
Our Milky Way is a barred spiral galaxy about 120,000 light-years in diameter. Light, the fastest thing in the universe, would take 120,000 years to cross its disk. For comparison, the oldest civilizations on this planet got their starts about 6,000 years ago, and our species first migrated out of Africa about 100,000 years ago.
We know that the Milky Way contains at least one intelligent species with a technological civilization, but most galaxies are not so hospitable as our own. Deep-sky surveys have revealed a background of dim, diffuse galaxies, in addition to the irregular, tiny dwarf galaxies which we already knew made up most of the readily visible galactic population. Neither the diffuse galaxies nor the dwarf galaxies are likely to give rise to living worlds.
Life requires a variety of elements. To name but a few, there's an iron atom at the center of every hemoglobin molecule, and a manganese atom in every molecule of chlorophyll. Organic molecules are built of carbon atoms, an element uniquely versatile in its combinations; silicon, a substitute often suggested, isn't even half as versatile. Calcium is needed for bones and cartilage, phosphorus for nerves, sodium and chlorine to maintain the water balance in our bodies, et endless cetera. Hardly a single non-radioactive element is not used by life on Earth in large quantities for many functions.
But only hydrogen and some helium were created in the Big Bang. More helium, and every other element up through iron, are created in the nuclear reactors we call stars. The lighter elements spread through the universe on the stellar winds that pour out from stars during their life times; the heavier elements are released only in novas, when moderately heavy stars blow off their outer layers and become white dwarves.
Still more massive stars end their lives by blowing themselves apart. These supernova explosions are the forges in which the elements heavier than iron are made and scattered to space. Elements heavier than iron, you see, don't form spontaneously; it requires extra energy to cram all their protons and neutrons into their nucleus. The supernova provides that energy.
The diffuse galaxies don't concentrate enough mass to form big stars; that's why they're so dim. With no large stars, these galaxies remain forever metal-poor (in astronomy, a "metal" is an element other than hydrogen and helium), without the materials to form rocky planets like the Earth, and the living things on it.
Dwarf galaxies are brighter, indicating a greater concentration of mass, but their low total mass and irregular shape make them poor prospects for life. They don't have the material for generation after generation of star, each inheriting the elements from earlier generations, nor does their shape concentrate future star material where it can be seeded by earlier stars, and collapsed by the shock waves of nova and supernova.
On the other end of the scale, really large elliptical galaxies, formed by the collision of two or more spirals, are poor bets because of their chaotic structure and high radiation. Large galaxies tend to have active centers, which is a polite way of saying the central portion is occupied by a huge black hole, or exploding, or doing both at once, spraying radiation throughout the galaxy. We mostly detect this radiation, millions or billions of light years away that we are, as vigorous radio sources; to a living thing within such a galaxy, radio would be the least of its problems.
For life, then, we look to a moderate-sized spiral like our own Milky Way, with regular structure, and enough mass to generate the whole range of stellar types and sizes, through enough generations to make all the elements plentiful. In our own Local Group of galaxies, there are only a few such galaxies; most are dwarves. In fact, the Milky Way is massive enough that there is a black hole making its center an uncomfortable neighborhood; if it were a little more massive, we might not be here to contemplate the issue.
Even in a quiet, stable galaxy like ours, every location is not equally desirable. The central bulge, and the globular clusters in the galactic halo, are Population II stars, that is, early stars poor in metals. This is a result of the high concentration of stars in those places, whose radiation expelled the gases that would otherwise have become later generations of stars.
The galactic halo otherwise is very diffuse, with stars enormously far apart compared with the disk. These stars are, on the average, smaller than the stars of the disk, and poor in metals because of the distances between them.
In the disk itself we find all kinds of stars, and the spiral structure funnels the product of each generation to the next. The disk is rich in star-forming regions, and the stars are close together. Of course, even in the disk it's best to be in the central plane, with stars all around you to provide material for your solar system. In chess, it's advisable to keep your knights in the center, where all their next moves are on the board, instead of at the edge where their moves are truncated; similarly a supernova explosion at the edge of the disk will be wasted into open space. It's interesting to note that our own solar system is only 20 light years from the central plane of our galaxy.
As you might have guessed by now, not all stars are alike, and most of them are poor candidates for hosting a living world. The brighter the star, the faster it burns through its material, and the shorter its life span. In addition, really bright stars tend to be unstable; flare stars, recurring novas, and variable stars would regularly blast their planets clean with radiation. But such stars are unlikely to have planets in the first place, because their radiation pushes matter away from them when they begin to burn.
Dimmer stars last longer and don't sweep their vicinities clear of matter, but the most common kinds are so dim that the habitable zone, where water is a liquid, is very narrow. This makes the odds of a planet orbiting within it very small.
The best candidates are stars of types F, G, and K, in descending order of mass and brightness. These stars live long enough for life to evolve on their planets, and are bright enough to have extensive habitable zones. Our own sun is a G star, and Venus, Earth, and Mars fall within its habitable zone.
Even our sun is not completely stable. There have been four occasions, between 750 million and 580 million years ago, when the oceans froze over and the whole planet was covered with ice for 10 million years. These episodes presented severe jolts to Earth's ecosystems, with temperature swings from near -58 degrees F. to 122 degrees F., and significant chemical changes to the ocean and atmosphere. The events are called "snowball Earth," and the last one occurred just before the Cambrian explosion, the dramatic rise of multicellular organisms in the fossil record. If advanced life was indeed jump-started by such a rare effect, it would underscore the difficulty of evolving animals even on the best of planets.
Another consideration is that our host star must be single. If the primordial cloud collapses into two or more stars, there will probably be no planets. The combined radiation will drive away the remnant matter once the stars begin to burn. The multiple gravitation fields will eventually slingshot solid objects out of the system, if any do form. It's sobering to note that a full half of the type G stars in our galaxy are members of a multiple system; Alpha Centauri, the nearest star, is a triple.
Even in our own solar system, the living Earth is unique. Though Venus, Earth, and Mars all formed within the Sun's habitable zone, only Earth is inhabited. Venus's atmosphere filled with carbon dioxide, and the surface temperature there is hot enough to melt lead. In addition, massive volcanic eruptions, 400 million years ago, on a scale never seen on Earth, have completely destroyed its surface. Venus, then, failed to maintain a stable atmosphere or geology.
The geology of Mars informs us that it once had running water, and possible microfossils have been found in material blasted off its surface by meteors. There is no sign of life today; Mars is one-third the size of Venus or Earth, and has only the faintest wisps of atmosphere or water vapor. Mars, then, failed to be massive enough for life.
Another reason mass is important is to keep the core liquid. Earth's rotating liquid iron core generates its magnetic field, which fends off radiation and charged particles from solar flares, as well as extra-solar particles ("cosmic rays"). Without it, the Earth's surface would be much more radioactive than it is. Earth is the only rocky planet in the solar system with a magnetic field.
Many factors contribute to Earth's long-term habitability besides mass, and we have no way of knowing which ones are critical, and how likely they are to occur in the right combination in another solar system.
The tilt of the planet's axis with respect to its orbit is critical. Earth tilts at 23 ½ degrees, and most life is found within the tropics. Even the human population, with clothes and fire and houses, falls off sharply more than 45 degrees north or south of the equator. The greater the tilt, the more drastic the seasons.
Earth's tectonic plates are essential to its long-term habitability, and no other planet in our solar system has them. Slag from our molten core rises to the surface, cools, moves across the surface, and then is consumed back into the interior in subduction zones. The lighter material, which we know as continents, remains on top. Thus plate tectonics builds landmass. It also enhances biotic diversity; most life exists even today in the shallow coastal oceans, and how much of that there is depends on the number and arrangement of continents as they come together, drift, and break apart. For land animals, too, the splitting of continents means the evolution of separate species in the different parts, while continental collisions lead to widespread interactions of species that had no access to each other before.
Subduction also balances the carbon dioxide in the atmosphere. The Sun is 30 percent brighter now than it was 4 billion years ago, and it's burning more brightly all the time; in about 5 billion years, the Sun will turn into a red giant burning thousands of times brighter. Excess carbon dioxide in the atmosphere leads to the creation of carbonate rocks, which are drawn into the Earth's interior in subduction zones. We live in that period of the Earth's history when the rate of the Sun's increase in brightness and the rate of carbon dioxide sequestration are approximately equal. Later on, the Earth's atmosphere may well go the way of Venus.
One of the most unlikely aspects of life on Earth is our Moon. The Moon was formed when a Mars-sized planetesimal (not Mars itself, but something its size) hit the Earth a glancing blow. The Moon is uniquely large; no other satellite in the solar system is anywhere near so large, compared with its primary. The Earth-Moon system's center of gravity is significantly offset from the center of the Earth, though still inside the Earth.
The Moon makes the tides, which stir the oceans out of stagnation, and churn the coastal margins of the continents, where most life has always lived. Its gravitation also stabilizes the tilt of the Earth, keeping it at a constant angle. Both of these effects contribute to the habitability of the Earth; if either is crucial, then life will be rare indeed, as Moons like ours must be.
The early history of the solar system is a catalogue of collisions, as planetesimals accreted to form the planets, which swept the inner solar system clean by pulling debris into them, or ejecting it through gravitational slingshot effects. Every body in the solar system is pockmarked with craters, even Venus and Earth, though atmospheric weathering and vulcanism have covered much. At least one mass extinction, 65 million years ago, is known to have been caused by a giant impact, and one or two more are possibilities. Another could wipe us out at any time.
Long term habitability, then, requires a fairly clean solar system without too many giant impacts. Giant Jupiter, largest and innermost of the gas giants (giant gaseous planets) of our solar system, has contributed greatly to our welfare by cleaning up the solar lanes with its gravity: forcing material into bands through gravitational resonance, drawing material to itself, and ejecting material from the solar system altogether. So having a giant planet nearby, but not so nearby as to perturb your orbit, is a good thing.
Abiogenesis is the self-organization of organic self-replicators, or the origin of life in non-living materials. Miller, in 1952, filled a flask with a "reducing atmosphere" (methane, mmonia, and hydrogen gas) over an "ocean" (water), and then subjected it to "lightning strikes" (electrical discharges). After a week, he analyzed the water and found glycine and alanine, two of the amino acids that are the building blocks of proteins. Subsequent experiments have produced a variety of amino acids and the building blocks of nucleotides.
This was very heartening, but more recent data suggests that the early atmosphere was not reducing, because Earth is too small to attract and hold the components. Instead, the first atmosphere was composed of volatile gases released from infalling planetesimals as they vaporized, plus volatiles outgassed from volcanoes. The main gases would have been water vapor, nitrogen, carbon monoxide, carbon dioxide, and hydrogen. The hydrogen would have escaped to space rather than accumulating. Any methane and ammonia produced would have been decomposed by the Sun's ultraviolet radiation. Organic molecules would be hard to form in this neutral or mildly reducing atmosphere.
While the chemists and geologists were realizing this problem, astronomers continued to probe the composition of gas and dust clouds in interstellar space. Over the years they found an astounding variety of organic materials there, including methane and ammonia. These form on the surface of dust grains, warmed by starlight but shielded from direct exposure. They would be carried to Earth on infalling meteors, comets, and dust grains. Indeed, this process continues today, though of course such materials are now consumed immediately by bacteria.
This theory helps explain why organic polymers in living beings uses only "left-handed" amino acids. Most methods of synthesizing them from monomers make equal numbers of "left-" and "right-handed" polymers. But the organic material found in meteorites (carbonaceous chondrites, a kind of meteorite, can contain up to 5% organic material) contains more left-handed than right-handed molecules. Mason (op. cit.) argues that left-handed molecules are a hundred trillion times more stable than right-handed forms, because of asymmetries in the weak force.
Eventually, then, we have oceans on the cooling Earth, built largely from the icy cometoids that continue to crash against the atmosphere even today (they were discovered from orbit, in photos where they appear black against the backdrop of the Earth). The water would have a "concentration" of about .0001% organic molecules, based on the time it takes to recycle the ocean's water through the deep-sea volcanic vents, which would destroy the molecules with 900-degree heat.
Two problems had to be overcome to join monomers (such as amino acids) into polymers (such as the proteins used by life). The low concentration of monomers was one, and the energy required to "push aside" the water molecules between the monomers was the other.
Catalysts assist in bonding by actively removing water molecules; but the catalysts used by modern organisms are proteins, which obviously didn't exist yet! Metal ions such as zinc, or clay minerals could have served as catalysts; not as efficient as proteins, but possibly good enough. The sheet-like structure of the clays would also concentrate the monomers.
In modern organisms, the machinery related to metabolism (proteins) and the machinery related to replication (DNA and RNA) are totally interdependent. How could two totally interdependent systems evolve separately?
RNA has dual functionality today — its sequential arrangement of monomers allows it to carry information (like DNA, but more prone to replication errors), while its single-strand arrangement allows it to fold into useful three-dimensional shapes (like proteins, but not as efficient).
RNA probably formed first, combining metabolism and replication functions in a single system, the "RNA World". The plausibility of this is supported by the fact that some RNA molecules can act as catalysts for chemical reactions involving organic molecules. However, RNA is very complex; it would be hard to build such a precise and complex molecule from scratch. Some sort of "protometabolism" (abiotic chemistry) must have developed first to provide the materials and energy needed. We don't yet know what preceded the "RNA World".
RNA replication made evolution (driven by natural selection) possible. Because of copying mistakes, many versions of the original molecule formed. Some variants replicated faster than others, or were more stable, and progressively crowded out less able molecules. The evolution of more advanced RNA requires some sort of protected environment, some kind of "protocell" enclosing a complex of cooperating RNA molecules (the forerunners of our grouped chromosomes) and amino acids in some sort of membrane. Experiments show that protenaceous microspheres, messy protein-like structures roughtly spherical in shape, form when amino acids are added to boiling water. They can be found now in pools around volcanic vents, and might have provided the membranes for the early protocells. More than one type of protocell probably developed, but only one prevailed, since all life on Earth is similar at the biochemical level.
As the RNA strands grew longer, replication errors became a problem. DNA is mutated RNA: a different sugar, and one different base, allows it to form a double-stranded structure. DNA consists of two complementary strands carrying identical information; one acts as a template, which the second double checks. This error-suppression mechanism allows storage of genetic sequences up to millions of nucleotides long, long enough for highly sophisticated protocells, which became the first true cells.
These cells were prokaryotes (bacteria and archaea). They had no nucleus in which their DNA stayed, nor any specialized organelles such as the mitochondria that provide animal cells with their energy, or the chloroplasts in which a plant's photosynthesis occurs. Some of them, however, discovered the acetylcholine energy cycle, while others learned to use the energy absorbed by xanthophyll pigments originally built as protection from the sun. They had no specialized plasmids in their bodies in which these processes occurred; they occurred throughout their cells.
Photosynthesis is the combination of carbon dioxide and water and sunlight into glucose and oxygen. The glucose is food for the cell; the oxygen is waste. Sunlight, carbon dioxide and water are plentiful; these organisms now have a great advantage over others that drift around, devouring chance-encountered molecules.
Later, chemical mutation formed chlorophyll from xanthophylls. Chlorophyll absorbs energy in the green part of the spectrum, where the peak output from the Sun occurs, and is thus more efficient than the yellow- and orange-absorbing xanthophylls. Even better is the discovery that energy absorbed by xanthophylls can be transferred with nearly 100% efficiency to the chlorophyll molecules, boosting energy output and giving the cell a wider range of options.
Photosynthesis provided a selective advantage, but only if the free oxygen it released could be tolerated. Early anaerobic life treated oxygen as the corrosive poison that it is, second only to chlorine as a corrosive gas. Had it been present on the early Earth, chemical evolution would have been impossible; the oxygen would be destroyed the chemicals. By about two billion years ago, free oxygen in the atmosphere threatened life on Earth.
(The early ocean was rich in iron and sulphides, which removed oxygen from water. If there had been less iron and sulphides, oxygen levels would have built up much faster, perhaps not giving bacteria and archaea enough time to evolve oxygen tolerance. This is just one more reason that life is less likely to evolve complexity on a planet that is metal-poor.)
Evidence for the increase of oxygen occurs in the form of banded iron formations (BIFs), common between two and three billion years ago, but not present in older or younger rocks (older rocks are also characterized by minerals which are unstable in the presence of oxygen). These distinctive formations consist of alternating dark bands, containing up to 30% iron, and light bands made of chert (silicon dioxide). They can be traced sideways for hundreds of miles. The dark bands were formed on the ocean floor from iron dissolved in ocean water, then deposited on top of layers of chert accumulating on the seabed.
These formations are significant because iron can't dissolve in water under today's atmospheric composition; it requires an atmosphere almost oxygen-free. So, as molecular oxygen slowly began to accumulate from photosynthesis, the amount of it dissolved in ocean water would have alternated between very low and not quite so low. Iron put into the water through the erosion of rocks would have stayed dissolved when oxygen levels were very low (represented by chert layers), but would have precipitated out of solution when levels were not quite so low (the iron-bearing layers). Thus the banded iron formations record the period when oxygen was building up in the atmosphere.
(Because bacteria and archaea evolved in an iron-rich environment, they're vulnerable to iron shortages. When you have a bacterial infection, your temperature rises and the iron concentration in your blood plasma drops. Bacteria can't take the combination, and die. This is why human beings and other animals have evolved "having a fever.")
Another distinctive feature called "Redbeds" are actually red sedimentary rocks, formed when iron weathered out of rock in the presence of oxygen. Only present after about two billion years ago, they overlap in time with BIFs for several hundred million years.
Various mechanisms arose for dealing with the oxygen. They all involve sticking the oxygen in a non-toxic compound before it can do any damage. Some of these compounds are toxic to other life; the "red tides" today are caused by dinoflagellates that have stored so much oxygen in toxic compounds that massive fish kills result when the fish swallow the protoctists, and humans die who eat the fish. Other oxygen-absorbing compounds produce phosphorescence as a byproduct.
Some organisms went beyond oxygen tolerance and learned to use the oxygen in aerobic chemical reactions, which release much more energy than anaerobic reactions (an example of an anaerobic chemical reaction is fermentation, in which sugars are broken down into carbon dioxide, alcohol, and energy).
Respiration, the release of stored energy by aerobic reactions, is the opposite of photosynthesis. Respiration uses up dangerous oxygen and provides huge amount of energy, enough to fuel real complexity. Aerobic respiration had evolved by about 2.0 to 1.5 billion years ago.
The accumulation of oxygen in the atmosphere led to the creation of the ozone shield. Ozone in the upper atmosphere now blocked short-wavelength ultraviolet photons from the Sun from reaching the Earth's surface. There wasn't enough molecular oxygen in the atmosphere to create an ozone shield until about two billion years ago. Water is effective at absorbing radiation, so organisms living in the water would have been protected from ultraviolet radiation before that. Water is less effective than ozone, however, which limited life's options before two billion years ago. And of course, life on land would have been impossible.
Once the ozone shield was in place, eukaryotes evolved. More complicated than prokaryotes, and thus more vulnerable to radiation, the oldest eukaryotes are found between 1.5 and 1.4 billion years ago.
All cells more complicated than bacteria and archaea are eukaryotes, which are characterized by:
The concentration of DNA in the nucleus, and its organization into paired chromosomes, allows mitosis (sexual reproduction). Mitosis ensures fewer copying mistakes, and offers a greater opportunity for realignment and recombination. It provides greater variation for evolution to select among in response to changing environmental conditions.
The mitochondria in eukaryotes are the organelles in which respiration occurs. They take the products of fermentation from the cell cytoplasm, and conduct a set of chemical reactions involving oxygen and the fermentation products to create the enormous phosphate-bond storehouse of energy characteristic of aerobic metabolism. The energy derived from ingested food is increased by a factor of almost 20. Chloroplasts are the plastids (a class of organelles) in which photosynthesis occurs.
Chloroplasts and mitochondria possess their own DNA, mRNA, tRNA, and ribosomes. They reproduce independently of the rest of the cell (which is why they can be used to trace maternal lineages in population genetics).
Today chloroplasts and mitochondria are dependent on some proteins coded in the nucleus and cannot survive independently. But this was probably not always the case. Eukaryotes are believed to have developed when a prokaryote, probably an archaean, lost its cell wall. The remaining membrane, more flexible than a cell wall, began to grow and fold in on itself, leading to the formation of a nucleus and other internal membranes, and enabling the proto-eukaryote to engulf and digest entire prokaryotes, instead of being restricted to small molecules in the environment.
Later, eukaryotes enhanced themselves through endosymbiosis (cooperation) between prokaryotes. A prokaryote capable of aerobic respiration began living in the cytoplasm of a single-celled eukaryote. The respiring prokaryote eventually became mitochondria. The mitochondria were probably similar to modern purple nonsulfur bacteria. Photosynthesizing bacteria became chloroplasts.
Bacteria and archaea (prokaryotes) exhibit a wide range of metabolisms used to derive energy from the environment: fermentation, sulfur metabolism, nitrogen consumption, or the aerobic combustion of hydrogen. In contrast, most eukaryotes have the same type of metabolism, because they rely on mitochondria. Eukaryotes have the high energy efficiency needed to develop complexity.
I have gone into some detail on the evolution of the first living things for a couple of reasons. First, because the material is inherently fascinating, at least to me (my apologies if it's less interesting to you). Secondly, because even in this greatly simplified form, you can see how complex the whole sequence of events was. Bacteria, that is, non-respirating prokaryotes, may well be inevitable or near-inevitable on any planet with the right conditions, that avoids catastrophes such as giant impacts, runaway vulcanism, and the like. Evidence of bacteria, in the form of micro-fossils and stomatoliths, occurs almost as soon as the Earth cools enough to permit life to exist; and bacteria have persisted ever since, through multiple episodes of world-wide mass extinctions. Indeed, from the viewpoint of numbers or mass, this has always been a world of bacteria.
All the other steps, however, could be anything from inevitable to nearly impossible. We have no way of knowing from a sample of one. Finding bacteria or bacterial micro-fossils on Mars and some moons in the solar system would tend to indicate that bacteria are nearly inevitable, but further steps are not.
About one billion years ago there was a burst of evolution in eukaryotes, possibly triggered by the development of sexual reproduction. Multicellular eukaryotes, or metazoans, arose. The first ones were colonies of unspecialized cells, like Volvox today. Other metazoans like Rotifer began to develop specialized cells, but were still microscopic. Others, like sponges, were large enough to see with the naked eye — had there been eyes to see them. Finally we get fully integrated individuals whose cells are too specialized to survive independently, like jellyfish.
The oldest metazoan body fossil so far found is about 600 million years old, but trace fossils (tracks and trails) provide evidence that metazoans developed much earlier (most fossils are calcified bone; soft bodies and soft body parts aren't easily fossilized).
What are the odds of multicellular organisms arising? For over 85 percent of its lifetime, Earth contained no creatures large enough to see without a microscope. Life has existed for at least 3.8 billion years, but for most of that time the planet was dominated by bacteria-like organisms so small that a hundred billion could fit in a teaspoon. Earth's dominant life-form was essentially slime! It was the realization of just how long it took life to progress beyond this stage on Earth that first led me, personally, to doubt the inevitability of this progression.
Even if advanced life does develop, it might not last long. Microorganisms are very tough, and have much less stringent requirements for survival than metazoans. Some microorganisms can thrive in extreme temperatures and chemical environments. A most spectacular ecosystem is the "deep hot biosphere," where some microorganisms thrive miles below the surface, living off the energy produced by the chemical breakdown of rock. In contrast, animals can only survive in a far more limited range of environments.
Because metazoans took so long to evolve on Earth, and because such organisms are so fragile and needy compared with bacteria, I believe that the evolution of metazoans is an unlikely step. This would mean that even most planets suitable for life would be inhabited by nothing more advanced than bacteria.
Without knowing the properties of alien life, we must assume that it will be similar to our own. Life on Earth is based fundamentally on two types of macromolecules: nucleic acids, which store genetic information, and proteins, which control chemical reaction rates and provide structures. While alien life will differ in its precise biochemistry, it will still be based on similar large molecules. Even the simplest bacteria are actually extraordinarily complex biological machines, and require DNA or something analogous to store their genetic information.
If alien life relies on large, complex organic molecules, it will have environmental restrictions similar to life on Earth. The stability of molecules depends on chemical bonds, which will be the same throughout the universe. All possible worlds will follow the universal laws of chemistry and physics we know. These laws are fundamental aspects of nature; astronomy has demonstrated that they apply throughout the vast distances of the observable universe, and back in time for at least 10 billion years.
What if alien life were based on a less fragile element than carbon, whose bonds were more resistant to heat and chemical attack? Are there other ways to make life that can metabolize energy, reproduce, and mutate to adapt to changes in a wider range of environments? Since large molecules can be made with silicon, can there be life based on silicon or even on silicone (silicon-oxygen) polymers?
Silicon isn't really in the same league as carbon, nor are any of the other 82 stable elements. Carbon is special in its ability to form a rich variety of complex compounds. It makes strong bonds and can form single, double, and triple bonds with itself. Although there are many known silicon-based compounds, the list of possible carbon compounds is nearly limitless. For this reason organic chemistry, that is, the chemistry of carbon compounds, is a subject as vast as the chemistry of all the other elements combined.
Carbon was also much more readily available than silicon on the early Earth, and any similar rocky planet of an innermost solar system. In the form of carbon dioxide, it was one of the main components of the volcanic outgassing that formed Earth's original atmosphere. As a gas, molecules of carbon dioxide were readily available for use. Silicon is not only more rare, but occurs naturally as hard, tough grains of silicon dioxide — sand grains — that would require much more energy to break down for use.
For all these reasons, silicon-based life is unlikely to be common, at best. It hasn't been found on Earth, in Moon rocks, in Mars rocks, or in the thousands of meteorites and interplanetary dust samples that have been studied in the lab.
Given a world on which metazoans have evolved, what are the odds of sentience — rational thought, self-awareness, symbol manipulation — arising? I believe they are low.
First of all, not all species are suitable vessels for sentience. (Sound familiar?) Thought may not require a brain built just like ours, but it does require a large number of connected units interacting, whether you call them nerves, ganglia, or something else. The brains of elephants, dolphins and whales, man and the other apes are large, and furrowed with deep folds to extend the area of connecting brain matter without increasing its volume, and the size of the head needed to contain it. It is just these species which have shown, to various degrees, the ability to produce art, recognize themselves in mirrors, understand speech and gestures, and learn to communicate with sign language, symbol blocks, and other means. In the mammals, at least, intelligence is not an on/off thing that man has exclusively, but a spectrum from alertness to full sentience.
The mammals, however, are a single late-arriving class of a single, not particularly numerous or successful phylum. Of the five kingdoms of life on Earth, only the animals have nerves, let alone brains (the other kingdoms are: Monera (bacteria), Protoctista (single-celled eukaryotes), Fungi, and Plantae). Of the animals, only our own phylum, Chordata, has large, complex brains. Even a large invertebrate, like an octopus, has a very small brain.
Even most of our own phylum could never evolve sentience. The chordates without true spinal cords might as well be invertebrates. Even a truly large fish, such as a whale shark, has a tiny brain with few lobes. Amphibian brains are little different from fish brains, and reptilian brains form the most primitive, innermost parts of our own. Birds are essentially smarter reptiles that have learned to fly, and some birds can mimic human speech and recognize themselves in mirrors.
Only in mammals has sentience emerged and come to full fruition in ourselves and our ape cousins, and in the cetaceans. Even the most primitive mammal brains have more lobes than a reptile, but a mouse hasn't the sheer number of connections for full sentience. You can't run Unix on an 8-bit chip with 64k of memory, and you can't write poetry with a brain the size of a superball with no grey cells. The joy elephants take in producing art, given the materials, or the ability of a chimpanzee to learn sign language, simply can't be experienced by a cat, with its tiny little brain in its narrow little head.
Even if a class or order similar to mammals evolves, most will remain too small and simple. Large size is a disadvantage because it requires a lot more food and water to sustain it; its greatest advantage is removing the animal from the larder of smaller predators. On islands in the Mediterranean without large predators, sailors used to find miniature elephants and miniature deer; the large size was not needed, and smaller herbivores needed less fodder, so they shrank over generations.
Moreover, greater intelligence has no survival advantage over simple alertness. A mouse or a deer is as smart as a mammal needs to be to escape predators; claws, teeth, horns, a tougher hide, are all more immediately valuable than sentience.
Even a species with full sentience needs a suitable environment. Of the cetaceans, the bottlenose dolphin, at least, has been studied enough to show its sentience. Outraged human pride to the contrary, dolphins cooperating with human scientists have shown the ability to understand human speech and gestures, manipulate symbols, and recognize themselves in mirrors (most animals either ignore mirrors, or, if advanced enough to process the image in it, treat it as a stranger). Without hands or their analogue, and stranded in an environment which rules out the fire needed to drive chemical reactions, melt ores, or even harden wooden spear points, the dolphin is sentient but can never develop civilization on its own, let alone science.
In conclusion, even if metazoans evolve and explode into all possible ecological niches, the odds are low that sentience will arise. Most species simply won't have "the right stuff".
The scientific revolution, the invention of true science out of a mass of unorganized data and trial-and-error methods, happened exactly once on Earth in mankind's 6,000 years of civilization. Looking at all the places where it didn't happen, and how long it took to happen, one isn't led to the conclusion that it was an inevitable development. The three centuries since the beginnings of science are only 5% of the time since the settling of the Nile, Indus, and Yellow River valleys.
European science owes its beginnings to ancient Greek thought, but the Greeks and Romans did nothing worth mentioning with their early start. Vending machines that dispensed holy water in temples (because slaves could not be used for so holy a purpose), and astrological calculating machines (the Anti-Kythera mechanism) mark the high points of their science.
Even that was lost when the Roman Empire fell and barbarian invaders destroyed civilization. The writings of the Greek theorists and Roman engineers were reintroduced to the West in the 12th Century, in places like Spain where the Christian and Islamic worlds interfaced. It was another five centuries before Europe had recovered enough, and amassed enough, to begin to put the pieces together and start up the exponential curve of knowledge.
India and China enjoyed a degree of learning equal to ancient Greece. China invented a huge list of things before anyone else, and ancient and early medieval India's astronomical observatories were wonders of the world, unequalled in accuracy until Tycho Brahe's work in the Renaissance. Both nascent scientific movements, however, were wiped out. India's fell victim to mysticism, the belief that the world is not real and not ruled by consistent, objective laws. In China the emperors instituted Buddhism for the masses, and put an end to change in the name of political stability.
Even today there is no guarantee that science will survive as the ruling paradigm in the minds of the world. Most people are poorly raised and poorly educated, and remain ignorant of science even while enjoying the medicine, industry, and consumer products that trial-and-error, cut-and-fit technology without science could never have produced. The import of Indian mysticism to the West in the 60's has ruined the minds of a couple of generations now, while stupid fundamentalism leads to the introduction of "Creation Science" in the schools. There are "post-modern" academics who seriously assert that science is a belief system, not the study of objective reality, and U.S. senators who denounce the cost of basic scientific research while ignoring the waste and cost, in money and death, of unregulated capitalism.
No, I don't believe that science is the necessary discovery of sentient beings. It only came about once on this planet, and its future is by no means certain even here.
The idea that we might be rare seems preposterous and presumptuous. The assertion that we and our planet are unlikely goes against the "principle of mediocrity," the common-sense expectation that we are ordinary and commonplace. Indeed, it goes against our historical experience: where once we believed that we inhabited a unique place inside the crystal sphere of the stars, we have learned that we aren't the center of the solar system, the galaxy, or the universe. Since we now go around the Sun instead of vice versa, surely the same must be so around every star?
As reasonable as that sounds, however, this optimistic outlook is almost certainly untrue. Our experience on Earth does not extrapolate easily to other planets. An analogy can be found in the roundness of the Earth. Everyday experience, and the direct evidence of our senses, says that the Earth is flat. Even the ancient Greeks, however, knew otherwise. The roundness of the Earth's shadow on the Moon during an eclipse, the difference between the stars visible in Greece and in Egypt, even the fact that a ship putting out to sea vanishes from sight from the hull upwards, proved that everyday experience was wrong. Similarly, while life is abundant on Earth and six billion humans crowd it, the examination of the Earth, the history of life on it, and the history of humanity strongly suggests that worlds like ours are rare.
Even if we are not unique, we might never know. The universe has inconceivably many stars and even more planets — separated by distances so vast that even light takes years, centuries, even millennia to cross them. Rarity can mean permanent isolation. If we are rare enough, we will be forever isolated in our little bubble of space and time. If the neighbors are too far away, then for all practical purposes we will effectively be alone. Unless advanced life is so common that it exists around the few thousand nearby stars, we may never have the opportunity to contact or even detect other intelligent species.
In this paper I have ignored the usual argument that goes, "If life is common, where are our visitors?" Rather than argue with UFO freaks about the nature of evidence, or debate with the SETI crowd what frequencies we should be monitoring, I have attempted to present the evidence of cosmology, galactic astronomy, solar and planetary astronomy, geology, chemistry, biochemistry, paleontology, evolutionary biology, and history. As a would-be science-fiction writer, I have paid close attention to this question all my life. Some of the facts presented here were researched in books or on the Web; others, like the survey that revealed half the G stars in our galaxy are multiples, or the infall of ice balls against the atmosphere even today, come from my memory of items read over the years.
If you're not convinced that sentient, technological cultures like our own are very rare, and that we may be the only one in our galaxy, you are invited to read further. There are whole college courses taught on this subject today, which once was the sole domain of a small circle of writers of speculative fiction. If you examine the literature, ignoring the special pleadings and concentrating on the facts as we know them, you will see that it is as I've stated, and supports my position.
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