Why one is Needed and How it might be Derived




НазваниеWhy one is Needed and How it might be Derived
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Summary

The universe is not quite as mysterious as it once appeared to be. Cosmologists and astronomers now understand a great deal about its past, present and future, as well as what creates and controls its contents. Using computers and instruments to analyze photons collected by different types of telescopes, it is possible to infer what was likely to have been happening when these photons were created, and even what energies and matters were encountered in their journey through space and time. As we progress in this manner, we find that the science of cosmology depends upon--in fact, mostly is--the physics of energy and matter. In other words, the behaviour of the gigantic is controlled by the properties of the minuscule, the ultimate test of reality's rational conformity to causality.215

However, there appears to be no method of peeking through a singularity. If this is so, then no future beings, wherever they might live in the universe, will ever discover what caused the Big Bang, or what existed before time in this universe began.

This could be where a god once lived--or even lives now. If one exists anywhere at all, then this is the only place that modern science would position him--before the beginning, and beyond any possibility of interfering with the present.216 Certainly, there is proving to be no physical need for a god's intervention within our universe. Once a universe possessing the properties we are discovering has been created, it will simply evolve in the manner our telescopes reveal, with stars, galaxies and planets (and life, as the next chapter explains) being formed along the way. A god's influence is seemingly not needed in the day-to-day operation of the physical universe. But isn't that to be expected of a creation if God Himself designed it?217

Well, so much for the universe. It started, and it evolves as time goes by. We don't know why it began or even if its existence requires a cause,218 but we do know a lot about how and why it evolves, and the forces that regulate its development.

It is time to turn our attention to the phenomenon of life, and review what scientists can tell us about it. Two questions are most important to our discussions: how did life begin, and how has it given rise to the forms we see today? The next chapter will review the answers that have been found.


"Gödel's Theorem," "General Systems Theory" and "The Conservation Laws" are postscripts to this chapter.


CHAPTER EIGHT: LIFE


Life's development has been much harder to definitively trace than that of the universe. This is because biology's countless combinations and permutations are exceedingly convoluted compared to the linearity of physics or the predictability of chemistry.

Experimental physics is often conducted by holding all variables constant, then determining the effects of changing just one. This procedure, together with the universe's constraints (which limit the number of particles [such as protons or electrons] there are to investigate and the number of ways they can behave) has simplified the discovery of many of the universe's secrets. Physicists now extrapolate with confidence from present to past, from past to future, and from here to the other side of the universe.

Chemistry is somewhat more complex than physics, because there are over a hundred chemical elements. This relatively large number means that millions of different combinations (as molecules and compounds) can exist. This complexity is being conquered, however, as demonstrated by the near-routine formulation of new and improved fabrics, explosives, alloys, drugs, plastics, and innumerable other products.

Progress in biology, on the other hand, has been considerably slower until just recently. Life's ability to mutate and change over successive generations has meant that investigators cannot simply extrapolate backwards to determine what previously existed, nor look forward and predict what might result.

Most of our knowledge about life's history has been obtained from fossils and preserved remnants of past life forms, but two factors greatly complicate the task. First, biotic matter provides food for other living things, so most of it never makes its way into the future to be studied. Second, the extensive (and ongoing) geophysical changes that the Earth has undergone during its more than four billion years of existence has left the story-telling remains of life's progress fragmented and incomplete. However, enough evidence exists and has been found for scientists to trace life's gross history on this planet. Moreover, each year new fossils and new facts (particularly genetic) about past and present inter- and intra-species relationships are discovered. These fill knowledge gaps and build confidence in the accuracy of what has already been deduced. What biologists now generally accept is related in the following pages.

1. Possible Origins of Life on Earth

One of the first theories to become widely known that described how life may have begun on this planet was proposed in the 1920's independently by Aleksandr Oparin, a Russian biochemist, and J. B. S. Haldane, a British biologist. They pointed out that some four billion years ago, conditions in the Earth's shallower seas and oceans would have resembled a chemical vat. This vat must have held a variety of ingredients that would have been warmed by sunlight, constantly stirred by tides and winds, bathed in ultraviolet radiation from the sun, and intermittently subjected to electrical discharges from thunder storms. This so-called called "primordial soup" would inevitably have become more complex over time, as interactions (mostly chemical) between the constituents occurred. Eventually, it would likely contain many of the molecules and compounds needed to create some elemental forms of life.

Stanley Miller, under Harold Urey at the University of Chicago in 1953, recreated many of these conditions in the laboratory. Together they subjected methane, ammonia and hydrogen (gases that very probably existed on this planet in its early years, and that are still present in abundance on Jupiter and Saturn) to electrical sparks in a sealed sterile flask. On later analyzing the flask's contents, they found many of the amino acids from which life's building blocks--proteins--are built.219 Similar experiments have since yielded molecular components of proteins that regulate carbohydrate and fat formation. In other words, some of the major constituents of life have been fabricated from scratch in the laboratory.

However, it is equally possible that life on this planet began thousands of meters beneath the sea's surface, in total darkness. Clues to this conjecture have been found in north-western Australia, in sulphur-rich rocks that retain micro-fossils of single-celled organisms over three and a quarter billion years old. These rocks possess mineral structures that reveal they originated close to hot springs on the sea floor.

In 1977, a mid-ocean ridge of hot springs was discovered encircling the globe. The wealth of chemicals and nutrients it supplies nourishes a complex ecosystem of over 500 species, from bacteria to tube worms and crabs.

The energy source that sustains this web of life is the oxidization of hydrogen sulphide in a process called chemosynthesis (as opposed to photosynthesis, whereby sunlight powers life on the Earth's surface). Experiments at the Woods Hole Oceanographic Institution determined that when similar physical (high-pressure, turbulence, completely dark, etc.) and chemical conditions are constructed in the laboratory, large organic molecules containing over thirty carbon atoms form in less than a day. Thus, life on Earth may have first begun in the sunless depths of its oceans.220

Alternatively, life may have begun as some form of methanogen.221 Methanogens are microbes that obtain energy by converting hydrogen and carbon dioxide into methane. Very few such organisms exist in any of Earth's typical, oxygen-abundant, environments (because methanogens are consumed by the more-efficient carbon life forms that now occupy these niches), but a complete food-chain community of them has been found in Idaho living in 58°C water two hundred meters underground. Presumably these have survived from very early times. Methanogen communities may have been common on this planet before oxygen in gaseous form became abundant (see section two of this chapter) and they may also exist where conditions are similar, such as upon some of the sun's other planets or moons, for instance.222

While several situations and mechanisms might have given rise to life on this planet, it may in fact owe its origins to extraterrestrial events that first happened in water, frozen in space, long ago and far away. Although space temperatures average just 3° above absolute zero, recent experiments have found that amorphous ice (the kind that forms when water vapour freezes in a vacuum) flows when subjected to ultraviolet radiation (as it would be in space). In the laboratory, when carbon monoxide, carbon dioxide, and methanol (gases all abundant in space) are dissolved in water before freezing to form amorphous ice, subsequent ultraviolet radiation produces hundreds of complex organic molecules. Moreover, if this frozen ice flows (or is melted or added to water), membranous vesicles (similar to those found in the 1969 Murchison meteorite223) are formed, together with even more complex compounds (some able to convert ultraviolet energy to visible light).

Laboratory findings such as these are reinforced through data collected by the European Space Agency's satellite Infrared Space Observatory (ISO). When scrutinizing selected objects, the ISO can detect the emission of infrared rays at particular wavelengths, revealing the presence of identifiable atoms, molecules and solids. These data show that complex, ring-structured, aromatic molecules form in the regions surrounding very old stars, over the relatively short period of a thousand years or so. Spectral analysis of interstellar dusts and gases have identified hundreds of different organic compounds, including amino acids of the type needed to build life's proteins.224 Recently, sugar molecules (glycolaldehyde) have been spectroscopically detected in the dust clouds near the centre of our Milky Way galaxy. (What makes this finding particularly interesting is that such molecules can combine with other molecules to form ribose and glucose; ribose molecules are utilized in the construction of DNA and RNA.)

All these findings strongly suggest that life's precursors, including cell-like sacs containing organic compounds, could have been formed many billions of years ago225 in space, and would therefore be part of all comets, asteroids and planets from their very beginnings.226

2. Development of Life on Earth

While we don't definitively know where life first developed, we do know approximately when it first appeared on Earth--it showed up less than a quarter of a billion years after the Earth's crust had formed. In other words, just about as soon as it could.227

For reasons noted in the introduction to this chapter, early evidence of life is hard to come by.228 Nevertheless, indirect evidence suggests that it was present at least 3.7 billion years ago. This has been deduced from an analysis of rocks dating to that age, found on an island close to Greenland. These rocks contain a higher carbon-12 to carbon-13 isotopic ratio than chemical and physical processes alone would create. (Life processes prefer the lighter isotopes, and this concentrates carbon-12 where life exists.) More direct evidence, in the form of fossil micro-organisms, has been discovered in sedimentary rocks from Iceland that are between 3.7 and 3.8 billion years old. (Iceland is particularly suitable for finding early life forms because its rocks have not been greatly disturbed by geological processes during the intervening ages.)

Many of us were taught in school that there are three kingdoms of life on this planet.229 The simplest and most ancient are called the Archaea (otherwise known as archaebacteria, the first cells).

Archaean kingdom representatives were first discovered in volcanic vents on the floor of the Pacific ocean,230 three kilometres deep off the Galápagos Islands.231 Archaea and very primitive bacteria are autotrophic (that is, they build their complex living molecules by chemosynthesis, a chemical process mentioned in the previous section).

The second kingdom, the Prokarya, are a later development; they consist of life forms whose cells lack internal membranes (and thus have no nuclei).

Prokaryotic life was flourishing within the Earth's shallow oceans as blue-green algae (a.k.a. cyanobacteria), over three and a half billion years ago. Once formed, the anaerobic232 cyanobacteria began dumping its photosynthetic by-product, oxygen, into the Earth's oceans and atmosphere, and continued to do so without much competition for over two billion years. Eventually a new form of bacteria evolved that was able to use this oxygen through a process we call aerobic respiration; this opened the way for the more complex (and more energy-demanding), nucleated, eukaryotic cells to evolve. (Bacteria, of course, still exist in abundance everywhere conditions permit, and they still lack cell nuclei.233)

The third kingdom, the Eukarya, first appeared about two billion years ago. The cells of eukaryotic life forms contain membrane-bound nuclei, and all plants and animals (including humans) belong to this kingdom.

While one billion years ago the continents were still barren (with the possible exception of primitive algae), the seas teemed with unicellular life. Many of these life forms reproduced asexually through division, although some used sexual means. About 700 m.y.a. (million years ago), multicellular sea plants appeared. They rapidly developed in form and prevalence as they made the most of their added capabilities. Multicellular sea plants stayed at the forefront of life's evolution until the beginning of the Cambrian era, about 540 m.y.a., when multiple forms of marine animals developed from simpler varieties of roundworms. This transition occurred because possession of body cavities and an alimentary canal allowed worm-like creatures to grow more than a few cells thick (as nutrients and waste materials could now be readily passed between internal cells and the external environment). Larger bodies meant that supporting structures would be valuable adaptations, and any that evolved would be retained. The first vertebrates developed soon thereafter (about 500 m.y.a.).

By 400 m.y.a., plants, fungi and primitive arthropods (invertebrates, similar to crabs or lobsters, having an external skeleton and jointed appendages) had colonized the ocean shores and moved inland. (The ongoing evolution of early arthropods eventually produced spiders, centipedes and insects.) Around this time, fish utilized their swim bladders and fins to spend temporary periods on land. These organs gradually evolved into lungs and legs, and the animal class known as amphibians arose. The fluid-filled amniotic sacs we call eggs allowed amphibians to reproduce and give birth on dry land, and some later evolved into reptiles, dinosaurs, lizards, snakes and turtles.

The earliest mammals appeared some 200 m.y.a., evolving from a group of reptiles called therapsids. These mammals were small (about five centimetres long) and possibly lived in trees during the dinosaur age. They remained rodent-like creatures234 until the dinosaurs became extinct 65 m.y.a. One branch of these early mammals evolved (some 30 m.y.a.) to become Proconsul, our hominoid ape ancestor, and their descendants became the gibbons, orang-utans, gorillas and chimpanzees we know today. About six million years ago, the ape and hominid lineages separated; today our closest living relatives are Central African chimpanzees (demonstrated and verified by comparative DNA sequencing235).

The genus Homo appeared about two and a half m.y.a. (although stone tools have been found that date to earlier periods). Artifacts left by "technologically advanced" clans of early humans (who used stone tools to chip bones and antlers into refined shapes) have been found in Israel's Dead Sea Rift Valley and dated definitively236 to 780,000 years ago.

Neandertals (who first appeared in Europe about 200,000 y.a. and whose ancestors were hominids who moved from Africa to Europe some 500,000 y.a.) holed up in valleys to survive the ice ages and so avoided the many challenges that constant moves would have brought. Perhaps as a result, their tools changed little during most of their existence, and this suggests that their intelligence also did not greatly change. However, fossilized bone structures show that Neandertals did have the means to utter words, and they probably developed and used simple languages.

The tools and ornaments of Homo sapiens, on the other hand, changed greatly over very short periods of time. Our species first appeared in Africa over 100,000 y.a. and moved into Europe (as Cro-Magnon) around 40,000 y.a., and they seemed to have confronted and surmounted the various challenges successive ice ages introduced.237

How do we know these things? Specimens of life and associated artifacts have been trapped in muddy sediments, chalk, glacial ice,238 peat bogs, dry sandy deserts, tree resin239 etc., for millions of years. These entombments often preserve complete specimens in date-stamped strata for scientists to examine.240 Painstaking observations over many decades combined with more recent sophisticated analytical techniques (such as DNA analysis and various imaging techniques) consistently show that life's development demonstrates an overall progressive trend from simple to complex.241

3. Evolution

Millennia ago, humans realized that greatly different animals (deer, birds and fish, for example) possess body organs and systems very similar to their own, but could not explain why. Over the centuries various explanations were proposed, some theological, some scientific; two centuries ago most people accepted the theological interpretation--that life in its different forms was Created. Papers read to the Linnean Society in 1858 written by Charles Darwin and Alfred Russel Wallace (who had independently reached similar conclusions) did little to change this situation--attendees simply did not understand the importance of what they were hearing. However, when Darwin's book On the Origin of Species by Means of Natural Selection, was published a year later, evolution became a topic of discussion for every learned person and things changed forever.242

It is easy to understand what immediately happened. Darwin and his ideas were ridiculed by almost everyone; scientists said he could not prove what he was saying, and the religious said that mankind was created--as proven by texts in the Bible. Humans simply could not be "descended from an ape."

Darwin's work, and its attendant publicity, resulted in widespread use of the words "evolution" and "natural selection." These terms are sometimes treated as though they hold the same significance--they do not. One is a fact, the other is a theory, and we should take a moment to discuss the differences between the two.

There is plenty of evidence to show that evolution occurred--is still occurring--and that all life on this planet is interrelated, with a common ancestry. Palaeontologists study fossils of once-living organisms, and their work demonstrates that the bones and structures of ancient life forms gradually changed over time. Comparative studies of the physical and systemic structures of living plants and animals uncover the same kind of gradual change. Genetic mapping243 adds to the information obtained, and shows beyond any possible doubt that links between living species, and between living and extinct species, exist.244 Weiner, after discussing work done by Seymour Benzer and his wife in the 1980's, noted that flies, worms, seeds, yeasts and bacteria possess thousands of very similar genes or gene sequences.245 This could only have occurred if they all had a common ancestor. (In fact, a pre-Cambrian common ancestor must have existed, well over 540 m.y.a., for such widely separated species to possess so many similar genes.) Moreover, as Weiner pointed out, the genomes of mice and men (and women, if it needs to be stated) are about the same size and contain corresponding genes.246 (de Duve actually states247 that the evidence showing all Earthly organisms to be descendants of one common ancestor is "overwhelming.")

That evolution occurs is something animal and plant breeders have known and profited from for centuries, and it is a fact that few educated people today dispute.248

However, we can never be certain that we know all of the factors that cause evolution to occur, so any explanation of why evolution occurs may someday be modified or even overturned. While we have what we think is a very good idea, there could be additional or different reasons why evolution happens, so scientists continue to call this very good idea a theory. Natural selection249 is Darwin's very good idea, and it has withstood all manner of challenges to its ability to explain and to predict. But it can be thrown into doubt, and even discredited, any time a fact of evolution is found that it cannot explain. (All scientific explanations are like this; any or all of them may one day be shown to be inadequate or inaccurate, and we remind ourselves of this limitation by calling many of them theories. All will remain theories forever.250)

Thus, that evolution occurs is a fact; however, the explanation why evolution occurs will always be called a theory. This, presumably, is why controversy continues. A few people, wilfully or mistakenly, capitalize upon the word "theory" to imply that the concept is untrue, and that evolution does not occur. What they might better state is that the natural selection explanation of why evolution occurs is a theory, good only until some better explanation is found.251

Returning to the theme of this section, it is estimated that some two billion species have evolved on Earth during the past six hundred million years (the period for which we have some of the best fossil records, and during which all of our land life developed). Today, about 99.9% of these are extinct.

The two million species that exist today exhibit a multiplicity of forms. Variations range from the large, most-obviously complex, multi-system animals, down to the minute, single-celled, relatively simple bacteria. As might be expected, it is the tiniest of these which demonstrate the greatest diversity and resilience.252 The habitats of bacteria range from the plus 91°C boiling hot springs of Yellowstone Park, to the minus 50°C super-cooled brines found in the Antarctic. Bacteria also flourish under tremendous pressures on the ocean floor, spread prolifically throughout the soil we walk upon, waft through the air we breathe, and luxuriate in every kitchen.

When the papers written by Darwin and Wallace were first read,253 no one had seen evolution occurring. Today, experiments demonstrating its thesis can be conducted using fruit flies in high school science laboratories. Evolution was thought to be too slow to be witnessed in nature, but the real challenge to demonstrating its ubiquitous occurrence comes from the need to detect and measure small changes over a number of generations while also recording every possible factor that might relate to (or be causing) such changes.

Some of the earliest decisive documentation of evolution occurring in the wild was obtained by Peter and Rosemary Grant through studies of "Darwin's" finches on the relatively isolated Galápagos island, Daphne Major. For more than two decades, the Grants, with the help of many colleagues, captured, numbered, precisely measured, banded, catalogued and released, almost every finch that lived on the island (sometimes only a few hundred, sometimes several thousand in a year). This period included years of drought, as well as wet and more normal years. In this manner, they recorded the features of close to 100,000 finches, together with many details about their varying habitats.254

These records were run through computer programs that sought correlations between the number and variety of finch, and changes in their environment such as rainfall, seed plant variety and abundance, and so on. Drought years drastically reduced the number of softer seeds, leaving the number of hard-shelled cactus seeds about the same. Finch species with large, strong beaks that were able to crack hard-shelled seeds, survived in stable numbers during those years, as might be expected. However, measurements of surviving members of the other finch species showed that only those whose beak was larger than average for their species were surviving and reproducing. The net result was that the beak size of each finch species drifted toward a larger and stronger shape during drought years. This drift continued in successive generations for as long as the drought continued. Wet years produced the opposite effect, and resulted in a drift within each species toward a finer beak structure (because the cactus plants began to die, and thinner beaks could better retrieve the smaller seeds of other plants that fell into the many tiny cracks in the island's volcanic rocks).

Others have conducted parallel work. John Endler, for instance, working with guppies in various South American countries, observed natural generational variations in colour which were brought about by changes in the environment. Dark water favoured brilliance (better to attract females); light water favoured camouflage (better for hiding from predators).

Any change in an environment may affect species living within that environment. An accumulation of adaptations within one species eventually produces what becomes described as a new species. Collections of plant, animal and insect fossils in museums and universities around the world show that time, environmental change, and geographical separation are all that are needed for species to evolve from old into new.

Changes over time cause descendants to either diverge (i.e. increase in difference, one from another), or converge (i.e., increase in similarity). The factors that promote species divergence are predominately food (which favours the development of tools--for instance, the beak-that better exploit the particular kind of environment which supplies that food) and sex (which favours the development of partner-attracting displays or like-attracting-like matings). The forces that promote species convergence include the presence of enemies (which favours the development of camouflage and herd behaviours, the latter because there is safety in numbers) and the physical features of the environment being exploited. Hybridization, whereby closely related species merge genes, can produce fairly rapid convergence or divergence. The prevailing environment determines which outcome predominantly survives.

Evolution, we now realize, is often not a slow and gradual process. It can occur in small or large bursts, and these may be followed by long ages of slow consolidation.255 A common sequence is as follows: an environmental calamity occurs, followed by a rapid collapse in food supplies. The calamity can be localized and relatively insignificant (a fallen rock, for instance) or something very pervasive (the eruption of an immense volcano, the impact of a large comet, or the rapid development of an ice age, for example). Each environmental change causes the death of some or perhaps almost all of the existing, previously well-adapted species.256 The decline in numbers of some species (or the environmental change itself) opens niches that were previously occupied, blocked, or non-existent, and this provides opportunities for suitably different members of surviving species to thrive.

The mutations that make life's evolution and continuance possible need not be large, as the work with Darwin's finches demonstrates. However, even drastically mutated offspring may survive and flourish under some kinds of environmental change. Evolutionary change following extinctions is rapid, because many of the previously well-adapted (and presumably competing species) completely die out. This may open niches accommodating to some of the more extreme variants (that might not otherwise have survived); without competition, they may now proliferate. Evolutionary change slows down again just as soon as successfully adapted species fill all available energy niches.257

Massive extinctions have not been uncommon in our planet's history. Two of the more infamous were probably caused by asteroids or comets hitting the Earth. The first of these occurred around 208 m.y.a., creating the environment that early dinosaurs exploited to become the Earth's dominant animals (this impact produced the changes that mark the junction between the Triassic and Jurassic periods). The second collision happened around 65 m.y.a., and ended the dinosaurs' supremacy. Other extinctions occurred around 438 m.y.a., 367 m.y.a., and 245 m.y.a. Each of these cataclysms resulted in the demise of more than fifty percent of the prevailing marine species, and an equal or greater percentage of the existing land species.258

Records of growth from tree rings as well as ice core samples show that large calamities, due to one cause or another, have also occurred relatively recently. The most prominent events happened around 3200, 2300, 1628, and 1159 BCE; the most recent took place in 535 CE.

Catastrophes rapidly and radically transform the planet's various environments, and life forms that do not adapt (i.e., evolve) do not survive. (And, as a corollary, it must be emphasized that if life does not continue to evolve it will not continue to survive, for it is certain that changes will occur to life's environment in the future just as often as they have in the past. Moreover, it is worth noting that the environment currently changing the most rapidly is not the Earth's biosphere, it is our human mental environment--a change brought about by the mix of facts, ideas, opinions, fantasies, beliefs, etc., that worms its way into our thoughts every day. To survive, we must discover how to adapt to the changes occurring there.)

Fossil records show gross evolutionary changes in Homo's body structures that took thousands of years to develop. Detecting subtle changes necessitates making fine measurements, and we currently lack the detailed records covering several generations that would unequivocally demonstrate human evolution in action. Doubtless, as computer record-keeping increases in scope and depth (particularly if DNA profiles are to be stored), we will soon have plenty of evidence to show that, like all else that lives, humans evolve, and evolve continuously.

Humans have always acted to minimize the effects of events that may influence their evolution. We store food and survive most food shortages; if we did not do this, the average body mass and size of H. sapiens would drift downwards. We capitalize on niches and specialize in occupations; if we did not do this, our numbers would decrease because there would be too many competing within each energy niche (read money, thus food, for energy) and fewer would survive. We stress universal literacy and education; if we did not do this, the total number of energy niches would decline over time. Together, forces such as these select for particular skills: musical, mathematical, artistic, and so on. In effect, ever since we have had the ability, we have acted in a manner that influences the way we evolve. Genetic engineering is about to vastly extend this ability.

4. The Probability that Life exists Elsewhere

It is not difficult to estimate the probability that life has developed on other planets in the universe. All we need do is calculate the total number of stars, the number of these that may support habitable planets, and the likelihood that any of these planets would support life.259

First, the number of stars in our universe. It is estimated that our Milky Way galaxy contains between two and three hundred billion stars (2-3 x 1011), and that there are about one hundred billion galaxies (1011) in the visible universe. If the average number of stars in other galaxies is similar to ours, then there are 2-3 x 1022 stars all told. Using the smaller number we have 2 x 1022 stars to start with.

Second, there are many inhospitable zones within all galaxies (the planets of stars too close to the centre of a galaxy or to radiating black holes, for instance, are being sterilized by microwaves260) and stars in these regions are unlikely to support life-bearing planets, at least, not life as we think of it.261 Let's guess that only one tenth of each galaxy's stars are clear of these areas, giving us 2 x 1021 stars in hospitable zones of the universe.

Third, using observations noted in section four of the previous chapter, we can guesstimate that some fifty percent of all stars possess planetary systems, so about 1 x 1021 (or 1021) stars are predicted to have orbiting planets.

Fourth, many exoplanets likely do not possess the conditions we consider necessary to support life (water, appropriate temperature ranges, appropriate elements and minerals, energy sources such as sunlight or planetary heat, etc.). A reasonable guess might be that of those possessing planetary systems, only one star in ten will hold a planet that is habitable. This gives us 1020 stars or 1020 habitable planets.

Fifth, we do not know if life will always arise on habitable planets.262 If, as is turning out to be likely, the molecules from which life originates can form in space-ice, then probably all of the universe's planets will have been inoculated by now. How much of this material then goes on to create life can only be a guess. Presumably, if the right conditions exist, eventually all will; but, to err on the conservative side, let us say that only one in a hundred habitable planets becomes a host to life.263 Thus about 1018 (1,000,000,000,000,000,000 or one quintillion) life-bearing planets possibly exist in the visible universe. Of these, about 107, or ten million, could be in our own galaxy.

As we learn more about the nature of life and our universe, we will undoubtedly revise our estimates of the number of planets that could be home to living entities. The number may decrease or increase, even significantly, but it is very unlikely that the number will turn out to be one. Statistically, therefore, it is highly improbable that our planet is the only one to bear life; the universe contains an incredibly large number of stars, and the conditions and ingredients required to start and support life probably exist in many, many millions of places. Furthermore, these places may include intergalactic space, within gases where life's precursors may first have formed, then evolved, to create living entities that waft through the heavens in forms vastly different from ones we might recognize.

5. Intelligent Life

The contention that life exists elsewhere holds a mystery. If ten million planets in our galaxy alone are likely to support life, why have we not heard from any of it by now? It has been estimated that about a thousand of these planets would be supporting life that has evolved to the state of communicating by radio transmissions. Why has the SETI Institute (Search for Extraterrestrial Intelligence) not yet detected intelligence-bearing signals?

Moreover, why have we not found unquestionable evidence264 that we have been visited by aliens, or by one of their devices? We have already sent probes far beyond our own solar system, and within a decade or two will likely send similar instruments to exoplanets orbiting neighbouring stars. Indeed, in less than a couple of generations we may be visiting these planets ourselves. (Current technology necessitates journeys limited to a dozen years or so, but ion-accelerated drives already in use promise an ability to travel great distances in that amount of time.) This being so, we will inevitably attempt to colonize any habitable exoplanets we find, and, from those bases, we will certainly move onwards and outwards. All intelligent life, needing to replenish the resources it consumes throughout its industrial development on its home planet, will want to do the same. A few plausible assumptions and a couple of calculations suggest that any such pioneering life form will have colonized the whole of its own galaxy within a period of five to fifty million years.

This may seem a long time relative to our life span, but it is minuscule in evolutionary terms (we have to go back farther than that to find living dinosaurs). And it is an almost infinitesimal period on the galactic time scale. Life on other planets could well have begun a billion or more years before it arose on ours. Why, then, have we not been colonized by any of the intelligent civilizations that should have been able to do so in the past hundreds of million years? Or, why have we not at least been visited by some form of von Neumann probe (see endnote 177 for a little more information about von Neumann probes) during that time?

Crawford265 suggests that the answer to this puzzle (called the "Fermi Paradox") could be that, although the formation and evolution of life may indeed be universal, its subsequent development into intelligent life forms may be rare. An alternative explanation could be that von Neumann probes have already visited, or even now be present. If they were small enough--developed by aliens using nanotechnology, perhaps--we would not have noticed them.

However, the explanation may be even simpler. Perhaps life forms intelligent enough to use von Neumann probes, or to travel and colonize hospitable planets, avoid planets whose "intelligent" inhabitants are constantly at war among themselves. Landing would certainly lead to a transfer of technology, and would eventually equalize abilities of the two life forms. Far easier for them to wait to see if we mature to a stage where we no longer make war; seeking contact before then only invites trouble and is simply not what an intelligent being would do.

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