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Chapter 28 – Origin and Evolution of Life: on Earth and Elsewhere________________________________________________________________________  We shall not cease from exploration.And the end of all our exploring.Will be to arrive where we started.And know the place for the first time.-- T.S. Eliot To see a world in a grain of sand,And heaven in a wild flower:Hold infinity in the palm of your hand,And eternity in an hour.--Blake, Auguries of Innocence (1757-1827)  Chapter Preview In the first 27 chapters of this text, we have reviewed the arguments for humans not occupying the central

place in the Universe. Historically, scientists discovered that the Earth is not at the center of the Solar System, the Sun is not at the center of the Milky Way, and the Milky Way is not at the center of the Universe, any more than any other galaxy. We now need to ask ourselves the question, "Are we the only example of intelligent life in our Universe?" One of the most risky but potentially rewarding pursuits in modern science is the search for extraterrestrial intelligence and the attempted communication with other life forms. This is an area of study bordering on biology and astronomy. In this chapter we will briefly review the origin and evolution of life on Earth and evaluate the chances for the existence of intelligent life elsewhere in the Milky Way. 

Key Physical Concepts to Understand: definition of life, Principle of Mediocrity, Anthropic Principle, evolution of primitive life on Earth, the Drake equation, SETI

I. Introduction 

Humans have had a tendency to feel that the Earth is a special place in the Universe, perhaps for no good reason, from ancient times until the present. From the time of the Copernican revolution in 1510, when the Earth was intellectually displaced by the Sun as the center of the Solar System, humans have become increasingly aware that we are not in a revered place. Scientists have gradually come to understand that humans have no central role in the Universe. This concept is called the Principle of Mediocrity, a principle that was resisted by entrenched theology at the time of Copernicus. But more recent philosophical/physical inquiry suprisingly leads counter to the Principle of Mediocrity and has resulted in the Anthropic Principle:

 The Anthropic Principle : Without intelligent life, the Universe is meaningless, for there is no one to study or contemplate its existence.

 This is not an unsupported philosophical statement. Recently discovered physical/cosmological coincidences have been pointed out by a number of physicists (Web Essay: The Anthropic Principle), including Brandon Carter, John Wheeler, Richard

Gott, and Robert Dicke. The existence of life in our Universe appears to be the result of a delicate balance among certain fundamental physical parameters such as the strength of gravity, the strong force, and the expansion rate of the Universe. A slight change in any one of these would have eliminated any chance of life in our Universe.  One example is the strength of the strong force. If the strong force had been only slightly weaker, fusion of hydrogen into heavier elements would never have taken place in stellar interiors and the heavier elements essential for life, including carbon, nitrogen, and oxygen, would never have formed in the Universe. If the strong force were only slightly stronger, it would have accumulated super heavy nuclei during the early moments of the Big Bang, preventing the further fusion of elements in stellar cores and eliminating stars as a source of energy for life in an evolving Universe.  If gravity were only slightly stronger, seen in a slightly larger gravitational constant, the Universe would have used up all its mass in forming enormous, short-lived stars in a brief period after the

Big Bang. These stars would have only lived for millions of years, not the billions of years necessary to keep a planet bathed in energy while life slowly evolves. If gravity were slightly weaker, only very low-mass stars would have formed in the Universe, too small to generate enough light to warm planets at any significant distance from them. The fact that we are alive in the 20th century constrains some of the most basic physical constants. There is a deep connection between particle physics, cosmology, and the origin of life.  One of the most fundamental questions in science is: "Are we unique and alone, or are we the result of the natural and relentless evolution of countless life-forms in a fertile Universe?" In this chapter, we will all too briefly examine the probability for intelligent life elsewhere in our galaxy. First, we must agree on a basic definition for life. Then we will examine the scarce clues that nature has provided to estimate the probability of finding life elsewhere.

 II. What is Life?  

Our first clue in being able to estimate (or guess) the probability of finding intelligent life elsewhere in the Universe… Clue A: The characteristics of the simplest forms of life on Earth. Oddly enough, there is no clear, widely agreed upon definition of life. We know it when we see it, but how do we make the classification? One way of constructing a definition is to make a list of properties of life and then review the list to determine how adequately one can use these properties to define life. The Russian philosopher and historian Alexander Koyre observed: "What is life? – this question concerns us all and is one of the most important in cosmology. We divide the world into living and nonliving things but still have no widely accepted meaning of the word life. Complex organisms are composed of cells, which are composed of molecules, which are composed of atoms; and it is not clear at what level of complexity life first emerges. The cell is a miracle of the physical world

and required billions of years to evolve; dare we exclude it, assert that it is nonliving, and claim that life is manifest only in complex multi-cellular organisms? "Living organisms feed, grow, move, reproduce, and behave in response to their environment. Many things admittedly non-living exhibit similar properties. An automobile moves and consumes food; a crystal grows; a candle flame needs nourishment, reacts to its environment, and self-reproduces with sometimes alarming consequences. Manmade automatons are extremely intricate, and computers now play chess with each other. With so many nonliving things mimicking the characteristics usually ascribed to organisms, it is difficult to pinpoint exactly what defines life. Are we to believe that self-reproduction and evolution are the hallmarks of life? According to biochemistry, self-reproduction is possible in highly organized chemical systems, and according to biology, evolution operates automatically by means of natural selection. The physical world, it seems, has an astonishing power for creating organized complexity, and there is nothing of a physical nature

that sets life apart from the rest of the physical world." Life on Earth is based on the process of reproduction where organisms inherit the characteristics of their ancestors via a chemical blueprint or genetic code that is reproduced whenever the organism is replicated and can be altered or mutated by unpredictable outside influences, such as impacts of high-energy photons. This altered genetic code leads to organisms with differing traits, some of which allow the organism to more successfully adapt to its environment. One working definition for life is that which carries a genetic code that serves as the blueprint for reproduction and evolution of successive generations of organisms. By evolution, we mean the change or modification in a biological entity in response to its environment, a somewhat different view of evolution than we have used in contemplating the evolution of planets, stars, galaxies, or the Universe, which often evolve without environmental stimulus. Life on Earth is based on complex carbon molecules, which can link to form gigantic molecules

containing millions of atoms. No other element comes close to forming molecules with the diversity and complexity that carbon can. The carbon compounds containing one or more carbon-hydrogen bonds are called organic molecules, even if they are not found in living organisms. Other important elements in the compounds found in living systems are nitrogen, oxygen, sulfur, and phosphorus; all are formed via nuclear fusion in stellar cores. Life, or at least life as we know it on Earth, is based on complex self-reproducing organic molecules, DNA in particular (Figure 1). DNA is a long chain-like molecule that can split and then each half is able to attract new atoms so it can rebuild the whole, thus reproducing itself. DNA stores information in terms of molecular sequences on its chain structure, genetic information used to biochemically construct a complete organism. Although the DNA in an oak tree, a toad, and a person are similar, the detailed differences in the DNA structure make the three quite different. Amino acids are large organic molecules used to build protein molecules, which link in different shapes and structures (Figure 2). Proteins are used in

living organisms to build cells and as enzymes to catalyze chemical reactions necessary for life. Cells are in turn organized into entire organisms. Groups of cells in multi-cellular organisms perform specialized functions essential to the organism as a whole. Cells in the human body, for example, include skin, muscle, nerve, and blood cells. III. The Miller-Urey Experiment & Chemical Evolution Our second clue...Clue B: The natural process of formation of amino acids on Earth. Is there evidence that the biochemical processes that occurred in the evolution of life on Earth happen naturally, so that we would expect them to occur again on another planet? In the early 1950s, Stanley Miller, a twenty-three year old graduate student at the University of Chicago performed a chemical simulation of the primitive Earth (Figure 3), inspired by his mentor, Nobel laureate Harold Urey. He constructed a closed

container in the laboratory, a network of glass chemical plumbing, which housed an atmosphere of hydrogen, methane, and ammonia gases, along with water, which they thought best represented the composition of the Earth’s primitive oceans and atmosphere, the result of volcanic outgassing. Through this mixture, Miller passed electrical sparks, analogous to the lightning that must have zapped the early Earth. After several days Miller halted his experiment and analyzed the reddish sludge coating the interior of his apparatus. He discovered an organic residue rich in amino acids. This experiment has been repeated many times using different compounds to represent the early atmosphere and different energy sources, ultraviolet radiation for example; amino acids are always created. It should be emphasized that neither Miller nor anyone since has created life in a test tube. The significance of the Miller-Urey experiment is that it showed how a simple chemical building block for life could be expected to naturally evolve in the environment provided by the primitive Earth.How life evolved from amino acids to single-celled organisms was still a mystery, but the Miller-Urey experiment produced an unwarranted wave of

optimism in biological predestination -- the notion that the evolution of life on Earth was preordained once the necessary ingredients were initially assembled under appropriate conditions in a primordial "soup", tidal pools filled with organic matter concocted in a real world version of this simulation. Once amino acids and other prebiotic (pre-life) organic molecules are formed, chemical and molecular self-organization would take over. In the decades after the Miller-Urey experiment, scientists began to appreciate the inherent ability of matter to self-organize -- assemble copies of itself as long as the supply of components lasts. Julius Rebek, Jr., a chemist at MIT, announced in 1991 that his research group synthesized an organic molecule, amino adenosine triacid ester (AATE), structurally related to proteins and nucleic acids, that would assemble replicas of itself. This is not the creation of a primitive life form, however. AATE reproduces in a contrived environment not related to the primitive Earth and reproduces too accurately, not allowing for mutations or mistakes that would allow the molecule to evolve toward one more efficient at reproduction. 

It was popular after the Miller-Urey experiment and its sequels to assume that the broad sketch of the origin of life had been drawn; it was only a matter of time before the details of the picture were filled in. Experiments would eventually lead to a picture including chemical evolution -- detailing the formulation of prebiotic molecules and molecular evolution, the assemblage of more complex precellular structures, membranes, protocells, and organelles (a cellular component that provides a specialized function). However, hypothetical speculation regarding the origin of life flourishes while hard experimental fact is scarce. We do not know what the prebiotic conditions were on Earth. It is far from conclusive that life was inevitable. Fred Hoyle, the renowned British astronomer, once commented that the spontaneous evolution of life from a mixture of chemicals was about as likely as the random assembly of a 747 from a tornado tearing through a junkyard.The atmospheric conditions on Earth at the epoch of the formation of life are uncertain. It is not clear that the Earth had an atmosphere dominated by hydrogen compounds, such as methane and ammonia, favorable for the synthesis of biomolecules as

assumed by Miller. It is possible that the Earth was a boiling inferno during the life-forming epoch, resulting from a large abundance of carbon dioxide in the atmosphere causing an intense greenhouse effect. It is also likely that the Earth was still in the throes of intense cratering events of the magnitude that would excavate dust high into the Earth's atmosphere, shutting it off from sunlight for long periods, similar to the hypothetical dinosaur-destroying impact. Because of the low yield of relevant biomolecules in the Miller-Urey experiment and their tendency to combine into more complex non-biological molecules, it is unlikely that life formed in a thick broth of organic molecules. Some essential biomolecules, including some sugars, have never been produced in the Miller-Urey experiment or its successors. Interactions between biomolecules and solid surfaces, such as clays or other minerals could have directed the production of prebiotic molecules. Although amino acids show a large degree of self-organization, they do not produce the specific reactions that would lead to efficient chemical evolution.

Chemical evolution, like biological evolution (discussed below) is based on the self-replication of molecules -- the use of a molecule as a template for the assembly of identical molecules. If the assembly is flawless, the daughter molecules inherit precisely the same traits as the mother molecule and evolution cannot take place. If the assembly process allows for assembly mistakes, the daughter molecules differ from the mother and will have somewhat different chemical properties or traits. Traits that allow for more efficient replication will result in a greater abundance of daughter molecules. Molecules that reproduce less efficiently will produce fewer progeny and will eventually die out. Although self-replicating molecules aren't generally regarded as life, they were the likely precursor of life. Clue C: The discovery of organic compounds in space. Do you remember discovery of amino acids in meteorites found on the Earth (Chapter 11, Section IV)? Sri Lankan meteorite chemist Cyril Ponnamperuma discovered all of the basic amino acids that code genetic information in DNA in a

single carbonaceous chondrite, but found no evidence of life itself. Emission from molecules of the amino acid glycine, floating free in interstellar space, has even been detected by radio astronomers. Juan Oro of the University of Houston suggested the possibility that life originated elsewhere in the Universe and was transplanted to Earth in a meteorite. This notion is not popular among scientists; microbes have never been found in the hostile environment of space. However, the omnipresence of complex orgnanic molecules in space, including amino acids, vividly illustrates the ease in which they are synthesized. In the 1950s, the American chemist Sidney Fox found that amino acids repeatedly heated and dissolved in water would form spheres of short protein chains, called proteinoids, with structural similarities to cells. Proteinoids fall short of primitive life: they don't reproduce or evolve. However, the Miller-Urey experiment and Fox’s work give us a tantalizing hint that life could easily have evolved elsewhere in the Universe, given a physically and chemically friendly environment, including carbon compounds and liquid water.

  IV. Primitive Life on Earth The building blocks for life on Earth are simple molecules of two types, amino acids and nucleotides. These molecules are made from carbon, nitrogen, oxygen, and a tiny abundance of other elements. Amino acids are joined together in living organisms into chains called proteins. Nucleotides are similarly joined to form the molecular chain of the complex organic molecule DNA, found in the nuclei of cells. DNA is the long molecule with the structure of a double helix that carries the genetic information of a cell. The English scientist Robert Hooke introduced the concept of a living cell in 1665. The theory that all organisms are built from cells was proposed in the 19th century. Single celled organisms vary from small bacteria with diameters only hundreds of times the diameter of the hydrogen atom to six-inch ostrich eggs (Figure 4). Multi-cellular organisms contain interacting cells of differing function, which depend upon each other for survival. Humans have roughly

1014 cells each of which contains 1014 atoms (Figure 5).  Clue D: The fossil record of evolution of life on Earth indicates that at least 1 billion years were necessary for the appearance of life. How cells originated and evolved is not known but a likely picture of their origin can be built through educated speculation. The first cells may have been primitive bacteria that consumed organic molecules and derived their energy directly from them. As the number of primitive bacteria rapidly increased the supply of organic chemical "food" on Earth dwindled. With the decline of chemical food sources, any simple, photosynthesizing, unicellular organisms that developed would have an advantage and would increase in population at the same time that the more primitive bacteria became decimated. The earliest form of life in the fossil record is the non-photosynthesizing bacterium found in Australian rocks dated at 3.5 billion years old.

(Figure 6). Photosynthesizing unicellular organisms are found in rocks dated at 1.9 billion years old. After the origin of photosynthesizing organisms, the Earth’s atmosphere was steadily altered with the production of oxygen by photosynthesis in preparation for the evolution of oxygen-breathing life forms that would come later. The Earth’s atmosphere also developed an ozone layer that protects the surface from life-threatening ultraviolet radiation. Six hundred million years ago the Earth experienced the so-called Cambrian explosion, an abrupt change from the slow steady pace of evolution of primitive life forms to a rapid evolution of a rich variety of organisms of widely varying complexity. Fossils are seen in every rock of the Cambrian epoch capable of supporting fossils. The cause of this evolutionary explosion is not clear. A variety of invertebrates, such as insects, appeared in the fossil record in the first hundred million years or so of the Cambrian era, with an explosive increase in the number of vertebrate (such as mammals) and plant species following. Humankind has only evolved in the last several million years.  

V. The Theory of Biological Evolution The modern theory of biological evolution was proposed independently by the English naturalists Charles Darwin and Alfred Wallace in 1858. Darwin has received most of the credit based on his famous book "Origin of the Species." Evolution is based on the ideas that organisms in a single species differ by small amounts. These small differences allow some individuals to adapt better to the environment than others. The environment continually stresses individual organisms. Organisms that have an edge will live longer and produce more offspring than the more poorly adapted. Offspring will in turn preferentially inherit those traits that are beneficial to survival. The environment favors some inherited traits and disfavors others. Disfavored traits will then tend to be lost in successive generations, while favored traits will tend to represent more and more individuals in the entire population. This process is called natural selection. One of the most well known examples of natural selection is seen in the evolution of some species of moth. In modern times, light-colored moths living in

highly industrialized areas have developed dark protective coloration in order to blend into dark, soot-covered surfaces. The theory of evolution proposed by Darwin and Wallace did not explain the mechanism of inheritance of traits or the origin of individual differences. We now know that inheritance occurs through the genetic code of DNA that carries traits from generation to generation through its chemical self-replication. Mutations are changes in the DNA, which happen when it is chemically or physically transformed, e.g. because of intense exposure to radiation, or through a glitch in the chemical self-reproduction process. VI. What Conditions are Necessary for Life? Clue E: The discovery of extrasolar planets, especially Earth-like planets. Recent discoveries of planets and protoplanetary dust disks orbiting nearby stars (Chapter 12) indicate to some astronomers that planets orbit nearly all

stars. However, how many of these planets could potentially support the evolution of life? Life as we know it requires organic compounds and liquid water for support and sustenance. If a planet is too close to its parent star its surface exceeds the boiling point of water and is too hot to harbor life. If it is too far from its parent star it is below the freezing point of water and would is cold to support life. In order for a planet to sustain life, it must meet the "Goldilocks Criterion": not too hot and not too cold.  Life on Earth has evolved over a time interval of several billion years. Stars more massive than about two solar masses would not live longer than one billion years, and would therefore be much less likely to evolve intelligent life than a one solar mass star. On the other hand stars with masses less than about 0.5 solar masses only have a very small volume surrounding them that would give a planet temperatures in the life-sustaining range.  What evidence do we have for the existence of other habitable planets in the Milky Way? In our own

Solar System, only the smaller terrestrial planets (Mercury, Venus, Earth, and Mars) and Pluto have solid surfaces suitable for the evolution of life. Mercury is too close to the Sun for liquid water and Pluto is too far. The dense atmosphere of Venus produces a large greenhouse effect to make this planet far too hot to permit life. Among the planets, only Mars and Earth appear habitable. Mars appears once to have had a warmer climate and flowing water and could have supported life. Viking Lander measurements of the Martian surface did not detect life, but these measurements were not definitive. Extrasolar planet detections have not yet produced the unequivocal detection of an Earth-like planet in the habitable zone of another star, but the detection game has just begun. VII. The Drake Equation: Life in Other Star Systems? How can we estimate the probability for life elsewhere in the Universe? Now we can combine our clues to guess the probability of finding intelligent life anywhere else in the Milky Way.

The Drake equation, Equation 28.1 below, provides an estimate of the number of civilizations in the Milky Way that are technologically advanced to the point where we can potentially communicate with them. The Drake equation is named after its proponent, the American astronomer Frank Drake. 

Equation 28.1: N = R fp ne fl fi fc L, where

R is the rate at which solar-type stars form, fp is the fraction of stars with planets, ne is the number of planets per star system

suitable for life, fl is the fraction of planets suitable for life where

life actually arises, fi is the fraction life-bearing planets where

evolution to intelligent life occurs, fc is the fraction of planets with intelligent life

that develop a communications technology and chooses to use it for the search for extraterrestrial intelligence, and

L is the expected lifetime of such a civilization. 

The Drake equation presents the number of civilizations that we can possibly communicate with as a product of terms that we can estimate. It separates a complex problem into the product of a number of terms that can be individually dealt with more easily. We will use the Drake equation to develop a pessimistic estimate and an optimistic estimate of the number of alien civilizations that we can potentially communicate with (Table 28.1)

 Table 28.1: Estimates of the Number of Detectable

Civilizations from the Drake EquationFactor Lower

Estimate

Higher Estimate

R 1 per year

1 per year

fp 0.5 0.5

ne 0.01 0.1

fl 1 1

fi 0.1 1

fc 1 1

L 100 years

1 million years

Number

0.05 50,000

  R, the rate of formation of stars with properties friendly to the evolution of life. Since life originated 3.5 to 4 billion years ago on the Earth, we can speculate that it would take intelligent life 3 to 4 billion years to evolve on any planet in the galaxy. Armed with only one example of the occurrence of life on a planet we can assume nothing else. If this assumption is correct it would be useless to search for intelligent life on any planet orbiting a star that is so massive that it will not live for at least 1 billion years. We would expect intelligent life-bearing planets to exist in orbit about mid-mass main sequence stars like the Sun. Statistical studies of stars in the Milky Way estimate that one such star forms in our galaxy each year.

 fp, the fraction of stars that form planets. Many think that all non-multiple star systems form planets with stable, nearly circular orbits. Any planets found in multiple star systems would have highly non-circular orbits, resulting in highly varying surface temperatures, unsuitable for life. An estimate of the fraction of stars without stellar companions is roughly 0.5 (Chapter 12). ne, the average number of environmentally habitable planets orbiting stars that have planets. The remaining factors in the Drake equation are uncertain at best. The mean number of planets with an environment suitable for the evolution of life orbiting stars that have planets is one if we use the number of planets that we know are hospitable to the formation of life in our own Solar System. A planet must be far enough from the Sun that the Sun’s energy doesn’t vaporize the planet’s atmosphere or destroy life with blistering heat. At the same time a planet can’t be so far away from its central star that it receives too little life-giving warmth to support life. ne could be 1 from our own experience, or much

less, say 0.1, if we want to make a conservative estimate. It is hard to estimate fl , fi , and fc, but we know they can’t be greater than 1. Let’s choose fl = 1, fi = 0.1, and fc = 1 for a conservative estimate, and fl = 1, fi = 1, and fc = 1 for an optimistic one. The expected lifetime of a technological civilization -- one that has developed the tools enabling it to communicate with alien civilizations -- cannot be estimated with any degree of reliability. We on Earth have had the radio communication technology for interstellar communication over the latter half of the 20th Century. We don’t know whether a technological civilization on Earth will be limited to a 100-year period by weapons of mass destruction, pollution, famine, and disease or whether we as humans will learn from our mistakes before it is too late. Estimates could legitimately range from 100 years to many millions of years. Let’s take L=100 years as a pessimistic estimate and one million years as an optimistic estimate. The resulting estimates for the total number of intelligent civilizations in our galaxy that we could

communicate with ranges from much less than one to one million. Our conclusion is that we have no idea whether there are no other civilizations in the Milky Way that we could communicate with, or countless numbers of such civilizations. We can only attempt such communications. Are there civilizations in other galaxies? The existence of such civilizations is academic, for the time required to send them a communication at the speed of light and await a return answer would be a minimum of millions of years, perhaps longer than our own civilization will endure.  IIX. The Search for Intelligent Life on Other Planets How do we search for extraterrestrial civilizations? How would they choose to communicate with us? The use of electromagnetic radiation is the logical choice. Humans already use radio waves for communication on Earth. Light is the fastest means of communication known. Radio waves in particular are an optimum means for

communicating across interstellar space, as they easily penetrate clouds of dust and gas. If we agree that radio waves are the logical choice for communication, how do we choose which frequencies to listen to? We might assume that aliens would use the same criteria as we would. We would choose a clear frequency with as little interference as possible. A band of relatively noise-free frequencies exists in the microwave region of the spectrum. Twenty-one centimeters is also a good choice because astronomers build large radio dishes for measurements at 21-cm to study the distribution of hydrogen atoms in space and alien astronomers might be expected to do the same. We currently have the technology to detect extraterrestrial communications, and have had it for several decades. One of the first searches for extraterrestrial intelligence, abbreviated SETI, was made in 1960 by Frank Drake using radio telescopes to monitor nearby solar stars for radio transmissions. NASA and private funding sources have sponsored the SETI work into the 1990s using computers and radio telescopes in an all-sky survey examining

hundreds of stars over a range of radio frequencies. The largest radio telescopes, including the 1000-foot diameter Arecibo radio telescope in Puerto Rico, have been used to monitor tens of millions of radio frequencies simultaneously (Figure 7). What is the motivation for us as a society to spend tax dollars on SETI? An extraterrestrial contact would be one of the greatest events in human history. The scientific discoveries that would be communicated to us in a short period could be enormously profound causing a quantum leap in technological development. The impact on society would change on our culture in ways that we can’t predict.  We have been sending radio and television communications into space for the last 75 years. Are other intelligent civilizations monitoring us for signs of life? Would they reply? Would they even recognize us as intelligent after receiving decades of sitcom transmissions such as Mr. Ed and the Dukes of Hazard? Will our descendants be here to receive transmissions in another 100 years? IX. Epilog 

In this book, we have studied the five great questions of Origin: 1. How did the Universe begin? 2. How did galaxies form? 3. How are stars born? 4. How did the Solar System originate? 5. How did life come about? In fact, these are not separate questions -- they are all connected and share the same physical laws. We can rephrase all five questions into one: How did we come about, from the time of creation until now? This is not an applied research question that will directly bring us a more comfortable life with more conveniences. It is a cultural question, a question emanating from our natural curiosity. Is it a scientific question? Parts of it are, in that they are testable. However, we will never be able to replicate the experiment as a whole. In the end, as far as science has advanced in the 20th Century we have only scratched the surface of these five fundamental questions. We are only beginning to frame the right questions; we are still far from answers.Summary In previous chapters, we have seen the historical development of the Principle of Mediocrity, which states that we as humans occupy no central place in the Universe. Counter to this principle is the Anthropic Principle, which says that the Universe has no meaning without intelligent life to witness its existence. Although there is no universally acceptable definition of life, a working definition is that it consists of those objects that grow, adapt, and reproduce and whose reproduction is governed by some genetic code. Life on Earth is based on organic compounds, amino acids, and nucleotides. That these complex molecules could have been produced naturally in the Earth’s primitive environment was shown in the Miller-Urey experiment, which produced

amino acids in a laboratory container. Amino acids have also been discovered in meteorites and in the interstellar medium. What conditions are necessary for life? In order for a planet to incubate life, it must be in a moderate temperature range where water can exist in liquid form. A planet can be neither too close nor too far from its parent star. The evolution of life also requires time, at least 1 billion years from the singular example of the evolution of life on Earth. This necessitates life forming on a planet orbiting a star with a main sequence lifetime of at least 1 billion years. The earliest appearance of an organism in the fossil record is a primitive bacterium seen in some Australian rocks dated as 3.5 billion years old.  Charles Darwin and Alfred Wallace proposed the modern theory of biological evolution in 1858. Evolution is based on the inheritance of traits and the preferred success of organisms that have traits that allow them to better adapt to the environment over organisms with less-favored traits. These traits are transmitted from generation to generation via DNA, the chemical blueprint for life on Earth.  The Drake equation estimates the number of extraterrestrial civilizations in the Milky Way that could potentially communicate with us. This number is the product of the rate at which solar-type stars form in the galaxy, the fraction of stars orbited by planets, the number of habitable planets per star having planets, the fraction of planets suitable for life where life actually arises, the fraction of life-bearing planets where evolution to intelligent life occurs, the fraction of planets with intelligent life that develop a communications technology and choose to use it for the search for extraterrestrial intelligence, and the expected lifetime of such a civilization. The resulting estimate of the number of developed civilizations in the Milky Way varies widely, from one to tens of thousands. It is likely that a civilized society would attempt to communicate with us by transmitting radio messages. Scientists are currently monitoring nearby solar-type stars for radio communications.

Key Words & Phrases 

1. amino acid - a type of organic molecule that serves as the building block for proteins.2. Anthropic Principle - Without intelligent life, the Universe is meaningless, for there is

no one to study or contemplate its existence.3. DNA - a complex organic molecule with the structure of a double helix that carries the

genetic blueprint of life.4. Drake equation – the equation that estimates the number of intelligent civilizations in

the Milky Way with which it is possible to communicate.5. invertebrate - animals without segmented spinal columns.6. natural selection – the process in the theory of evolution in which organisms with

favorable inherited traits reproduce with more abundance that those with unfavorable inherited traits.

7. nucleotide - a family of organic compounds out of which DNA is composed.8. organic molecule - a molecule containing one or more carbon atoms.9. Principle of Mediocrity - humans have no central role in the Universe.

10. protein - a family of organic molecules used in living organisms to build cells and as enzymes to catalyze chemical reactions necessary for life.

11. self-organization - the ability of molecules to replicate.12. SETI – an abbreviation for Search for Extraterrestrial Intelligence.

 13. vertebrate - animals with a segmented spinal column, including mammals and reptiles.

 

 Review for Understanding 

1. Explain the Anthropic Principle in your own words.2. Explain the Principle of Mediocrity in your own words.3. What is the Drake equation? Write it, explaining each of the terms in your own words.4. Give your own definition for life.5. Why would 21-cm radio radiation be an excellent wavelength for extraterrestrial

communication?6. What is natural selection? Give an example.7. What is the scientific significance of the Miller-Urey experiment?8. Why would communication with extraterrestrial civilizations be difficult?9. Why are planets orbiting high-mass main sequence stars unlikely to support intelligent

life?10. What is the evidence of earliest life on Earth?

  Essay Questions 

1. Is the Anthropic Principle a scientific principle? Explain.2. Do you think that it is likely that we are the only civilization in our own galaxy? Discuss

your reasoning.3. Do you think that a space-faring civilization would recognize us as intelligent life, worthy

of communication? Discuss.4. Define evolution. How are biological evolution and stellar evolution fundamentally

different?5. What is molecular self-organization? How does it differ from biological evolution?6. Why do scientists believe in a common origin for life on Earth?

 Figure Captions

 Figure 1. A model of the structure of DNA. Modify. Arny. Figure 2. Schematic of amino acids and proteins. Figure 3. A schematic diagram of the Miller-Urey experiment. In the 1950s, this experiment showed that amino acids, one of the chemical building blocks of life, could easily be produced in an environment similar to the primitive Earth. FMW 589-E.4 Modified. Figure 4. Photos of a bacterium and an ostrich egg. Figure 5. Fossils of blue-green algae, and other primitive life.

 Figure 6. Timeline of the development of life forms on Earth. Modify. Arny.

 Figure 7. The Arecibo 1000-foot diameter radio telescope in Puerto Rico. Arecibo is the largest telescope on Earth. P 320-19.8.