This is a chapter from my book The Emergence and Nature of Human History, Volume One. The main section of the book is a chronology of the events that led to the emergence of human consciousness. The chronology employs Carl Sagan's device of condensing the Universe's age down to one year, with the Big Bang occurring on 1 January. The section also measures time through the use of an imaginary timeline one million meters in length.
I am not a scientist; I am a (very minor) historian and former history teacher. But I am writing a "deep history" of the world, and so I was forced to venture into subjects which are typically the province of physicists, chemists, and biologists. If I have made egregious errors of any kind, or even minor ones, please correct me.
You can read the first chapter of the chronology here.
You can read the second chapter of the chronology here.
You can read the third chapter of the chronology here.
You can read the fourth chapter of the chronology here.
You can read the fifth chapter of the chronology here.
THE EARLIEST LIFE ON EARTH
PERHAPS AS EARLY AS ABOUT 18 SEPTEMBER; ABOUT 715,000 METERS UP THE LINE
The world of living things we know today is the product of an inconceivably long, complex, and varied process of organic evolution. But how and when did this evolutionary process start? How did life arise from non-life, and is it possible for us to know with any degree of certainty how this happened? And there is a more fundamental question: what is life, in the biological sense of the word? What definitions make the most sense and possess the greatest empirical support? And when we look for the deep origins of life on the Earth, to see how the biosphere of which we are now a part came into being, what exactly are we looking for?
Credible Definitions of Life
As we saw in the previous chapter, the early Earth developed conditions favorable for the emergence of life. But defining the line between the living and the non-living is more difficult than it looks at first. There are self-organizing patterns that appear spontaneously throughout nature, as we have seen, patterns brought about by the passage of energy through a set of particles or other units of matter. If by such a process a pattern grows and develops, such as formations of crystals, would we say that such a pattern is alive? I think we would be hard-pressed to do so. So what is the fundamental defining characteristic of life? Many of us would probably say that it is the potential for self-reproduction. A living thing reproduces by gathering energy from the environment. It does this in order to maintain both its own physical equilibrium and to produce a new living entity with characteristics similar to its own. So it would seem, at first, that the ability to reproduce is the rock-bottom, absolute defining characteristic of living things.
And yet, viruses live in the shadowlands between the living and the non-living. They are incapable of self-reproduction, but by seizing a portion of a living object they can indeed reproduce prodigiously. Are they alive, or are they merely potentially alive? And what of living things that cannot reproduce during their lives? Are they not alive, after all? What is the specific set of properties all living things share in common that distinguish them from the non-living?
Biologist William Schopf points out that all living things in the world today share a common chemical heritage. Living things are primarily constructed out of hydrogen, oxygen, nitrogen, and carbon, all very simple elements, very ordinary stuff. Living beings may have other elements in them, such as sulfur and phosphorous, but these four essential elements (what Schopf simply calls CHON) make up an astounding 99.9% of all living systems.1 So perhaps this is where our search for a definition of life can begin. Is life simply that which possesses a specific kind of chemical composition?
Obviously, it is not enough to simply know the chemical composition of life. What do living things characteristically and uniquely do? In the early 1940s Erwin Schrödinger said that living things, above all, are entities that gather energy to temporarily stave off the effects of entropy. Entropy in this instance is understood as total physical equilibrium—in biological terms, death.2 Schrödinger said that the organization and structure produced by life were distinctive in the physical realm, different from the behavior of any other thing, and yet the behavior of life and its tendency to resist disorder were fully consistent with the known laws of physics.3 So is the defining characteristic of life the ability to temporarily stave off the effects of entropy?
In 1944 Oswald Avery discovered DNA, and in 1953 James Watson and Francis Crick, assisted by Rosalind Franklin, elucidated DNA’s structure. It was now clear that DNA was the “molecule of life”. DNA is the polymer (complex molecule) that, in conjunction with ribonucleic acid, RNA, holds the “instructions” by which structure and organization in living things are brought about. So can we argue that living things are physical objects that by definition possess nucleic acids? Certainly every living thing or potentially living thing in the world today does. But is there more to living things than the mere possession of the requisite nucleic acids? After all, living things are active participants in the environment, entities which perform a variety of functions and which are affected by multitudinous events. How can we make our criteria more rigorous?
Hungarian biologist Tibor Gánti, in a 1971 work, laid out a very convincing set of criteria by which life can be defined. He distinguishes between the absolute criteria of life and the potential criteria. The absolute criteria, in his view, are the following:
1. Living systems are individual units which have properties in combination that the individual components that comprise the system do not possess in isolation. [In my view, Gánti is saying that living entities are essentially synergistic.]
2. Living systems are engaged in metabolism. Metabolism is the use, by a living entity, of energy-matter ingested or absorbed from the environment, energy-matter which is changed into forms the living thing can use to maintain its life functions. Certain forms of dormant life, such as seeds, may, however, be possible exceptions to this criterion.
3. A living system displays a degree of stability. Changes in its surroundings do not change its basic functions. [Of course, Gánti is discussing environmental changes that fall within a living thing’s survival parameters. Obviously environmental disruptions can kill something off.] Living things display both the capacity for homeostasis [the maintenance of internal stability through the operation of internal feedback mechanisms] and excitability [the ability to respond to stimuli].
4. Living systems carry information within themselves that is necessary for their construction, development and functioning. This information is contained in subsystems within the living thing, and this information is acted on by other subsystems capable of “reading” and using it.
5. Living things possess internal processes that need regulation. These internal processes must be regulated so that the living thing can go on living. Even a living entity that is not reproducing requires such regulated systems.
Gánti then describes the criteria that define the potential properties of life:
1. Living systems have the potential for reproduction. Not all members of a group of living things possess this ability, nor do all cells in an animal body, but at least some individual entities in each instance must have this capacity if the group or individual animal is to survive.
2. A living system must have the capacity for hereditary change and evolution. Obviously, since an individual organism does not evolve, this criterion applies only at the group or population level.
3. Living systems tend to be mortal. This is not an absolute criterion because it is thought that certain cells (such as cancer cells) and some kinds of bacteria may be immortal. But in general, living things die, and it is definitely one of their potential traits.4
I think these criteria are as rational and complete as any I have seen, and I think them a reasonable basis on which to proceed. They incorporate much of the thinking of other biologists as well. So in looking for the origin of life, we will be looking for physical entities that were the chemical precursors of the first objects to display these criteria, and then finally the first objects that actually exhibited such characteristics themselves.
Hypotheses About the Origins of Life
By the end of the 19th century the advent of evolutionary ideas in biology and the advances made in the studies of chemistry and geology had made it evident to most scientifically-educated people that the Earth was very old, that life forms had changed over time, and that the stories handed down by the various religions about the origins of the Universe and humanity were nothing more than creation myths, the products of particular cultures and their narrative literary traditions. These myths, frequently allegorical in nature, were colorful and of literary and historical interest, but they were of no scientific value whatsoever. (See the chapter Myths: Biological in Origin, Cultural in Dissemination in a later volume)
As the decades of the 20th century passed, evidence about the natures of the Universe, the Earth, and life poured in from every science. Evolutionary concepts deepened, augmented by the tremendous discoveries being made in genetics, which ultimately led to what is known as the Neo-Darwinian synthesis. But one intractable problem remained: how had life itself originated?
The search for a fully naturalistic solution to this problem has been underway for many decades. The Russian biochemist Alexander Oparin began writing about this issue in the 1920s. Oparin originally was seeking to refute vitalism, the view that living things consist, in part, of non-material or ultimately mysterious features. Oparin wanted a fully materialistic explanation of life. He ultimately concluded that the early Earth must have had a non-oxygenated atmosphere, and that the crucial processes that brought about life on Earth had begun in the ocean. He hypothesized that the coagulation of organic compounds in that setting led to the formation of droplets that displayed rudimentary metabolic functions. It was by this process, Oparin believed, that the transition from non-life to life had begun.5 British biologist J. B. S. Haldane also thought that the early Earth had had a non-oxygenic atmosphere, and he conjectured that the earliest life was actually a viral-like entity, half-way between life and non-life, that displayed primitive gene-like properties. Ultimately there emerged the Oparin-Haldane hypothesis. It rejected vitalism completely and asserted that life had a totally materialistic basis. It assumed the early Earth had a strongly reducing atmosphere and this atmosphere, in conjunction with a “primordial soup” of elements in the ocean and inputs of electrical energy, had produced first organic monomers (simple molecules) and then organic polymers.6
As we saw, the ultimate organic polymers are DNA and RNA. We have come to understand that DNA and RNA are so complex that both must have appeared only after an extensive process of chemical evolution. To explain how life really began, it is therefore necessary to find the chemical processes and precursors from which nucleic acids evolved. Gradually, as our knowledge of nucleic acids has grown, the issues surrounding the beginning of life on this planet have come into sharper focus. It is now clear that the challenges are greater than the first investigators into life’s origins had imagined. Consequently, it now appears that the following questions must be answered in order to determine how life originated:
1. How did amino acids and nucleotides (a chemical base, a sugar group, and a phosphate group) come to exist?
2. How were amino acids and nucleotides assembled into macromolecules like proteins and nucleic acids, a process which requires the presence of catalysts?
3. How were these macromolecules able to reproduce themselves?
4. How were these reproducing macromolecules assembled into systems which had boundaries that separated them from their environments (in the manner of cells)?7
If we begin to consider these questions solely by looking at modern life forms, we run into a thorny issue immediately:
The essential problem is that in modern living systems, chemical reactions in cells are mediated by protein catalysts called enzymes. The information encoded in the nucleic acids DNA and RNA is required to make the proteins; yet the proteins are required to make the nucleic acids. Furthermore, both proteins and nucleic acids are large molecules consisting of strings of small component molecules whose synthesis is supervised by proteins and nucleic acids. We have two chickens, two eggs, and no answer to the old problem of which came first.8In response to such difficulties, certain researchers have argued that life did not originate on this planet at all, but rather was brought here by objects from space that crashed into the Earth, objects which contained organic molecules. This hypothesis is known as panspermia, although its current advocates say that it should be understood more broadly. It can be traced back to a Swedish chemist named Svante Arrhenius, who in the early 20th century hypothesized that microorganisms from extraterrestrial worlds had drifted to Earth and seeded this planet with its first living things. But panspermia’s most ardent defenders have been the late Fred Hoyle, a noteworthy astronomer who argued comets were the chief source of our planet’s earliest life, and one of his former students (and a prominent scientist in his own right) Chandra Wickramasinghe. (Hoyle, as we have already seen, made major contributions to the study of stellar nucleosynthesis; see The First Stars) Unfortunately, in promoting panspermia, these two researchers published a series of increasingly tendentious scientific papers in the 1970s and 1980s. These papers made claims so shaky and so open to criticism by other scientists that some researchers concluded sadly that Hoyle’s convictions had become a kind of “cometary religion.”9
There are major objections to the panspermia hypothesis, notable among them the observation that it merely pushes the questions about the rise of life back to an unknown extraterrestrial location without explaining how life arose there. And if life can originate beyond the Earth, and survive journeys of enormous distances through inconceivably inhospitable environments, why would the Earth itself not be a suitable environment for life’s origins, given the Earth’s many advantages? Although its advocates are adamant in their contention that they have accounted for the presence of life on this planet, and although their studies of what are known as extremophiles (life forms that can exist in extremely harsh conditions, such as conditions found in interplanetary or interstellar space) demonstrate the possibility of panspermia, most researchers believe that there is no need to look beyond our own world for an explanation of terrestrial life. Yes, many researchers think extraterrestrial organic compounds may have augmented or complemented the development of life on the Earth, but these same scientists contend that these compounds were not the origin of terrestrial life. Additionally, advocates of panspermia sometimes have the unfortunate habit of asserting that if a prominent scientist states that some life forms could have arrived from space then these same scientists are, in effect, saying that they must have done so, and that these scientists have therefore “endorsed” panspermia.10 This is a deliberate twisting of the facts, and it only makes panspermia’s claims to be objectively scientific more dubious.
If we assume a terrestrial origin for life on this planet, there must have been some simple and direct way by which pre-organic monomers, the simplest kinds of molecules, were formed. In order to understand how such monomers must have been produced, we need a credible picture of conditions on the early Earth. The first real experiments on the origin of life in fact tried to replicate such conditions, based on certain assumptions about the early Earth’s atmosphere, primarily those of the Oparin-Haldane hypothesis. These experiments were conducted in the 1950s by a University of Chicago graduate student, Stanley Miller, and U. of C. Professor Harold Urey. Miller and Urey used an apparatus that employed water, methane, ammonia, and hydrogen gas. Electrical energy was passed through the apparatus periodically. The water represented the early ocean. The methane, ammonia, and hydrogen represented the postulated early atmosphere. The electrical discharges represented the lightning that was assumed to be commonplace on the early Earth. The two investigators reasoned that among the chemical compounds produced in this process there might very well be organic ones. The net results of these experiments were, however, ambiguous. Certain amino acids were produced, but only two of them are significant in the production of proteins. Moreover, Miller and Urey’s assumption that the early atmosphere was dominated by methane and ammonia is no longer thought to be valid. So although there were indeed intriguing results from these experiments, they do not appear to lead us in the right direction.11
Among origin-of-life researchers there is a fundamental divide which seems to exist between those who argue that metabolic-like processes, through the harnessing of catalyzing chemicals, must have preceded replication, and those who argue that replication itself must be given primacy. There are eloquent arguments in favor of each position. In assessing these viewpoints, we must consider hypotheses about self-organizing tendencies in the physical world, the nature of the early Earth’s atmosphere, the role of the primitive world ocean, the surface conditions prevalent on the Earth in its first several hundred million years, the nature of molecular evolution in general and the evolution of enzymes in particular, the effects on nascent life forms of hostile environmental factors such as intense radiation, the ability of membranes to form and in so doing to encompass small quantities of liquid (the origin of protocells), and a variety of other factors, all of which must be considered. Most significantly, hypotheses about life’s origins must be tested through the use of the most rigorous experimentation. What are some of the scenarios that have emerged, therefore, since the days of the Oparin-Haldane hypothesis and the Miller-Urey experiments?
Natural selection, as we will see in more detail below, is the process by which organisms flourish or decline based on their reproductive success in a given environment. It is usually thought of only in association with life forms. Manfred Eigen, a Nobel laureate who has devoted much of his career to studying the origin of life, has argued that natural selection, rather than being the result of the existence of living things, brought about the emergence of living things by acting on non-living, but self-replicating molecules. Natural selection, he says, is a physical principle that operates in defined situations. When non-living entities begin to replicate, he believes, natural selection will begin to work.12 Eigen contends that the prebiotic world was chemically diverse enough to allow for the emergence of a cycle where very short sequences of RNA and very basic kinds of enzymes evolved and in effect drove each other’s development in a reciprocal manner. However, Eigen’s critics claim that he has not adequately accounted for the emergence of his postulated short RNA sequences, and that the production of proteins by these simple kinds of replicators would be difficult. So while Eigen has given us valuable insight into how a system can evolve once nucleic acids exist and the genetic process has begun, his postulated scenario may not have occurred.13
The emergence of RNA was a crucial event in the evolution of complex life forms. In 1986 Harvard biochemist Walter Gilbert coined the phrase The RNA World in an essay. Gilbert hypothesized that RNA could have been self-organizing and self-catalyzing, and indeed ribonucleic acid has the interesting property of being able to both store information, as DNA is able to, and catalyze reactions, as proteins are able to. But the production of full-fledged RNA from a prebiotic environment is simply too unlikely, given the extreme difficulties inherent in constructing nucleotides. The generation of ribose, a simple sugar, from a non-organic setting seems to be particularly problematic as well. Further, nucleotides display chirality, or “handedness”. They come in forms that mirror each other but which cannot be superimposed on each other. (Through the application of specific tests, chemists assign a designation of left-handed or right-handed to particular molecules.) If the building blocks of nucleic acids are nucleotides, and if they can produce chains that have both left-handed and right-handed varieties, this presents a problem. RNA can be built from either variety (although in our cells it is entirely of the right-handed kind) but it cannot grow from a mixture of left- and right-handed varieties. This has led many scientists to abandon the idea that RNA was the original molecule of life. They are instead looking for prebiotic molecules that are nonchiral, ones which can be readily synthesized and which can form polymers. Important progress has been made in this area of research, and it is now thought that RNA replaced its antecedents and began functioning independently.14
In an article published in American Scientist in 2009, physicist James Trefil, biologist Harold J. Morowitz, and physicist Eric Smith laid out a hypothesis about the origin of life that does not require the presence of RNA or DNA in its earliest steps, and may not require the presence of even a rudimentary cell. Their hypothesis is an example of the metabolism first viewpoint. The three scientists postulate that the process that led to life may have begun with basic, simple chemical reactions taking place in porous varieties of minerals. These reactions would not have required the presence of complex enzymes. Small molecules, arranged in simple networks in certain minerals, would have served as catalysts for these reactions. If a network of small, simple molecules generated its own constituent parts, it would be the core of a recursive chemical system, a system these researchers call self-amplifying. [I would call it synergistic.] These scientists hypothesize:
that such a system arose and that much of that early core remains as the universal part of modern biochemistry, the reaction sequences shared by all living beings. Further elaborations would have been added to it as cells formed and came under RNA control, and as organisms specialized as participants in more complex ecosystems.15Feedback mechanisms that would eliminate side reactions, mechanisms these researchers call “self-pruning” features, would be absolutely essential. They would concentrate reacting molecules to a limited series of pathways in the same way metabolism does, and advocates of metabolism first are seeking these mechanisms.
And why must these initial chemical steps on the path toward life be of the most basic kind? To eliminate the need for highly improbable random events, such as the appearance of complex nucleic acids out of a prebiotic environment. In the metabolic process today, we can see preserved the actions of simple networks of small molecules. Why, the authors ask, did the non-living world bring forth such reactions to begin with? It may have been something as simple as the fact that the laws of physics tend to “prefer” low states of energy to high ones. Just as water seeks the lowest point in whatever area it is found in, moving from a high energy state to a lower one by forming and flowing in channels down the side of a hill, so chemical channels were created on the early Earth. The researchers contend that “reservoirs of energy” accumulated in the non-living world of our planet’s first eon, masses of electrons and certain other ions “seeking” to release their energy. The earliest chemical reactions that led in the direction of life facilitated the release of this energy by establishing biochemical channels acting in concert with each other.
The authors point out that in metabolism as seen in the modern world the citric acid cycle, or Krebs cycle, breaks down organic molecules into carbon dioxide and water. Oxygen is used to effect this break-down, and the outcome is energy. Organic molecules are, in a sense, being “burned” as fuel. But the cycle can work in reverse, taking in high-energy electrons from carbon dioxide and water and using them to construct large molecules out of smaller ones. Trefil and his colleagues maintain that this reverse-cycle, known as the reductive mode of energy transfer, must have operated on the early Earth since the earliest atmosphere was non-oxygenated. (This reductive process still operates in certain anaerobic organisms.) The reductive mode gives electrons a method of lowering their energy content and allows for the efficient organization of molecular networks operating in a cyclic fashion. This simple cycle can then interact with other chemical cycles. The process that produces the essential oils that are used in the construction of cell membranes is a pathway that starts through such interaction.
These simple chemical reaction systems are an early manifestation of order (or in my view, emergence). This order, the authors believe, existed prior to the onset of replicating molecules. The three researchers argue that there logically must have been a stage during which these primitive systems evolved in the direction of molecules with replicative abilities. It was these early replicating molecules that allowed for the processes of natural selection (see below for a discussion of this process) to begin operation.
The authors believe that research along these lines, looking for the rise of chemical and biological complexity by examining the most basic form of metabolism, will show us the true pathway to life. They conclude:
If this notion turns out to be true, it will have important implications for a deep philosophical question: whether we should understand the history of life in terms of the working out of predictable physical principles or of the agency of chance. We are, in fact, arguing that life will appear on any planet that reproduces the environmental and geological conditions that appeared on the early Earth, and that it will appear in order to solve precisely the sort of ”stranded electron” problem [electrons that are unable to lower their energy state] discussed above. [Emphasis added]16One of the most arresting hypotheses about the origin of life comes from chemist A. G. Cairns-Smith. The title of his 1982 study Genetic takeover and the mineral origins of life tells us that he approaches this subject from a unique perspective. He begins his study by reviewing the various hypotheses that had been offered up to that time about the origin of organic polymers, and pointing out their implausible aspects.17 Cairns-Smith argues that the essence of the origin of life problem is a genetic one: “What would have been the easiest way that hereditary machinery could have formed on the primitive Earth?”18 He believes that the genetic mechanisms we see in operation today are the advanced replacements for the first, ultra-simple hereditary mechanisms.19 It is his contention that these ultra-simple systems first emerged as crystals in clays, forming what he calls “inorganic genes”.20 He argues that crystals can store information (through structural defects of various kinds) and replicate structures.21 Remarkably, he further contends that these clay structures were subject to natural selection and began self-propagating. Later, he argues, as carbon and nitrogen became incorporated into such systems, energy from sunlight (a primitive form of photosynthesis) helped form chemical structures that ultimately permitted the formation of amino acids, which began the road to nucleotides and true organic evolution.22 (By the way, the rather advanced chemistry in this text forces a layperson like me to depend heavily on the sections which summarize the evidence.)
It is a remarkable contention: the first living things were replicating minerals, clays which possessed crystalline genes. Many observers have examined this hypothesis, but it must be said that very few of them have agreed with it. Cairns-Smith himself has always hoped that experiment can produce (or discover) clay-based life, but for now his ideas must be considered simply interesting conjectures, without any major empirical support.
Could life have emerged around the hydrothermal vents of the deep oceans? Geneticist Paul Lurquin has examined the issue in some detail. Hydrothermal vents are formed when cracks in the Earth’s crust, which is considerably thinner under the oceans, bring water in contact with magma. The water becomes superheated, but because of the immense pressures at those depths it does not boil. Rather, underwater “chimneys”, the hydrothermal vents, form. These vents are frequently the homes of distinct ecosystems. Lurquin points out that the combination of high pressure and heat can produce chemical conditions in which the production of every amino acid and the formation of small proteins is possible. Experiments which replicate the conditions surrounding hydrothermal vents have been done, and, in Lurquin’s words they have found that,
a whole catalog of organic molecules, including amino acids and pyruvic acid (an important metabolite ubiquitous in living cells) could be formed at high pressure and temperature from H2S,[hydrogen sulfide] CO, and CO2, as well as ammonia (from nitrate) and nitrogenated hydrocarbons. Interestingly, iron sulfide (FeS) was absolutely necessary to catalyze these reactions, to generate hydrogen for reduction reactions, and to concentrate and stabilize the reaction products. This mineral is abundant in the earth’s crust in the form of pyrrhotite. Although the formation of nitrogenous bases as found in RNA and DNA has not been reported, hydrothermal vent chemistry seems to be much more than just paper chemistry [a hypothesis without supporting evidence].23So perhaps it is possible that chemical evolution began in the depths of the world ocean, a product of the geological conditions of the Hadean Eon. The evidence is indeed intriguing, and research along these lines is promising.
There is a debate among biologists and other scientists over the issue of autocatalysis, the ability of molecules to manufacture their own catalysts. Stuart Kauffman, whom we encountered earlier, in our discussion of self-organizing systems, has contended that once particular kinds of pre-biotic polymers reach a critical stage of complexity their interactions will become autocatalytic. Because of this tendency, in Kauffman’s view, life is an expected phenomenon in such a chemical setting as the Earth. Kauffman’s hypothesis has come under serious criticism, and even if such autocatalytic systems did emerge their exact nature and function have still to be determined. Moreover, if they existed they may have been much simpler than Kauffman originally proposed. However, the contention that some sort of autocatalytic molecules were a necessary (but not wholly sufficient) condition for the emergence of life on this planet seems to be gaining support. Indeed, it is difficult to see how pre-organic molecules could have begun to evolve without the ability to utilize energy from the surrounding environment to sustain their reactions.24
Origin of life research continues unabated. As evidence about the nature of the early Earth continues to accumulate, as research into hydrothermal vents expands, as increasingly sophisticated tools of mathematical analysis are used more and more extensively to evaluate such issues as the reproductive capacity of autocatalytic sets, and as all the diverse aspects of this problem are probed, more progress will be made. Most crucially, as the scientific community’s members continue to review and question each other’s work, suggesting avenues of research, asking hard questions, demanding rigorously-conducted experimentation, and expanding lines of communication among various disciplines, the likelihood grows that a definite, empirically-demonstrable explanation of the origin of life on Earth will be discovered at some point in the 21st century. Such a discovery is not a certainty, because as this very brief survey of hypotheses has shown (and I have omitted several others), the problems surrounding this issue are as difficult as any faced by the sciences. But if we do discover the processes of chemical evolution that led to the emergence of the universal common ancestors of all living beings in the world today, it will be an event as momentous as any in human intellectual history.
1. Schopf, J. William. Cradle of Life: The Discovery of Earth’s Earliest Fossils, p. 107
2. Schrödinger, Erwin, What is Life?, pp. 69-71
3. Schrödinger, pp. 76-82
4. The Principles of Life (Summary of Tibor Gánti’s work) located here: http://home.planet.nl/...
5. Fry, Iris. The Emergence of Life on Earth: A Historical and Scientific Overview, pp. 66-71
6. Fry, pp. 71-77
7. Kimball’s Biology Pages, located at http://users.rcn.com/...
8. Trefil, James Harold J. Morowitz, and Eric Smith. “The Origin of Life” in American Scientist, May-June 2009.
9. Shapiro, Robert. Origins: a Skeptic’s Guide to the Creation of Life on Earth, pp. 224-247
10. Introduction: More Than Panspermia, located at: http://www.panspermia.org/...
11. Shapiro, pp. 98-106
12. Fry, 104-105
13. Fry, pp. 107-111
14. Knoll, Andrew H. Life on a Young Planet: The First Three Billion Years of Evolution on Earth, pp. 76-80
15. Trefil, et al.
16. Trefil, et al.
17. Cairns-Smith, A. G. Genetic takeover and the mineral origins of life, pp. 45-60
18. Cairns-Smith, p. 70
19. Cairns-Smith, p. 120
20. Cairns-Smith, pp. 160; 257-258
21. Cairns-Smith, pp. 264-273
22. Cairns-Smith, pp. 357-365
23. Lurquin, Paul F. The Origins of Life and the Universe, pp. 102-104
24. Wim Hordik, Jotun Hein, and Mike Steel, “Autocatalytic Sets and the Origin of Life”, from Entropy, 2010; Hordik, Kauffman, and Steel, “Required Levels of Catalysis for Emergence of Autocatalytic Sets in Models of Chemical Reaction Systems” in International Journal of Molecular Sciences, 2011.