In the Beginning was Abiogenesis

In current biological terminology "abiogenesis" refers to the emergence of life as self-replicating assemblages of organic chemicals able to control chemical energy from non-life assemblages of chemicals. This is the hypothesis that primordial life originated within the first billion years of Earth's history as a result of chemical reactions that generated larger and larger organic polymers, which ultimately attained control of bioenergetics and the property of chemical self-replication. This modern conceptualization is reasonable in view of what is now known of the biochemical basis of all life.

The modern conceptualization replaces earlier notions of "spontaneous generation" that viewed life as arising fully formed from non-living matter, or de novo. Such Aristotelian notions of archebiosis probably arose because much of life exists at the microscopic level of prokaryotes and unicellular eukaryotes, and because some organisms living on detritus reach visible proportions after having been of microscopic size.

The term 'life' in relation to abiogenesis embraces the notion of utilization of energy sources and biochemical self-replication. This requires that an assembly of molecules and macromolecules acquired the ability to convert energy into growth and that informational macromolecules directed self-replication.

The processes that led from inorganic chemicals to the panoply of organic, carbon-based chemicals in primordial cells would have been governed by the non-random physical laws of chemistry, specifically those physical properties related to organic molecules. Since the processes involved would be widespread across the primordial planet, enormous numbers of molecules and macromolecules would have been generated in the first 10 billion years of Earth history. Because modern cells contain only L-amino acids, it is believed that a single primitive cell line, the universal cenancestor is the ancestor of all living cells. According to most theorists, conditions on Earth are no longer suitable for abiogenesis to occur. Unfavorable conditions apply because oxygen is 'toxic' to molecules.

The elucidation of abiogenetic mechanisms would be of enormous interest, yet this has not been a particularly active area of research because it has limited applications. The advent of space exploration has infused life into Astrobiology and the search for the Origin of Life.

The lack of a full scientific explication for abiogenesis does not mean that chemical evolution was not the origin of life. Rather, the difficulty in deciding between theoretical scenarios reflects difficulties in recreating conditions and the impossibility of recreating the time frame for biopoiesis. Certainly, biopoiesis is a much better explanation than the God of the Gaps, which is merely simplistic magic-thinking.

From NASA's Astrobiology Institute
Spitzer Telecope Data Suggest that Life's Building Blocks are Abundant
Finding Life in Mars Analog Sites on Earth
More on Astrobiology:
Evolution and Ecology Beyond the Planet of Origin
Evolution and Development Workshop
Evolution and Ecology Beyond the Planet of Origin
Evolution and Development Workshop
New York Center for Studies on the Origins of LifeAd Astra Magazine: Astrobiology Issue (January/February 1999)
The Astrobiology Web
Free audio on WAMC's Origins of Life Segments
Origins on Abiogenesis
Recommended on the Talk Origins Archive:
Lies, Damned Lies, Statistics, and Probability of Abiogenesis Calculations
Borel's Law and the Origin of Many Creationist Probability Assertions
Spontaneous Generation and the Origin of Life

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What Is Life?

What are the minimal descriptors of life – when do we decide that a collection of chemicals has crossed the line to that of inhering life processes?

The minimal essential features are compartmentalization, self-reproducibility, incorporation of energy controls, and adaptability to the environment. The essential features of biochemical systems include compartmentalization, genetic programming, and enzyme systems that both adapt to the environment (including feedback controls) and utilize or defy energy (generation of products in excess of non-enzymatic equilibria). In other words, if we put the most basic chemical constituents into a flask, we would not have life processes unless we also had the essential features listed above.

Compartmentalization within lipid-bilayer membranes permits seclusional selectivity and chemical control of reactants and products. Compartmentalization also permits the localization of energy storage/utilization, and the control of translation of genetic instruction. Further, membrane-bound systems control internal responses to the external environment. Self-reproducibility – regeneration – utilizes genetic programming. Molecular programming likely first utilized RNA, and later evolved DNA as a more stable system for storage of instructions.


PLoS Biology: What Is Life--and How Do We Search for It in Other Worlds?: " Koshland (2002) lists seven features of life: (1) program (DNA), (2) improvisation (response to environment), (3) compartmentalization, (4) energy, (5) regeneration, (6) adaptability, and (7) seclusion (chemical control and selectivity). A simpler definition is that life is a material system that undergoes reproduction, mutation, and natural selection. Cleland and Chyba (2002) have suggested that life might be like water, hard to define phenomenologically, but easy to define at the fundamental level. But life is like fire, not water--it is a process, not a pure substance. Such definitions are grist for philosophical discussion, but they neither inform biological research nor provide a basis for the search for life on other worlds."

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Universal cenancestor

The universal cenancestor is hypothesized as being at the ancestral root of all living organisms. Not the earliest or simplest living organism, and not necessarily the sole example of its type, this organism possessed the genetic material that diverged (about 3.5 Ga) into all current living organisms.

A number of terms are employed to refer to the universal cenancestor – last universal ancestor (LUA), last common ancestor (LCA), or last universal common ancestor (LUCA).

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Origin of Life and Early Life on Earth

Thematic Issue: Origin of Life and Early Life on Earth
1. Origin of Life on Earth: Prebiotic Synthesis
2. Continental Crust and Oceans on Ancient Earth.
3. On Carbon Monoxide in Comet Hale-Bopp.
4. On the Earliest Traces of Life on Earth.
5. On Earth's Earliest Fossils.
6. On the Evidence for Earth's Oldest Fossils.
7. On the Akilia Rocks and Earth's Earliest Life.
8. Early Earth: Carbonaceous Meteorites as a Source of Sugars
9. On the Origin of the Earth.
10. Life and the Evolution of Earth's Atmosphere.
11. On the RNA-World Hypothesis
12. Origin of Life: The Present Status of Chemical Theory

Evolutionary oscillation in prebiology: igneous activity and the origins of life. Entrez PubMed: "The processes of chemical evolution are responsible for the origin of life. Three such processes have special importance: oscillation, creation, and competition. An oscillation from one kind of environment to another provides a mechanism for instituting processes that can only take place under conditions far removed from equilibrium. Oscillating evolutionary processes are likely to have played an important part in the origin of life. It is a mistake to assume that life originated in any one environment. It did not arrive in a moment of time. It was the result of a long period of chemical evolution during which it passed through a variety of environments. Biopoesis took place in an environment in which a variety of different kinds of protolife were assembled and concentrated. One essential form of protolife involved in these processes is the protocell. The experiments of Fox suggest that the creation of protocells involves violent oscillations of temperature and hydration. Igneous activity is especially characterised by oscillating conditions. Volcanic eruptions consist of violent changes from one extreme condition to another. Temperatures, pressure, phase, concentration and hydration all oscillate violently, and are subject to shock pulses of many kinds. Protolife may well have passed through extremes of environment for wider that those that life itself can sustain. The most probable environment for the assembly of the various forms of protolife would be on mudbanks forming either at the mouth of streams draining regions of active vulcanicity, or round the edge of hot volclanic pools. In this situation one could fins concentrated not only the various stands of protolife necessary for the final act of biopoesis, but also perbiologically formed nutrients necessary as for the first eobionts. As soon as the first protocells start to grow, they start to compete with each other, and so initiate a new additional evolutionary process, that of natural selection. Only after such competition has been initiated is life itself likely to be established."
Sylvester-Bradley PC. Evolutionary oscillation in prebiology: igneous activity and the origins of life. Orig Life. 1976 Jan;7(1):9-18.

Impact frustration of the origin of life. : "One possible definition for the origin of life on Earth is the time at which the interval between devastating environmental insults by impact exceeded the timescale for establishing self-replicating proto-organisms. A quantitative relationship for the Hadean (pre-3,800 Myr ago) and Early Archean (3,800 to 3,400 Myr) impact flux can be derived from the lunar and terrestrial impact records. Also, the effects of impact-related processes on the various environments proposed for abiogenesis (the development of life through chemical evolution from inorganic materials) can be estimated. Using a range of plausible values for the timescale for abiogenesis, the interval in time when life might first have bootstrapped itself into existence can be found for each environment. We find that if the deep marine hydrothermal setting provided a suitable site, abiogenesis could have happened as early as 4,000 to 4,200 Myr ago, whereas at the surface of the Earth abiogenesis could have occurred between 3,700 and 4,000 Myr."
Maher KA, Stevenson DJ.Impact frustration of the origin of life.Nature. 1988 Feb 18;331(6157):612-4.

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Before the Beginning

Based on radio-dating of meteorites, the solar system is about 4600 Ma – 4600 million years, or 4.6 billion years old (Ga). This occurred 10 Ga after the Big Bang. The sun and planets condensed from a large, hot accretion disk.

When early Earth was cooling from its molten state, conditions would have been like hell on Earth – hence the term "hadean" for this earliest segment of the precambrian. Meteors rained down on the hot Earth, ultraviolet radiation was unchecked by an ozone belt, and volcanos belched fumes into a reducing atmosphere, which sparked with lightning.

Life, of course, had not yet evolved, but it may have had an earlier start than thought previously (see below).

The term "hadean" was coined to designate the time before the earliest known rocks. However, rocks have been found that are older than the 4550-3800 Ma time-frame of the Hadean.

The oldest rocks so far discovered on Earth are:
1) Jack Hills, Western Australia, a 4.4 Ga detrital zircon (sample W74) in the Jack Hills metaconglomerate, Eranondoo Hill. More at Earliest Piece of Earth
2) The Acasta Gneisses near Canada’s Great Slave Lake (4.03 Ga)
3) The Isua Supracrustal rocks of West Greenland (3.7 to 3.8 Ga)
4) Northern Michigan (3.5-3.7 Ga)
5) Swaziland (3.4-3.5 Ga)

“No known rocks have survived from the first 500 million years of Earth history, but studies of single zircons suggest that some continental crust formed as early as 4.4 Ga, 160 m.y. after accretion of the Earth, and that surface temperatures were low enough for liquid water. Surface temperatures are inferred from high d18O values of zircons. The range of d18O values is constant throughout the Archean (4.4-2.6 Ga) suggesting uniformity of processes and conditions. The hypothesis of a Cool Early Earth suggests long intervals of relatively temperate surface conditions from 4.4 to 4.0 Ga that were conducive to liquid-water oceans and possibly life. Meteorite impacts during this period may have been less frequent than previously thought.”A Cool Early Earth

More: Introduction to the Hadean / Palaeos Hadean: The Hadean Eon / Geol 2C Hadean lecture / Evolution: Change: Deep Time / Geologic Time: Age of the Earth /

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The RNA world on ice: a new scenario for the emergence of RNA information.

Entrez PubMed: "The RNA world hypothesis refers to a hypothetical era prior to coded peptide synthesis, where RNA was the major structural, genetic, and catalytic agent. Though it is a widely accepted scenario, a number of vexing difficulties remain. In this review we focus on a missing link of the RNA world hypothesis-primitive miniribozymes, in particular ligases, and discuss the role of these molecules in the evolution of RNA size and complexity. We argue that prebiotic conditions associated with freezing, rather than 'warm and wet' conditions, could have been of key importance in the early RNA world."
Vlassov AV, Kazakov SA, Johnston BH, Landweber LF. The RNA world on ice: a new scenario for the emergence of RNA information. J Mol Evol. 2005 Aug;61(2):264-73. Epub 2005 Jul 13.

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Primordial Atmosphere.

A hydrogen-rich early Earth atmosphere. : "We show that the escape of hydrogen from early Earth's atmosphere likely occurred at rates slower by two orders of magnitude than previously thought. The balance between slow hydrogen escape and volcanic outgassing could have maintained a hydrogen mixing ratio of more than 30%. The production of prebiotic organic compounds in such an atmosphere would have been more efficient than either exogenous delivery or synthesis in hydrothermal systems. The organic soup in the oceans and ponds on early Earth would have been a more favorable place for the origin of life than previously thought."

Tian F, Toon OB, Pavlov AA, De Sterck H. A hydrogen-rich early Earth atmosphere. Science. 2005 May 13;308(5724):1014-7. Epub 2005 Apr 7. Comment in: Science. 2005 May 13;308(5724):962-3.

The evolution of the prebiotic atmosphere : "High CO2 levels are required to warm the primitive earth in the face of decreased solar luminosity. The atmosphere should have had an effective stratospheric cold trap, which would have limited the abiotic production rate of oxygen to relatively low values. Photostimulated oxidation of ferrous iron in the oceans should have been the dominant source of atmospheric H2. Rainout of H2O2 would have kept the atmospheric H2 content high and the O2 content low, even if other sources of H2 were small."
Article / PubMed / ChemPort
Kasting, J.E. The evolution of the prebiotic atmosphere. Origins of Life 14, 75−82 (1984).

Nitrogen fixation by corona discharge on the early precambrian Earth. : Entrez PubMed: "We report the first experimental study of nitrogen fixation by corona discharge on the anoxic primitive Earth. The energy yields of nitric oxide (NO) and nitrous oxide (N(2)O) were experimentally determined over a wide range of CO(2)-N(2) mixtures simulating the evolution of the Earth's atmosphere during the Hadean and Archean eras (from 4.5 ba to 2.5 ba). NO, the principal form of fixed nitrogen in lightning and coronal discharge in early Earth, is produced ten times less efficiently in the latter type of electrical discharge with an estimated maximum annual production rate of the order of 10(10) g yr(-1). For N(2)O the maximum production rate was estimated to be approximately 10(9) g yr(-1). These low rates of syntheses indicate that corona discharges as point discharges on the clouds and ground did not play a significant role in the overall pool of reactive nitrogen needed for the emergence and sustainability of life."

Nna-Mvondo D, Navarro-Gonzalez R, Raulin F, Coll P. Nitrogen fixation by corona discharge on the early precambrian Earth. Orig Life Evol Biosph. 2005 Oct;35(5):401-9.

Neon isotopes constrain convection and volatile origin in the Earth's mantle.: "Identifying the origin of primordial volatiles in the Earth's mantle provides a critical test between models that advocate magma-ocean equilibration with an early massive solar-nebula atmosphere and those that require subduction of volatiles implanted in late accreting material. Here we show that neon isotopes in the convecting mantle, resolved in magmatic CO2 well gases, are consistent with a volatile source related to solar corpuscular irradiation of accreting material. This contrasts with recent results that indicated a solar-nebula origin for neon in mantle plume material, which is thought to be sampling the deep mantle. Neon isotope heterogeneity in different mantle sources suggests that models in which the plume source supplies the convecting mantle with its volatile inventory require revision. Although higher than accepted noble gas concentrations in the convecting mantle may reduce the need for a deep mantle volatile flux, any such flux must be dominated by the neon (and helium) isotopic signature of late accreting material."Ballentine CJ, Marty B, Sherwood Lollar B, Cassidy M. Neon isotopes constrain convection and volatile origin in the Earth's mantle. Nature. 2005 Jan 6;433(7021):33-8. Comment in: Nature. 2005 Jan 6;433(7021):25-6.

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Efficient near ultraviolet light induced formation of hydrogen by ferrous hydroxide.

Entrez PubMed: "The formation of hydrogen on irradiating ferrous ion in aqueous solution or suspension was studied over a wide range of pH. In addition to the known reaction in acid solution which decreases in yield with increasing pH and required far UV light, there is an efficient reaction occurring between pH 6 and 9 which utilizes near UV light. The latter reaction is an approximately linear function of both the concentration of ferrous ion and the light intensity. The quantum yield of hydrogen from the suspension of Fe(OH)2 at pH 7.2 is very high: > or = 0.3. The quantum yield decreases by a factor of five at 1 mole percent of ferric ions. To explain these observations it is proposed that an intermediate formed on excitation of the Fe(OH)2 polymer is further reduced by a neighboring Fe(+2) to form H2. These results support the work of Braterman et al. (1983) which claimed that the near UV driven photooxidation of ferrous ions could be responsible for formation of the Banded Iron Formations on the early earth. The efficient photoreaction observed in the present work could also serve as a source of active reducing equivalents to reduce CO2 and thus provide a solution to a dilemma in the arguments on the role of reduced carbon in the origin of life."
Borowska ZK, Mauzerall DC. Efficient near ultraviolet light induced formation of hydrogen by ferrous hydroxide. Orig Life Evol Biosph. 1987;17:251-9.

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