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02-27-2002 4:57 AM
Apologies in advance for the length of this post. Enjoy!
Even though abiogenesis – the origin of life from non-life – is not related to the validity or falsehood of evolutionary theory, it is an interesting subject in its own right. Although evolutionary theory does not rest on the truth of abiogenesis, creationists in particular seem to demand that a non-supernatural origin of life be “proven” before evolution can be accepted. It is in that sense that I will undertake to provide a brief synopsis of the various hypotheses, and discuss in general terms both the positive aspects and the potential problems with each. Consider this the “Reader’s Digest Condensed Version” of abiogenesis.
The discussion of the origin of life is one of the most complex and contentious issues in science today. Because the issue is so complex, there are many, including even some scientists (almost all from outside the biological sciences), who claim that it was, in fact, impossible for life to have arisen through solely natural processes. They believe there was required some Divine Spark, or Supreme Designer, standing outside all known universal laws to initiate the process. Finally, they believe that life contains some level of organization below which recourse to merely physical laws cannot explain how it came into being – a First Event from which all else flows.
Although our knowledge in some areas may be weak or we may be missing some details today, natural abiogenesis studies begin with the premise that there is nothing unknowable in nature. Life, in the final analysis, makes perfect sense using known physical laws. The actions of a Supreme Being are not required to explain it. Reason, analogies drawn from modern organisms, and the results of scientific research from disciplines as diverse as astronomy, astrophysics, microbiology, parasitology, chemistry, biology, genetics and geology, as well as dozens of others, provide sufficient explanation. There was no First Event.
There are currently three main scientific hypotheses for how life arose on Earth. All three have their adherents, and all three are actively being researched by some of the finest scientific minds on the planet. All three have both empirical and inferential support (for elements of the theory), but all three contain certain assumptions that must be true for the theory to be valid. All three contain elements that have been shown either in the lab or in nature to lend credence to their hypothesis.
There is one unifying thread that ties all three hypotheses together which must be understood at the outset: all three hypotheses rest on a foundation of organic chemistry. Don’t get confused. “Organic” here does not mean “living” or “coming from life”. Organic chemistry is nothing more than the chemistry of carbon. It happens to be enormously richer than the chemistry of other elements - and thus able to support life - because of the unique associative properties of the carbon atom. In all likelihood the first building blocks of life arose as do all natural chemical compounds - spontaneously, according to the rules of thermodynamics. In one way of looking at it: we ARE carbon.
1. The Biotic Soup Hypothesis
This is arguably the most well-known (and least understood!) hypothesis. In essence, the hypothesis argues that chemicals available on the Hadean Earth from either terrestrial or extraterrestrial sources combined via standard chemical reactions to form biologically significant macromolecules. The biotic soup hypothesis uses an ammonia-methane atmosphere as a starting point, and naturally occurring electrical storms and radioactivity as the energy source.
Spectroscopic analysis by astronomers has revealed that space is permeated by an extremely tenuous cloud of microscopic particles, called interstellar dust, containing a variety of combinations of carbon, hydrogen, oxygen, nitrogen and, sometimes, sulfur or silicon. These are mostly highly reactive free atoms and small molecules that would hardly remain intact under conditions on earth, but in space could interact to form more stable, typical organic compounds, many of them similar to substances found in living organisms. That such processes indeed take place is demonstrated by the presence of amino acids and other biologically significant compounds on celestial bodies – for example, the meteorite that fell in 1969 in Murchison, Australia, Comet Halley (which was analyzed by the Giotto spacecraft during its 1985 passage), and Saturn's satellite Titan, the seas of which are believed to be made of hydrocarbons (based on the Voyager fly-bys) and which contains an atmosphere with significant organic compounds.
The modern chemical composition of the Earth is mostly Fe, Mg, Si, and O, with the other elements contributing 5% of the total. Life originated as a result of chemical reactions occurring (largely) in the atmosphere followed by reactions in the primeval oceans and lakes. The atmosphere at the end of the Hadean Period (~4-4.2 gya) is primarily composed of variable amounts of CO2, N2, SO2, H2S, S, HCl, B2O3, and smaller quantities of H2, CH4, SO3, NH3 and HF (but no O2), due partly to outgassing from volcanoes, and partly to the reaction of condensing water vapor (formed as the Earth cooled) with minerals such as nitrides (hence NH3), carbides (hence CH4, CO, etc.) and sulfides (hence H2S). There was no free oxygen (any free O2 would have reacted with P, Si and metals such as molten iron to give minerals e.g. iron oxides, silicates, phosphates, etc.). This atmosphere readily lends itself to the formation of small organic molecules, which in turn readily combine to form more complex macromolecules.
In the lab, tantalizing experiments attempting to re-produce the atmospheric conditions of the early Earth have produced astonishing results. As early as the 1950’s Harold Urey used simple electrical stimulation of a hydrogen-methane-ammonia atmosphere and – in just a few days – saw over 15% of the methane/carbon converted to amino acids: one of the key building blocks of proteins and hence life. Although his postulated atmosphere was probably inaccurate, the same amino acids in nearly the same proportions have been discovered in the Murchison asteroid. Since Urey, besides amino acids and other organic acids, experiments have yielded complex sugars as well as purine and pyrimidine bases, and even adenine: some of the components of the nucleic acids DNA and RNA, the genetic repositories of the codes of all life. Stanley Miller is STILL working on the problem at UC San Diego.
There are a few problems with this hypothesis. In the first place, it is impossible at this remove to determine the exact chemical composition of the early atmosphere – hence whereas the chemical reactions are quite straightforward, the relative yield is open to interpretation. In addition, many of the small organic molecule precursors such as HCN and HCHO are volatile and would break down readily in the atmosphere. It is postulated that these precursors were absorbed into the primeval ocean where they would be shielded from the damaging UV (<300 nm), creating the “organic soup” of the Urey-Miller experiments.
2. Cairns-Smith Crystal Matrix Hypothesis
One of the problems with the biotic soup hypothesis, even assuming the chemical reactions were as stated, is how these macromolecules – randomly distributed as they were – were able spontaneously to form the key biological macromolecules such as peptides and nucleic acids. The probability of the formation of these crucial biological molecules from a random mixture of organic chemicals is vanishingly small. In addition, these molecules had to be self-replicating. The chemist Alexander Cairns-Smith proposed that inorganic materials, rather than organic, represented the first replicators.
The fundamental problem he was trying to address is the requirement that the first "life" (using the term very loosely) had to have been self-replicating. Cairns-Smith speculated that the earliest replicators were not organic at all, but rather were self-replicating crystals that were later superseded by the rise of the far-more-efficient organic replicators. In this view, the first replicators were crystals of the type that exist in clay or mud along riverbanks; they transmitted their "genetic" information through the natural tendency of these types of molecules to fit together into a geometric pattern.
The fundamental characteristic of crystals as replicators must be hereditary variation, or inheritance. Fortunately, crystals in nature display this pattern: they may be perfectly aligned until a specific point is reached, in which a flaw has accumulated (these are quite common in natural crystals). This flaw has a tendency to percolate down the subsequent layers of crystal, setting up a rudimentary system of heredity. Furthermore, atoms of the crystal's substance may be more attracted to certain geometric patterns than they are to others. This sets up a kind of "differential reproduction" which then leads logically to a form of natural selection.
The hypothetical crystals described above may very well begin a basic process of cumulative selection. Certain crystals may have the property of altering streams or other water sources for their own "benefit", such as by increasing the likelihood of more of the same material being deposited in the same location. Crystals may also encourage the formation of "spores" by breaking easily into subsequent "generations" Those crystals that broke into generations most easily would be selected for; these generations would invariably contain mutations on occasion and would intensify the competition between rival variants.
In time, the crystals could evolve a sort of "phenotype" by altering other materials in their environment. These materials could be used to further the crystal's replication by inhibiting rival crystals from forming or promoting the parent crystal's reproduction. Cairns-Smith's hypothesis is that the materials used by the crystals for self-replication later turned out to be even more efficient replicators in their own right – the earliest peptide-RNA – which ultimately replaced their inorganic substrates. This process of replacement might repeat for several cycles, or the first products used by the crystals may have been the ancestors of modern replicators - i.e., RNA and eventually DNA.
The principal difficulty with Cairns-Smith’s hypothesis is the fact that clay doesn't necessarily form a lattice/matrix that is perfectly designed for the arrangement of biologically significant molecules. Since there are a rather large number of potential arrangements, getting the precise arrangement necessary to act as a catalyst for a specific molecule is pretty problematic. Finally, the type of clays best suited for this type of “inorganic evolution” are usually found in riparian zones – the smaller biological molecules are fairly unstable when subjected to unshielded UV. It remains to be seen whether such processes could occur in such a way that these molecules could persist long enough to form stable compounds.
However, as with Miller, Cairns-Smith’s organic replicator overthrow of the inorganics only needed to occur once…
3.a. Submarine Hot Springs Hypothesis – Electrochemical Variant
In this hypothesis, life is believed to have begun at the sites of warm submarine springs where chemical energy was focused and the mixing of spring water with seawater could lead to the precipitation of chemicals. The precipitation of chemicals on mixing of solutions can form a barrier preventing further mixing and precipitation. This barrier can also provide a template for the assembly of chains of organic molecules, and act as a catalyst for electrochemical reactions. This hypothetical precipitate, again operating in a naturally occurring biotic soup, consisted mainly of small groups of iron and sulfur atoms. Iron-sulfur groups still play an essential electrochemical catalytic role in all living cells.
As a boundary, the precipitate concentrated organic molecules such as amino acids. These formed at depth below the spring where water and its dissolved chemicals reacted with rocks containing Fe and iron-rich minerals. The boundary also concentrated other chemicals that could participate in chemical reactions.
As a catalyst the groups of FeSiO4 and Fe3O4 could activate molecular hydrogen (and probably methane which consists of carbon and hydrogen) which also formed at depth in the spring. The hydrogen is essential for the synthesis of organic molecules. Electrons produced as a by-product (and representing the dissipation of energy) are transferred to a type of iron, known as ferric iron, dissolved in seawater. (The ferric iron is produced from dissolved ferrous iron (richer in electrons) at the ocean's surface by sunlight. The same processes cause the reddening of the surface of Mars as iron-bearing minerals have “rusted”.
As a template, the iron sulfide precipitate (consisting of small crystals of only a hundred atoms or so), could bond chemically to, and assemble a sequence of, the molecular components of RNA. Acid springs of high temperature, coupled to emergent magma plumes, emit ferrous iron and other transition metals to the ocean. Solar energy oxidizes some iron to the ferric state, generating a dispersed positive terminal. Cooler alkaline waters emanate from the deep ocean floor, bearing hydrogen, methane, ammonia, formaldehyde, cyanide and hydrosulfide - molecules reduced from water and carbon oxides by reaction with ferrous silicate, residual nickeliferous iron and ferrous sulfide. Where these waters seep into the ocean, mounds, comprising layers of ferrous sulfide and green rust flocculants and films, arise. These mounds are where the reduced molecules are filtered and adsorbed. Concentrated, they react to form glyceraldehyde, amino acids, and the components of nucleosides.
The fluids are prevented from mixing thoroughly with the surrounding ocean by the spontaneous precipitation of a barrier of colloidal iron compounds. Nucleotides can then assemble in green rust. The thermal potential begins to be dissipated but the chemical potential is dammed. Though the hydrothermal solution is constrained, electrons escape from adsorbed hydrogen through the conducting layers of iron monosulfide, drawn to reduce the photolytic ferric iron.
There is invasion of the iron sulfide/hydroxide barrier by protons, pyrophosphate and carbonic acid, through iron sulfide-walled micro-channels. The newly formed nucleotides poison the iron sulfide but combine with peptides, producing pRNA. The side chains of particular amino acids register to fitting nucleotide triplet clefts. Keyed in, the amino acids are polymerized, through acid-base catalysis, to alpha chains by invading protons. The resulting short protopeptides sequester ready-made iron sulfide clusters to form ferredoxins, ubiquitous proteins with the longest evolutionary pedigree. These take over the role of catalyst and electron transfer agent from the iron sulfides, promote further chemosynthesis and so support the electrochemical reactor from which they sprang.
The principal problem with this hypothesis is the reliability of the invasion and precipitation scenario. To wit, how effective is the sulfide barrier and the green rust substrate at providing a template for biological macromolecules? In addition, to be more plausible, the hypothesis must assume a fairly high concentration of chemical precursors. Especially since, unlike the evaporation-concentration element of Miller’s biotic soup hypothesis, there is no specific mechanism for concentrating these molecules into sufficiently close proximity for the electrochemical bonding to take place.
3.b. Submarine Hotsprings Hypothesis – Flow Reactor Variant
Similar to the above, however instead of a postulated electrochemical mound as one of the poles, this theory using the high-temperature (300-400 C) energy found in the cracking front of submarine steam vents (“black smokers”) to provide the necessary energy.
In a hot spring, the flow of heat is constrained by the structure and constitutive properties of magma, rock and water; the gravity field, etc. The model proposes that in addition to the coherent flow of fluid, these constraints produce phase space trajectories which lead to the creation of high-energy molecular and macromolecular structures in which the particles are locked, or frozen, into coherence. The cooling particles fall and are trapped into the potential energy wells, i.e. attractors, provided by the constraining forces that bind matter together. In other words, life is an emergent property of the high-temperature chemistry and physical constraints present in the flow reactor of a submarine magma pipe.
The products of rapid heating and quenching at the cracking front follow a highly constrained trajectory, rapidly mixing with cool sea water flowing upward through a matrix of fractured rocks of enormous surface area, lined with a highly catalytic surface of clay minerals (note the use of Cairns-Smith’s clay matrix). They began their ascent as hot (~350° - 600°C), acid (pH~3.6), highly reducing fluids, and approach a low temperature end member which is cold (~2° - 20°C), slightly alkaline (pH 7.9), and oxidizing. A fraction of the thermal energy traveling from mantle to ocean is trapped into the high energy bonds of organic molecules, which remain as static equilibrium structures. The process is thermodynamically analogous to the emergence of matter from energy during the expansion and cooling of the early Universe.
As with the other hypotheses, this one also relies on the presence of organic molecules in the primeval ocean. However, unlike the biotic soup idea, these precursor molecules would be concentrated by the geophysical properties of the mantle at the site of the hotsprings.
Regardless of which of the above hypotheses ultimately leads to the creation of self-sustaining biomolecules, all show that life is merely an inherent property of chemical reactions. Any time conditions are appropriate, life (as we know it) should arise. And once we get self-replicating molecules, evolution (heritable variation, random mutation, and natural selection) + time are sufficient to explain the amazing diversity of modern life.
Science has yet to provide evidence for any of these hypotheses beyond reasonable doubt. But since all are “brand new” ideas, the only thing lacking is time… Stay tuned!