The Origin of Life

by Chemical Evolution?

 ( the science of natural abiogenesis ) 

by Craig Rusbult, Ph.D.
 


This page contains three descriptions of my views — short  medium  long — plus other views.

 
The Science of Chemical Evolution (short)

      In an attempt to explain the origin of life, scientists propose a two-stage process of natural chemical evolution:
      1) formation of organic molecules, which combine to make larger biomolecules;
      2) self-organization of these molecules into a living organism.
For each stage, scientists are learning that what is required for life seems to be much greater than what is possible by natural process.  This huge difference has motivated scientists to creatively construct new theories for reducing requirements and enhancing possibilities, but none of these ideas has progressed from speculation to plausibility.

The simplest "living system" we can imagine, involving hundreds of components interacting in an organized way to achieve energy production and self-replication, would be extremely difficult to assemble by undirected natural process.  And all of this self-organization would have to occur before natural selection (which depends on self-replication) was available.

Philosophical responses to this science are summarized
in my FAQ about Creation, Evolution, and Intelligent Design
and, in more detail, The Origin of Life: A Test-Case for Naturalism

and more science is in Thermodynamics and an Origin of Life by Chemical Evolution.
 


 
The following version is Section 6 in my "Test-Case for Naturalism" page:

The Science of Chemical Evolution (medium)

      In an attempt to explain how the origin of carbon-based life on earth occurred by abiogenesis (a natural non-biological production of life), conventional theories of chemical evolution propose two stages in the transformation of lifeless chemicals into life:  1) the formation of small organic molecules, which then combine to form larger biomolecules;  2) the self-organization of these molecules into a primitive living organism.
      Despite initial optimism following the famous Miller-Urey experiments in 1953, closer investigations have revealed major problems that have not been solved (and perhaps cannot be solved) in both stages of the proposed scenario:
      In the first stage, chemical equilibria are usually unfavorable (they are "energetically uphill") for the formation of small biomolecules and also for their synthesis into larger biomolecules.  During experiments in which there is a realistic simulation of the atmosphere on an early earth — using the probable starting molecules (H2O, plus N2 and CO2 which are stable and unreactive) instead of the improbable starting materials (H2O, plus reactive NH3 and "explosive" CH4 and H2) in the reducing atmosphere used for the Miller-Urey experiments — the yields of essential biomolecules are extremely low.  {note: Since writing this I've discovered that views on the early-earth atmosphere have changed; I'll check it and will change this part of the page if necessary.}
      Even if biomolecules could form in Stage 1, these lifeless chemicals would have reached only the starting point for the most challenging part of their journey toward life in Stage 2.  The simplest "living system" we can imagine, involving hundreds of components interacting in an organized way to achieve energy production and self-replication, would be extremely difficult to assemble by undirected natural process.  And all of this self-organization would have to occur before natural selection (which depends on self-replication) was available.

      Basically, what scientists are learning is that the complexity required for life (in terms of biomolecule formation and self-organization) is much greater than the complexity available by natural process (beginning with lifeless matter).  This huge difference has motivated scientists to stretch their imaginations, to creatively construct new theories for reducing requirements and enhancing possibilities.
      For example, in an effort to avoid a "chicken and egg" problem — in modern cells, DNA is required for protein synthesis, but protein is required for DNA synthesis — scientists have proposed that RNA (which combines the replicating ability of DNA and the catalytic activity of proteins) was the key life-producing molecule in the earliest cells.  But this "RNA World" theory now seems implausible due to the apparent impossibility of pre-biological RNA synthesis, and because the catalytic activities of RNA have not matched initial optimistic hopes.  In response, scientists are now proposing "pre-RNA World" theories with key functional roles played by other molecules, and with metabolic energy sources that would be easier to use.
      Other alternatives include variations on the classic "soup" scenario, with new environments such as a semi-evaporated pond, a seafloor hydrothermal vent, the surface of a clay-like mineral, or even another planet.  Or the first life in the universe might have been totally unlike familiar carbon-based life on earth.
      Scientists are trying to develop principles for a prebiological selection of molecules, analogous to the biological selection of genes in living organisms.  And they are continuing to explore the self-organizing properties of complex chemical systems, and to search for ways of reducing the minimal complexity required for a living system.
      What is the status of these alternative theories?  So far, none has progressed from speculation to plausibility.  But they do stimulate experimental and theoretical research, and offer the hope that eventually scientists may discover new principles to serve as the basis for new theories, and may develop a plausible explanation.  But the major reasons to question chemical evolution come from what we do know about chemistry and life, not from our lack of knowledge.

 
      Irreducible Complexity and Minimal Complexity
      In Darwin's Black Box: The Biochemical Challenge to Evolution (1996), Michael Behe illustrates the principle of irreducible complexity with a mousetrap that has five interacting parts: a base, hammer, spring, catch, and holding bar.  Each part is necessary, and there is no function unless all parts are present.  A trap with only four parts has no practical function.  It doesn't just catch mice poorly, it doesn't catch them at all.
      What are the evolutionary implications?  Behe says, "An irreducibly complex system cannot be produced directly... by slight, successive modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional.  An irreducibly complex biological system, if there is such a thing, would be a powerful challenge to Darwinian evolution." (DBB, page 39)
      For a nonliving system, the implications are even more challenging, because natural selection — which is the main mechanism of Darwinian evolution — cannot exist until a system can reproduce.  For the origin of life, we can think about the minimal complexity that would be required for reproduction and other basic life-functions.  Most scientists think this would require hundreds of biomolecular parts, not just the five parts in a simple mousetrap.
 


 
Here is the original version (from 1998, with minor revisions) before it was condensed:

The Science of Chemical Evolution (long)

To explain the origin of the first carbon-based life on by abiogenesis (a natural non-biological production of life), scientists have proposed theories of chemical evolution.  (chemical E can also be called prebiological E, prebiotic E, or abiotic E).

      Theories: Old and New
      In the 1860s, careful experiments by Louis Pasteur falsified a theory of spontaneous generation, which had claimed that life arises spontaneously from nonliving matter.  By contrast with this outdated theory, modern theories of chemical evolution propose that life formed in a process that might have been allowed by different conditions on the early earth when there was no molecular oxygen (to interfere with the formation of organic chemicals) and no existing life to out-compete (or eat) the less efficient newly-emerging primitive life forms, or to consume the organic chemicals from which life might emerge.

      A Three-Stage Theory
      Currently, conventional theories of chemical-E proposes three major stages:  1a) the formation of small organic molecules,  1b) which then combine to form larger biomolecules, and  2) the self-organization of these molecules into primitive living organisms.  Despite initial optimism following the famous Miller-Urey experiments in 1953, since then a closer investigation of important details has revealed major scientific difficulties that have not been solved, and perhaps cannot be solved, at each step of the proposed natural process:
      STAGE 1a.  In Miller's original experiments and in the variations that followed, many organic molecules were formed in small amounts.  But these early experiments used an unusually reactive mixture of ammonia, methane, and hydrogen, plus water vapor.  Since then, scientists have concluded that the major molecules in the early atmosphere were nitrogen, carbon dioxide, and water vapor.  When these less reactive chemicals are used, the yield of organic molecules is much lower (and less varied) than the small amounts in the earlier experiments.  In principle, a lower yield is expected because, for example, forming amino acids from the reactive starting molecules (ammonia, methane, and hydrogen, plus water) is energetically favorable (with a "downhill" energy change of -800 kJ/mole), but with the more authentic atmosphere (nitrogen, carbon dioxide, and water) the reaction is unfavorable (with an "uphill" energy change of +200 kJ/mole).   {note: Since writing this I've discovered that views on the early-earth atmosphere have changed; I'll check it and will change this part of the page if necessary.}
      STAGE 1b.  When we look at the proposal that these small molecules combined to form larger biomolecules, we find that in water these reactions are also energetically unfavorable.  And new difficulties arise.  For example, during protein synthesis a prebiotic reaction mixture would contain many different chemicals (L-amino acids and R-amino acids, plus a variety of more reactive molecules), and the majority of newly formed bonds would not be the special peptide bonds (linking only L-amino acids) that are found in natural proteins.  The scarcity of peptide bonds is partly due to the fact that in a watery "soup" the formation of these bonds is energetically unfavorable.  Similar difficulties would arise in the prebiotic formation of other important biomolecules.  Attempts to synthesize RNA — or even the smaller molecules (such as ribose sugars) that combine to form RNA — have been especially unsuccessful.
      STAGE 2.  Even if plenty of biomolecules could form in Stage 1b, these lifeless chemicals would have reached only the starting point for the most challenging part of their journey toward life -- organizing themselves into a living organism in Stage 2.  The simplest self-sustaining "living system" we can imagine, involving hundreds of components interacting in an organized way to achieve energy-producing metabolism and accurate self-replication, would be extremely difficult to assemble by undirected natural processes.  It is important to remember that this self-assembly would have occurred before "Darwinian" natural selection was available.  Therefore, molecules would have to organize themselves into a system that fulfills the minimum requirements for natural selection (i.e., the ability to replicate itself accurately yet with occasional minor variations that allow evolutionary change while maintaining a capability for survival and self-replication) without the benefit of natural selection.

      Alternative Theories
      The big difference between what seems required (for life) and what seems possible (beginning with lifeless matter) has motivated scientists to stretch their imaginations, to creatively construct new ideas for reducing requirements and enhancing possibilities, to somehow make them match, thus producing a more plausible theory for the origin of life.
      In Stage 1a, for example, since the early atmosphere seems unfavorable for Miller-Urey syntheses, perhaps organic compounds were imported from space by comets, meteorites, and interstellar dust.
      And in Stage 2, an effort to avoid a tough "chicken and egg" problem — in modern cells, DNA is required for protein synthesis, but protein is required for DNA synthesis — inspired theories proposing that RNA (which combines the replicating ability of DNA and, to a small degree, the catalytic activity of proteins) was "the main molecule" in the earliest cells.  These primitive RNA-based systems could then evolve, gradually developing the ability to use proteins and DNA.
      This RNA World scenario now seems less appealing than when it was originally proposed, largely due to the apparent impossibility of RNA synthesis in prebiotic conditions, and also because RNA functionality (catalytic activity, self-replicating ability,...) has not matched the initial optimistic hopes.  In response, recent theories have proposed a simpler pre-RNA World with key functional roles played by other molecules (pantetheine or coenzyme A, pyranosyl RNA,...), and with metabolic energy sources (such as high-energy thioester bonds) that would have been easier to use, although less plentiful.
      Other alternatives include variations on the classic "soup" scenario, with new environments such as an isolated semi-evaporated pond, or a seafloor hydrothermal vent that might supply heat and sulfur compounds to serve as energy resources for primitive organisms.  Or maybe the original biogenesis occurred in an extremely different environment -- on another planet.  Or instead of limiting the possibilities to a self-contained cell in a watery organic soup, another theory proposes that inorganic clay-like minerals played an important function by interacting with organic molecules in the earliest forms of life.  Or the first life might have been totally unlike familiar carbon-based organisms.
      Scientists are trying to develop principles of a prebiological "molecular selection" that was analogous to biological natural selection.  /  And there is a continuing search for ways to reduce the minimal complexity that would be required for a system with self-sustaining metabolism and replication.  /  But instead of imagining a simplification, some scientists are looking toward complexity for answers to abiogenesis.  Stanley Kaufmann speculates that within a complex mixture of chemicals there can be a spontaneous production of an organized autocatalytic network of reactions that is a self-replicating system, and the beginning of life.  {details}

      What is the status of these alternative theories?  So far, none has progressed from speculation to plausibility.  Their main practical functions are to provide ideas for continuing experimental and theoretical research, and to offer hope for proponents of chemical evolution.  This hope takes two forms:  maybe a modification of a current theory, or a combining of these theories, can be developed into an explanation that is at least moderately plausible;  or scientists may discover new scientific principles that will form the basis for improved new theories.
      These hopes are the basis for a conventional response to criticism: "Please be patient; we don't have a plausible theory now, but eventually we will."  Maybe.  Currently, however, this optimism is based more on assumption than evidence;  and the major reasons to doubt the possibility of chemical evolution come from what we do know about chemistry and life, not from our lack of knowledge.
      Or perhaps life did originate once by a natural mechanism, but this unique event was so unlikely and strange (and therefore difficult for scientists to imagine) that we will never develop a theory for natural abiogenesis even though it did occur in nature's history.
      On the other hand, if scientists do eventually construct a plausible theory for chemical-E, supported by experiments that create self-replicating "life", we should not abandon critical thinking.  Why?  Experiments for "simulation" of biogenesis usually are carefully designed with great cleverness by intelligent scientists, and the simulation conditions are often unrealistic due to adjustments that make the experiment "work better."   Therefore, it would be appropriate to ask:  In realistic prebiotic conditions, what is the probability of the proposed scenario?  Without the intelligent experimental design, would the observed natural-creation still occur?  On the early earth, would the variety of environments and the vast amount of time (maybe more than a hundred million years) offset the effects of design in the experiments, thereby making what is observed in the lab a reasonable approximation of what could have occurred in the real world?
      And even if a scenario seemed plausible, this might not constitute a strong argument that it really did happen this way because — in contrast with biological-E, whose general "succession of species" theme is supported by abundant evidence in the fossil record — there is no historical evidence for the process of abiogenesis.  This absence of evidence would make it difficult to show that biogenesis occurred by a particular natural process.  And even though, as discussed earlier, there are empirically based reasons for thinking it is extremely improbable that life did arise by a natural process, it is difficult to argue for a "negative conclusion" in science.  On the other hand, even if scientists were convinced that chemical E is almost impossible, theories about "Many Worlds in a Multiverse" could provide an escape, a way to rationalize any degree of improbability without acknowledging the need for a designer/creator.  Whether or not miraculous-appearing Theistic Action has ever occurred in nature (in the origin of life or elsewhere) is a fascinating question, but our attempts to answer this question will not yield a "proof" that will satisfy everyone.  And a logical standoff where "nothing can be proved" should be no cause for alarm for anyone, theist or atheist, pantheist or agnostic.

 


 

Other Views of Abiogenesis by Chemical Evolution

Most of this page describes my views about an origin of life by a natural "chemical evolution" abiogenesis.  But other scientists have other views.  Most scientists agree that current naturalistic theories have weak support, but estimates for the construction of future theories — or the further development of current theories, especially the "complexity" theory outlined above and described in more detail below — vary more widely.

      This section begins with a view that doesn't seem scientifically justifiable.
      Although a natural origin of life by chemical evolution seems highly implausible, the National Academy of Sciences confidently asserts (in Science and Creationism, 1999) that "For those who are studying the origin of life, the question is no longer whether life could have originated by chemical processes involving nonbiological components.  The question instead has become which of many pathways might have been followed to produce the first cells."  Why does the NAS make this claim, since it doesn't seem consistent with the scientific evidence?  Maybe they are influenced by an assumption, which is not based on science, that everything in the history of nature happened due to natural causes.   { Despite the waffle words — "could have" and "might have been" — the strong implication is that scientific evidence-and-logic strongly supports a naturalistic origin of life. }

      And it continues with a view that seems more justifiable.
      • Loren Haarsma and Terry Gray are more cautious (and more realistic and scientifically responsible, I think) in their description of a "complexity" theory for the origin of life:
      Is it possible that simple organic molecules could self-organize into a living, reproducing organism?  Given our current scientific understanding, it is far too premature to definitely answer either yes or no.  There are many hypotheses for how first life might self-assemble on the early earth.  All of these hypotheses are still speculative. .....  Origin of life investigators have had a difficult time envisioning a proteins-only solution.  The RNA world scenario has fared somewhat better, but it is not clear how proteins get integrated.  The replicating closed auto-catalytic system described by Stuart A. Kaufmann has the advantage that the complex web of interactions is built in from the outset.  In essence this view acknowledges irreducible complexity, that is, the system has to be sufficiently complex in order for auto-catalytic behavior to emerge. .....  Stuart Kaufmann (The Origins of Order: Self-Organization and Selection in Evolution, p. 285) writes, "...this new view, which is based on the discovery of an expected phase transition from a collection of polymers which do not reproduce themselves to a slightly more complex collection of polymers which do jointly catalyze their own reproduction. ..."  { these excerpts — which are 35% of their full-lenth excerpts about this topic — are from pages 297 and 307-308 of Chapter 9 — "Complexity, Self-Organization, and Design" — in Perspectives on an Evolving Creation }
      a comment by me:  According to Kaufmann, his view "is based on the discovery of an expected phase transition" but the "discovery" exists only in his mind.  It would be more accurate to say that his view "is based on the expected discovery of a phase transition" or "is based on my prediction of a phase transition."

You can see a wider variety of views in the links-page for
ORIGIN OF LIFE — ABIOGENESIS BY CHEMICAL EVOLUTION?
 




 
This website for Whole-Person Education has TWO KINDS OF LINKS:
an ITALICIZED LINK keeps you inside a page, moving you to another part of it, and
 a NON-ITALICIZED LINK opens another page.  Both keep everything inside this window, 
so your browser's BACK-button will always take you back to where you were.

Here are other related pages:

four pages I've written about
science and thermodynamics,
methodology and philosophy,
regarding The Origin of Life

THE ORIGIN OF LIFE
(views from a variety of authors)

This page is
http://www.asa3.org/ASA/education/origins/chemical-cr.htm

Copyright © 1998 by Craig Rusbult
all rights reserved

SEARCH THE WEBSITE