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:
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:
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.
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. |
four pages I've written about THE ORIGIN OF LIFE |
This page is
http://www.asa3.org/ASA/education/origins/chemical-cr.htm
Copyright © 1998 by Craig Rusbult
all rights reserved