To: All Msg #146, Sep0293 10:28PM Subject: The Abiogenesis MiniFAQ Abiogenesis. What is ab
From: Usenet
To: All Msg #146, Sep-02-93 10:28PM
Subject: The Abiogenesis Mini-FAQ
Organization: evolv-o-tron, inc.
From: Deaddog
Message-ID:
Newsgroups: talk.origins
Abiogenesis.
What is abiogenesis? It is commonly construed to
mean the origin of 'life' from 'non-life,' but both
terms are vague and difficult to define. While we
recognize 'life' (dogs) when we see it, and think
that we recognize 'non-life' (rocks), this
distinction may not have been as clear cut on the
Archean Earth. Therefore, I choose to talk about
the propagation of heritable information:
basically, how chemical patterns (dogs and rocks)
are replicated and selected over time.
The Physical Setting
(really not my forte, so I'm brushing up a bit
first)
The Earliest Replicators
Modern theorists can be broken up into several
different camps. Apologies to those camps that I
miss.
Clay People: Minerals can 'replicate' their
crystalline structure. Their chemical pattern may
change with time; for example, as a clays 'grow'
they incorporate ions into their structure. If the
environment in which a clay is found changes, and
the source of ionic 'food' also changes (magnesium
gives way to, say, aluminum), the matrix can
continue to grow, but will be chemically different.
Because of the difficulties inherent in imagining
how organic replicators arose, some theorists,
notably A.G. Cairns-Smith, have advanced the
following hypothesis: inorganic replicators
(clays) seeded organic replicators (of whatever
sort). This hypothesis is called "genetic
takeover." The serious book is "Genetic Takeover
and the Mineral Origins of Life, Cambridge
University Press, New York, 1982, but the
fun book is "Seven Clues to the Origin of Life,"
written in an easy-to-follow Sherlockian style.
Advantages: No need for large pool of complex
organic molecules to start; can be gradually added.
Deftly avoids all questions of the sort 'where did
the ribose come from'
Disadvantages: No evidence nor plausible mechanism
for 'genetic takeover.' However, it should be
noted that clays have been seen to catalyze a
number of prebiotic reactions.
Chemical cycles: Chemical cycles replicate their
components and are self-sustaining. Interacting
cycles can arise [e.g.
D<-->B<-->C<-->D added to A<-->B<-->C<-->A], are
equivalent to 'mutations,' and are themselves
heritable. The most prolific popularizer of these
theories is G~nter W{chtershe~ser, a German patent
attorney with a strong background in chemistry
(Micro Rev., 52, 452 (1988)). His most basic
chemical pathway is related to a reductive Krebs
cycle, is dependent on pyrite, fool's gold, and
gives rise to a rich tapestry of additional
reactions. Modern cells again took over the
chemical reactions by blebbing off organic
coacervates from mineral surfaces.
Advantages: Can derive almost all modern
metabolism; genetic (as opposed to chemical)
inheritance does not become involved until quite
late; most reactions are postulated to
occur in, well, slime layers on the surface of
rocks (pyrite). This both reduces the
dimensionality and dilution problems associated
with normal prebiotic chemistry, and
provides an 'identity' for chemical organisms
(i.e., 'self'= your rock).
Disadvantages: Some slight evidence for the ur-
cycle, but the chemistry is pushed well beyond the
bounds of anything known today. If he turns out to
be right it will be an intellectual achievement
ranking with the Theory of Relativity. My personal
assessment is that the reactions have been severely
strained to produce 'modern' compounds and
pathways, while it seems far more likely that the
chemical cycles that initially evolved may have
looked nothing like what exists today. That is,
this theory seems to be guided by biological
preconceptions rather than chemical plausibility.
Genetic replicators. Note that these are actually a
subset of chemical cycles, but rely on some form of
structural complementarity rather than reactivity
for their propagation. Although all styles of
hypothesized replicators exist, most are based on
Watson-Crick style base complementarity. However,
see work from Rebek's lab for replicators that have
been designed de novo (JACS, 112, 1249 (1990)).
Short, self-replicating nucleic acid molecules
have been constructed by both von Kiedroswki
(Angew. Chem.(English) 98, 932 (1986)) and Orgel
(Nature, 327, 346 (1987)). This is why I always
get a kick out of folks babbling on about how we
don't yet have self-replicating nucleic acids, this
will be the key to life, etc., etc., etc. What
they mean, of course, is that we don't have a
nucleic acid polymerase that can use nucleotide
triphosphates ... which were unlikely to have been
around in abundance in the prebiotic milieu anyway.
Advantages: Obvious and direct relation to modern
life. Polymers can act as catalysts for their own
replication and can create what Eigen has called
'hypercycles' (Eigen, Naturwissenschaften, 65, 341
(1978) is the classic paper in this field).
Disadvantages: 'Food' tends to be relatively
unstable molecules (see, for example, Pace, Cell,
65, 531 (1991)) that are prepared in extremely low
yield by prebiotic pathways currently known. In
order to get around this, there are a wide range of
nucleic acid 'like' compounds that are proposed to
have preceded real nucleic acids on the
evolutionary stage (Joyce, Nature, 338, 217 (1989)
for a brilliant review).
Overall: it is generally believed that one of these
replicators must have evolved to a nucleic acid
based genetic system. The initial complexity of
the metabolism in which the original nucleic acid
replicators arose is open to question; for chemical
cycles it can be quite complex, for genetic
replicators it was almost certainly dirt simple.
Extrapolating Backwards
It is extremely important to try to discern how
these replicators would have led to modern systems.
What do we know about the ancestors of modern life?
Despite the almost complete lack of a conventional
fossil record, a great deal. In fact, the
molecular details of the progenitor of modern life
are almost certainly much better known than, say,
the physiological or morphological characteristics
of a protobovoid. This is because every organism
on the planet contains a wealth of metabolic
fossils that have been far better preserved by
genetic replication than any mineral fossilization
could have hoped to have achieved. I will
elaborate with only one example, but there are many
(Benner, Ellington, and Tauer, PNAS, 86, 7054
(1989) is a tour de force, but you may disagree
with many of their conclusions).
There are three domains of life: archaebacteria,
eubacteria and eukaryotes. For practical purposes,
these three domains can be considered to have
diverged from one
another at more or less the same time (that is, the
evolutionary distance between any two of them is
huge). NAD is the redox equivalent of metabolism,
just as ATP is the energy equivalent (I realize
this is an oversimplification, but let it stand for
the nonce). The structure of the NAD is the same
in the three domains. Therefore, the last common
ancestor of modern life had NAD.
Using logic similar to this, we can discern which
molecules, pathways, and cofactors are ancient, and
which are modern. Taken together, these inferences
allow us to draw a picture of the last common
ancestor of modern life, a cell that existed
roughly 2.5 billion years ago. This cell had DNA,
RNA, proteins, and a complex metabolism. It had
probably invented translation relatively recently,
and may still have had many reactions that were
catalyzed by nucleic acid enzymes, as opposed to
proteins.
The Big Gap
What happened between ca. 3.5 billion years
ago, when the first replicators arose, and 2.5
billion years ago, when a cell that was in many
respects similar to those that course our veins
today existed? This is the field of
abiogenesis: what mechanism is most likely to
move through this tortuous 1 billion year
history. There are literally hundreds of
hypotheses competing to fill this era. For
example, if you choose to believe Wachtershauser, I
can draw you a detailed and complete map of
virtually every major evolutionary event at the
molecular level over the last 3.5 billion years.
But, and I can't emphasize this strongly enough,
all of these hypotheses are based on one theory,
the theory of evolution. Because we know how
modern nucleic acid replicators (viruses and cells)
and their attendant metabolisms have evolved, and
because we know that the earliest replicators
evolved towards a nucleic acid-based replication
system (the last common ancestor of modern life),
we can extrapolate into this gap. The
extrapolations that prebiotic chemists /
abiogeneticists perform are no different than the
extrapolations that paleontologists perform in
tying together the lineages of creatures long dead.
We use molecular bones, but they are no less real.
-------------------------------------------------------------
From: Usenet
To: All Msg #162, Sep-02-93 11:29PM
Subject: A Disappointing Revelation
Organization: evolv-o-tron, inc.
From: Deaddog
Message-ID:
Newsgroups: talk.origins
I have posted the mini-FAQ in the hopes of providing some
snippets of evidence for those who say there are none.
Perhaps this will sway some of you, perhaps not.
I have tried to avoid advancing or advocating specific
mechanisms for abiogenesis because I still believe that
the following should be enough for us all:
Evolution is both a theory and a fact. In accepting this,
we must follow its tenets to their logical conclusion.
Organisms evolve; their lineages can be traced. Their
lineages lead back to a cell. Just as all cells evolved
from this cell, so this cell must have evolved from
something. The cell must have evolved from some set
of molecules. These molecules can be seen to evolve even
today. All of the events that led from the molecules to
the cell were governed by the theory of evolution. It is
the same theory that governs the evolution of all organisms,
indeed of all self-replicating systems.
Some of you clearly agree with this, and yet still insist on
dividing abiogenesis from 'evolution.' I can live with
Robert Derrick's argument, which is a very good one: in
essence, the propaganda value of evolution as a theory is
greater in the absence of abiogenesis. I agree
wholeheartedly. But know this: this position is
intellectually dishonest.
I am surprised and disheartened to find that abiogenesis is
in fact an acid test of how you view evolution. Either you
can extrapolate into the past based solely on *the
intellectual force of the theory alone* or else you must
caterwaul for facts and assurances. Darwin had nothing
like the molecular techniques available today, and yet he
could of necessity imagine a 'warm little pond' (I
am doubtlessly historically inaccurate in claiming this).
Yes, with facts you can batter the religious Creationists.
But it is not the facts that give you explanatory power;
they merely give you evidence. It is the theory that
explains the facts. And I fear that any mound of facts
will not erase the new vitalism that I find amongst those
who study evolutionary biology. The position that
abiogenesis deals with non-living things, and therefore
is separate from evolution automatically imparts a mystical
force to organisms that separates them from other evolving
systems, such as the molecules of which they are composed.
This seems a philosophical point, and so it is. But I think
it has consequences in how you defend the theory and fact of
evolution against religious Creationists. Perhaps I am
mistaken, but I think that while you would insist on
removing them from a biology class, you might be quite
comfortable in allowing them to teach an origins class that
gave 'equal time' to chemical evolution, panspermia, and
divine intervention of a non-sectarian sort -- since we
really don't *know* which of these it was, right?
Certainly I am being alarmist; but the recent attempts to
gain a toehold in social sciences classes can easily be
expanded to natural science based on precisely this sort
of philosophical distinction. I post for my own amusement,
certainly, but I believe it matters a great deal whether or
not abiogenesis is considered a logical and necessary
consequence of the theory of evolution.
Non-woof
[Thank you, Eric, for your concern, but I have said this
about as many ways as I can think to say it. I fear that
some folks will just have to remain ... incorrect.
Diatribes: off. Responses (on this thread) by E-mail only.]
E-Mail Fredric L. Rice / The Skeptic Tank
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