Caveat: I'm into organismal biology. This biochemistry stuff
is not my bag. I'm trying to pass on what I understand from
what others have written. I particularly want to thank
Kevin O'Brien for his many posts detailing biochemical
esoterica in an accessible fashion, and I hope that I
haven't bent the concepts stated by Kevin and others too
badly in handling.
BV>In which case, one common ancestor would be obvious. If on
BV>the other hand, life is the inevitable result of
BV>complexity, as Stuart Kauffman suggests, or as Michael
BV>Denton appears to believe, life is a natural phenomenon in
BV>the universe, life must have arisen many times. In that
BV>case one common ancestor (or even 3) would hardly be
BV>likely, would it?
WRE>Life can commonly arise without the necessity that it commonly
WRE>arise many times *in the same ecosystem*. The scenario
WRE>described conforms to a "winner-take-all" situation. Chemical
WRE>resources would exist prior to the first self-replicating
WRE>system that would thereafter be sequestered in instances of
WRE>that self-replicator. While not an absolute bar to further
WRE>novel self-replicating systems, it would certainly reduce the
WRE>likelihood of further ones arising.
MB>But there is no reason to think there should only be one ecosystem.
On earth, there are enough means of transporting small
chemical packages from place to place and enough relatively
inaccessible places that neither a view that there could only
have been one ecosystem nor a view that there must have been
more than one ecosystem can be held to be evidently true. I
was arguing against the claim that "life must have arisen many
times" with apparent application to life arising *here on
earth* many times. While I cannot rule out multiple instances
of abiogenesis, neither does it appear that I am obligated to
accept that multiple instances occurred here.
MB>If a self-replicating system arose, I agree this would decrease the
MB>likelihood of another self-replicating system arising *IF* the
MB>original uses the resources needed to generate the other.
So far, so good.
MB>But this doesn't effect *different* life forms from arising.
Hmm. I don't see how this can be supported. Sequestration of
significant chemical resources surely will have an effect on
other abiogenesis events, whether they would result in similar
results or not. Also, to accept the assertion as given one
must ignore the possible products of such chemical systems,
which may inhibit the formation of other self-replicators.
MB>For example, supposedly a self-replicating system arose that uses
MB>L-amino acids. Why didn't one arise that uses D-amino acids?
Modern organisms predominantly utilize L-amino acids. In the
(rare) cases where a D-amino acid is used, there seems to be
a specific conversion process to go from an L-amino precursor
to a D-amino acid. This situation, though, does not constrain
the initial self-replicating system to exclusive use of L-amino
acids. The instantiation of the biochemistry needed to
generate amino acids from other chemical precursors may have
come later, and with it the observed reliance upon one set of
amino acids rather than their chiral opposite numbers. Even
based upon consideration of biochemical systems made from
mixtures of amino acids of both L- and D- chirality, one could
argue that self-selective effects could lead such systems to
accentuate any imbalance in the proportion of chiral isomers
utilized until either L- or D- isomers were predominantly or
exclusively utilized.
The existence of specific conversion mechanisms in modern
organisms to make the odd D-amino acid from a L-amino acid
is suggestive that at some point there might have been such
mechanisms to convert a surplus of free D-amino acids to
their L-amino acid counterparts.
MB>The self-replicating system that uses L would not take anything
MB>away from the environment that would prevent the appearance of a
MB>D-life-form.
This assumes chirality dependence as a characteristic of the
first self-replicating system. That might be true, but it is
not necessarily so. There are several useful properties for
protein construction that are available only when the proteins
are composed entirely of either L- or D-isomers, but this does
not establish that those properties were necessary to the
first self-replicating system or even to early
self-replicating systems. If no such dependence is necessary,
then an initial self-replicator which scavenges both L- and
D-amino acids would reduce available amino acids to non-useful
levels for other self-replicator productions wherever the
first one reached.
MB>What's more, life uses only a small subset of all possible
MB>amino acids (only 20; and some of these are thought to have
MB>been acquired into life after its appearance). Given that
MB>abiogenesis experiments produce many other kinds of amino
MB>acids not used by life (often in greater concentrations),
MB>why didn't a life form appear that used these amino acids?
Why, indeed? Question: How many of these non-biological amino
acids are present in, say, our modern oceans? Our modern
lakes? Marine hydrothermal vents? If they are not present
in useful concentrations, why aren't they there?
Possible answer: Non-biological amino acids may have been
*used* by an early self-replicator, but not simply grabbed
and incorporated into protein structure. These may have
been processed into other, more useful, chemical products.
Second possible answer: Perhaps these non-biological amino
acids also commonly have R-groups that interfere with protein
stability. If so, then these amino acids would make
relatively unpalatable "leftovers" for a second or further
self-replicating system.
MB>The bottom line is that there would be lots and lots of
MB>leftovers after the "winner takes all."
The bottom line is that given certain favorable assumptions,
one could postulate plenty of useful leftovers, while under
other assumptions favorable to a hypothesis of a single
earthly abiogenesis event, there may not have been much
leftover or much leftover of any great utility after the first
self-replicator made its appearance. While I cannot in
principle exclude the possibility of multiple origins of life
on earth, neither am I obligated to accept as a given that
multiple origins of life actually took place here, contrary to
the assertion made by Berthajane.
Of course, common descent is not restricted to the idea of a
single origin of life -- all that common descent requires is
that modern organisms are derived from one or a few original
forms (where "few" is whatever number the evidence indicates).
These may have themselves been derived from the first
self-replicator, or from a second or subsequent
self-replicator which eventually displaced its competitors.
Even if life originated in multiple various forms here on the
early earth, what the evidence supports is that only one or a
few closely related instances of these initial systems
eventually gave rise to all life on earth. The fact of the
canonical genetic code and the similarity of the genetic
apparatus across all life argues strongly for a very
restricted set of common ancestors for all life.
Wesley