MB>But there is no reason to think there should only be one ecosystem.
Wesley:
>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.
I agree with you that there isn't enough evidence to argue
that life must have arisen many times, but I think Bertvan
has a point. With abiogenesis, there are two basic schools
of thought. Christian DeDuve (and I suppose Denton and
Kauffman) favor the view of chemical determinism where,
given the appropriate conditions, abiogenesis is inevitable.
Others view abiogenesis as a highly unlikely event that
depends essentially on the luck of the draw. If the chemical
determinists are correct, we still can't say that life must have
arisen many times, but this does indeed appear to be a natural
implication of this line of thinking. Inevitable things tend
to be likely and likely things tend to happen often. Thus,
I do think the fact that the evidence indicates life is
monphyletic is a real problem for chemical determinism
(but it is not a problem for the lucky accident view).
One can, of course, raise ad hoc scenarios where
the first life forms outcompete subsequent life forms,
but these are easily canceled out by other ad hoc
scenarios where this would not happen.
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.
Wesley:
>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.
All of this is possible. And given that abiogenesis is largely
imaginary, we can invent scenarios that go either way.
For example, sequestration may prevent other life forms
from arising *nearby*, but the earth is a huge laboratory.
If sequestration is thus to be significant, these primitive
systems must have been quite efficient very early on
and the raw resources provided by the planet must have
been scarce. If, on the other hand, the primitive systems
were not efficient (and I see no reason to think they would
be) and the earth was providing plenty of abiotic material
(a common assumption), there would seem to have been
plenty of abiotic precursors to go around. And since inhibitory
byproducts would quickly dilute out through diffusion and
degradation, newly forming systems on the other side of
the planet would appear to me to be quite safe.
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?
Wesley:
>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.
Given this initial self-replicating system is entirely hypothetical,
I don't see how we can put any constraints on it.
>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.
Yes. Let's say a population of self-replicating systems begins
to favor the L-isomers and begins to predominate. But if a competing
population begins to favor the D forms, it will quickly carve out
its own niche so it is no longer competing with the L forms.
This is how evolution typically works. Why would these
initial life forms all jockey for the same niche? Why wouldn't
they simply specialize along different trajectories to avoid
direct competition and stake out their own niches?
>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.
What makes you think these conversion mechanisms did not
arise long after the origin of life? Yes, bacteria use a D amino
acid to make their cell wall, but would you argue the bacterial
cell wall is older than the ribosomes?
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.
Wesley:
>This assumes chirality dependence as a characteristic of the
>first self-replicating system. That might be true, but it is
>not necessarily so.
I'm not making any necessary claims.
>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.
Once again, since the first self-replicating system is
imaginary, one can not make any such necessary claim
about it. I simply question your "winner take all" claim
as it is neither necessary that the winner would take all.
>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.
Do you have one little piece of data to support this claim?
What if the global rate of formation of amino acids is many
times greater than the rate at which this primitve self-replicator
sucks them up?
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?
Wesley:
>Why, indeed?
Who knows? But it is a problem for the view of chemical
determinism.
>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?
Many would argue that the oxygenic atmosphere has essentially
shut down abiotic amino acid synthesis.
>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.
And unless they were very, very good at using these and
there were very few amino acids to begin with, I don't
see this as likely.
>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.
If this is the answer, the implications are mind-boggling.
For what this means is that the twenty biological amino
acids are essentially the only ones that can form stable
(and thus functional) proteins. This then means that
the universal use of these twenty amino acids is NOT
evidence of a universal common ancestor (as is commonly
claimed). It would mean that if bacteria were found on
Mars that also used this same set, we could not use this
as evidence that they are related to Earth forms.
It would mean that once we begin to explore the
universe, we would expect to find life forms that
used this same set of twenty amino acids. And that's
the ironic thing about chemical determinism - in seeking
to explain the features of life as chemically inevitable,
it essentially paints all possible life forms into one
category - ours.
MB>The bottom line is that there would be lots and lots of
MB>leftovers after the "winner takes all."
Wesley:
>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.
That's fair. But you do then realize that your winner-take-all
scenario is dependent on a small subset of these assumptions?
Mike