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>>>DNAunion: NEW STUFF.
Since we are discussing origin of life stuff again, I recently read something
I would like to point out and comment on.
In the chapter being quoted from, David Deamer has just presented several
paragraphs of detail concerning the following summary. This detail material
has been omitted for brevity's sake.
"To summarize, an abundant source of long-chain hydrocarbon components of
prebiotic membranes is not obvious. On the other hand, one might argue that
because the origin of cellular life absolutely requires lipidlike hydrocarbon
derivatives, such molecules must have been available on the early Earth from
a yet unknown source." (David Deamer, Membrane Compartments in Prebiotic
Evolution, Chapter 8 of The Molecular Origins of Life: Assembling Pieces of
the Puzzle, edited by Andre Brack, Cambridge University Press, 1998, p194)
Can anyone else see the circular reasoning in this? If not, let me give you
an analogy.
PROSECUTOR: Ladies and gentlemen of the jury, the state believes that Mrs.
Jones' disappearance is not due to her being abducted by a stranger, but
rather is due to her husband's discarding her body after he savagely killed
her. Let it first be noted that the state can't present any actual evidence
that shows Mrs. Jones is dead, but we strongly suspect so: call it a working
assumption. In addition, we don't have a murder weapon with Mr. Jones'
fingerprints on it, despite our best searches. But we argue that since Mrs.
Jones could not have been murdered by her elderly husband without a murder
weapon being involved, then we can assume that the murder weapon with his
fingerprints does exist somewhere, and that we just haven't come across it
yet. The state asks that you sentence Mr. Jones to life in prison without
the opportunity for parole for his committing of such a brutal and
unthinkable act."
Okay, a bit over-dramatic (to say the least), but I think everyone gets the
point. Take away the preexisting assumption that Mrs. Jones was murdered (or
analogously, that life arose here on Earth) and the assumption based on it -
that a murder weapon with Mr. Jones' fingerprints on it must exist (or
analogously, that an abundant source of long-chain hydrocarbon components
must have existed) is unfounded, meaningless, and bankrupt. If one is trying
to demonstrate a basic, underlying assumption, then one shouldn't use
assumptions drawn solely from that final underlying assumption that he or she
is trying to validate to argue into existence, from thin air, evidence that
supports any part of that final underlying assumption.
>>>Chris: I don't see that the argument is circular at all, but rather that
it is *not* an argument ...
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DNAunion: But it is an argument.
"...one might *argue* that because the origin of cellular life absolutely
requires lipidlike hydrocarbon derivatives, such molecules must have been
available on the early Earth from a yet unknown source."
First, it contains the word argue. Second,it takes the form of a logical
argument.
Premise 1: Cellular life originated on Earth
Premise 2: Cellular life absolutely requires lipidlike hydrocarbon derivatives
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Conclusion: Therefore, lipidlike hydrocarbon derivatives must have been
present on the early Earth
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[...]
>>>Chris: Also, I'm not at all convinced that they are necessary for the
origin of cellular life, though they might be necessary for the origin of the
kind of cellular life that *we* are familiar with. We need to be careful not
to *assume* that the first life on Earth was much like any life we know today.
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DNAunion: But it is a safe assumption. By 3.5 billion years ago, life had
already developed a long way in cellular form. Schopf has identified several
types of bacteria in the Apex chert from Western Australia, including
*photosynthetic cyanobacteria*, which were found in complex arrangements.
The fossils - the oldest found so far - clearly indicate a long evolutionary
history prior to 3.5 Gya (for example, photosynthesis is considered to not be
prebiotically plausible, so it had to evolve through very many intermediates
via slight, successive, incremental enhancements. This would have taken a
many, many generations, and had to be completed no later than 3.5 Gya).
In addition, the universal tree of life must be rooted prior to those 3.5
billion year old fossils, and there are many cellular universals that must
have appeared near the root: all cells use ribosomes to make proteins, all
cells' ribosomes are composed of a small and a large subunit, all cells use
the same amino acid (whether modified or not) to initiate translation, all
cells (with very few exceptions) use the same 20 amino acids, all cells (with
very few exceptions) use only left-handed amino acids, all cells use only
right-handed sugars, all cells (with very few exceptions) use the same
genetic code, all cells use DNA to store genetic information, all cells
polymerize DNA in the 5'->3' direction, all cells use the same unidirectional
flow of biological information (from nucleic acids to proteins, and never in
the opposite direction), all cells use ATP as energy currency, all three
domains of cellular life use RNase P, etc. These (and other
"universalities") all must have existed a good bit BEFORE 3.5 bi
llion years ago.
The first cellular life here on Earth was very likely much like todays (at
least at the most fundamental levels). And depending on the amount of time
available, the first non-cellular life (a contradiction in many people's
view) could not have been vastly different if they had to evolve into cells
so quickly.
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