DNAunion: The last of the mutli-post material from my personal notes:
Panspermia?
Could there be a single logical solution that both solves the small
step/large step paradox that follows from Lynn Margulis's statement and
smooths out the assumed burst-flatline history of early life? Yes, but to
draw the fairly-obvious conclusion that comes from these two apparent
problems, one needs to free his or her mind from the unsupported assumption
that life actually originated here on Earth.
Consider this scenario: several types of bacteria traveled through space as
endospores (which are highly-protective, resting states) inside an asteroid
or comet and arrived on Earth. As the bacterial endospores germinated and
attempted to resume normal functioning, those types that were able to survive
take hold. From that point forward, for both Panspermia and the mainstream
scenario, the rest is the same standard Darwinian evolutionary processes.
This hypothesis can be either Panspermia (if the bacteria arrived on Earth by
chance after having arising naturally elsewhere) or Directed Panspermia (if
the bacteria were intentionally seeded on Earth by intelligent
extraterrestrials, which arose naturally elsewhere).
A form of Panspermia is in fact considered to have taken place (this not in
reference to the controversial claim that ancient fossils of nanobacteria
were found in the Martian meteorite ALH84001 that landed in Antarctica, but
rather the opposite: the transport of early Earth life to Mars via
impact-ejected materials).
Note the even the more far-fetched Directed Panspermia has at least one
prominent advocate: Francis Crick (of DNA-double-helix fame).
"Finally, a word of heresy: As Moore has pointed out (Chapter 5), the real
fossil record suggests
that our present form of protein-based life was already in existence 3.6
billion years ago and
evolved rather slowly for a billion or so years after that. This leaves an
astonishingly short time to
get life started. Moreover, the three main lines of descent (see Fig. 3 in
Woese and Pace, Chapter
4) seem a very long distance from their hypothetical common ancestor.
The rather far-fetched hypothesis of Directed Panspermia would predict that
life was sent here in the form of "bacteria" suitable for growth in anaerobic
conditions, and that several somewhat different forms would have been sent at
the same time, in the hope that at least one would survive." (emphasis in
original, Francis Crick, Forward to the First Edition, in The RNA World:
Second Edition, Cold Spring Harbor Laboratory Press, 1999, pXVI)
Crick continues by asking what appears to be a perfectly legitimate set of
questions.
"Therefore, the final question about the RNA World and the pre-RNA World (if
it existed) is:
*Where* did it occur? Are we totally confident that our form of life started
here, or did it perhaps
originate elsewhere in the universe? It might have been easier to start
elsewhere because, for
example, the atmosphere there was more reducing that the Earth's early
atmosphere appears to
have been." (Francis Crick, Forward to the First Edition, in The RNA World:
Second Edition,
Cold Spring Harbor Laboratory Press, 1999, pXVI)
In addition to Crick's point that “planet X” might have had a
more-reduced prebiotic atmosphere, making life’s origin there more
probable than for Earth, life also would have been more likely to arise from
scratch on “planet Y” if that planet had a wider window of
opportunity than Earth did. But what if “planet X” and
“planet Y” were the same? A single planet that had both of these
attributes - a highly-reduced atmosphere containing ammonia and methane, and
a time window for the origin of life of say 2 billion years - would have been
doubly-blessed.
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