DNAunion: Continuing from my personal notes:
A different potential time constraint - which assumes that life originated by
chemical evolution in the oceans - is the rate of recirculation of the oceans
through hydrothermal systems, which "zaps" complex organic molecules. This
estimate, worked out by Stanley Miller and Antonio Lazcano, is about 10
million years.
"Interviewer: What about submarine vents as a source of prebiotic compounds?
Stanley Miller: I have a very simple response to that. Submarine vents don't
make organic
compounds, they decompose them. Indeed, these vents are one of the limiting
factors on what
organic compounds you are going to have in the primitive oceans. At the
present time, the entire
ocean goes through those vents in 10 million years. So all of the organic
compounds get zapped
every ten million years. That places a constraint on how much organic
material you can get.
Furthermore, it gives you a time scale for the origin of life. If all the
polymers and other goodies
that you make get destroyed, it means life has to start early and rapidly."
(StanleyMiller as quoted
by Sean Henahan,Access Excellence, From Primordial Soup to the Prebiotic
Beach: An Interview
with Exobiology Pioneer Dr. Stanley L. Miller, October, 1996)
"Antonio Lazcano of the University of Mexico and Stanley Miller felt that the
origin of life could
not have been stretched out over such relatively long spans of time. They
point out that the
primordial soup would have been destroyed by circulation of the ocean through
hydrothermal
vents in a mere 10 million years. The origin of life must have taken less
time than that,
particularly if it had to happen while the primordial soup was still rich and
nourishing."
(Christopher Wills & Jeffrey Bada, The Spark of Life: Darwin and the Primeval
Soup, Perseus
Publishing, 2000, p258)
Others suggest that even a small window of 10 million years is far too long
to allot for prebiotic chemical evolution.
"It also seems unlikely that chemical evolution took hundreds of millions of
years, for most of the
key organic chemicals could not have persisted for so long. Our own hunch is
that even 10
million years is probably a substantial overestimate. Perhaps life appeared
over a span of a mere
few thousand years." (Christopher Wills & Jeffrey Bada, The Spark of Life:
Darwin and the Primeval Soup, Perseus Publishing, 2000, p258)
So we can see that the available time window for the origin of life went from
about 3.7 billion years in the 1950s, down to 400 million by the mid 1990s
(based on changes in beginning and ending boundaries - when the oceans formed
and the oldest signs of life, respectively). Additional insights have
further reduced this window: based on the calculated periodicity of impacts,
some researchers decreased the window to a mere 20 million years, while the
propensity of hydrothermal vents to "zap" complex organic molecules has
forced other origin of life researchers to narrow it to just 10 million
years. Taking an overall view, we see that the original 3.70 billion years
shrunk by 3.69 billion years down to just 10 million! And even this 99.73%
reduction does not reflect the lower limit, as still other researchers
believe life probably arose in as little as a few thousand years!
It is interesting to contemplate why the window has been narrowed so
drastically. The first question is, does this time window represent an
estimate of how long it would take for life to arise, or is it an estimate of
the amount of time that was available for life to arise? It is clearly the
latter: there is no real estimate on how long it would take life to arise
from non-life. Another way to examine the time window is to ask, has it been
continually decreasing because researchers have had such continual success in
the laboratory that they now know that life could arise in a mere few
thousand or few million years? Again, the answer is no. The ever-narrowing
window of time available for life to arise is forced upon researchers by
findings that indicate life’s origin could not have been allotted more
time. It’s not a case of "we know it could occur in 10 million years",
but rather "we know it had only 10 million years to occur".
But is this tiny window enough? Since there are no real estimates of how
long it would take life to arise (as opposed to the independently-determined
windows that impose time constraints on it), only indirect logic can be
applied.
"To go from a bacterium to people is less of a step than to go from a mixture
of amino acids to a
bacterium." (Lynn Margulis, interviewed in The End of Science, by John
Horgan. Addison-Wesley
Publishing Company, Inc., 1996. p 140-141).
If we accept Lynn Margulis's statement, and we know that it took over 3.5
billion years "To go from a bacterium to people", then logic would dictate
that the larger step of going "from a mixture of amino acids to a bacterium"
would have required more time – yes, more than 3.5 billion years, not a
mere 10 million or few thousand.
Another line of reasoning that suggests life might require more than 10
million years to arise is based on the rate of evolution just after the
appearance of life on Earth. The oldest fossilized cellular structures are
those of cyanobacteria-like organisms and eubacteria found in the
3.5-billion-year-old Apex chert of Western Australia, which were examined and
catalogued by Schopf. As mentioned previously, these bacteria "were arrayed
in long filaments, mats, and other complex structures" and "must already have
had a long evolutionary history". Yet they "looked very much like the common
photosynthetic bacteria that live in the oceans today". Apparently, in a
very short period of time, life not only arose but also evolved to very
complex levels. Yet immediately thereafter, and going forward for the next
3.5 billion years, changed little. In fact, life in general remained
single-celled until about 1 billion years ago. So again, for about 2.5
billion years after the origin of life, life apparently evolve
d very slowly.
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