DNAunion: The following multi-post material is from my personal notes.
Time Window for Abiogenesis
Scientists estimate that the Earth was repeatedly bombarded by
extraterrestrial objects (asteroids, comets, and meteorites) from the time of
its accretion 4.55 billion years ago up until about 3.9 - 3.8 billion years
ago. Many of these impactors were monstrous, with diameters of up to 60
miles or more. Compared to the bolide that putatively caused the extinction
of the dinosaurs 65 million years ago at the KT boundary (Cretacious/Tertiary
boundary), these impactors had diameters 10 times as large, which correlates
to spherical volumes (and therefore, masses and inertial forces) 1,000 times
that of the hypothetical dinosaur-killer. Such enormous impacts would have
boiled away the oceans and sterilized the face of the planet - life is not
likely to have originated, or survived, during such times. What does all of
this imply? That any initial steps towards biopoesis, or even the
persistence of life itself had it arisen, would have been repeatedly
"frustrated" by bolide impacts.
"Earth, being a bigger target with a stronger gravitational pull [than the
moon], would have
suffered blows from hundreds of objects of that size between 4.0 billion and
3.8 billion years ago,
geophysicist Norma Sleep of Stanford University and planetary physicist Kevin
Zahnle of
NASA's Ames Research Center in Mountain View, California, noted in the
Journal of
Geophysical Research last year. A few of these impactors were probably 500
kilometers in
diameter - big enough to create a superheated atmosphere of vaporized rock
that would in turn
have vaporized the oceans for 2700 years and sterilized even the subsurface,
say Sleep and
Zahnle." (Richard A. Kerr, Early Life Thrived Despite Earthy Travails,
Science, June 25, 1999,
v284 n5423, 2111)
"The Earth was being bombarded heavily and frequently in its early history by
numerous
impacting objects.... The size of bolides hitting the Earth during this time
interval, of roughly 4
billion to 3.7 billion years ago necessary to melt ice of 300 meters thick,
would need to be on the
order of 80 km in diameter. This is what was happening." (Dr. Jeffrey L.
Bada, Professor of
Marine Chemistry University of California, San Diego, and the Scripps
Institute of
Oceanography, La Jolla, California 92093-0216. Organic Chemistry of the
Early Earth, Based
on a presentation to the American Association for the Advancement of Science,
February 20,1994,
S. Brown, ed)
"Astronomers and geologists were discovering that Earth had a violent
infancy--hundreds of
millions of years after the planet had formed, giant asteroids and comets
still crashed into it,
burning off its young atmosphere and boiling away its oceans. In the process,
they also destroyed
all the chemicals that researchers assumed were in liberal supply on the
early Earth, including the
building blocks of lipids." (Carl Zimmer, First cell, Discover, Nov 1995 v16
nl 1 p7(9))
"The rates of hydrolysis [of the RNA nucleobases] at 100 [degrees] C also
suggest that an ocean-
boiling asteroid impact would reset the prebiotic clock, requiring prebiotic
synthetic processes to
begin again." (Stanley L. Miller, Matthew Levy, The Stability of the RNA
Bases: Implications for
the Origin of Life, Proceedings of the National Academy of Sciences of the
United States, July 7,
1998 v95 n14 p7933(6))
"Looking to the moon is nothing new for investigators trying to reconstruct
Earth's history of
bombardment. The moon's history of impacts presumably parallels Earth's, but
unlike Earth, it
doesn't experience the erosion and other geologic processes that tend to
erase the evidence -- rocks
melted in impact cataclysms. Radioactive dating of rocks brought back by the
Apollo astronauts
suggests that the bombardment of Earth and the moon didn't abate until about
3.8 billion to 3.9
billion years ago, implying that life could not have arisen much earlier than
that." (Ricki Lewis,
Primordial Soup Researchers Gather at Watering Hole, Science, August 22, 1997
v277 n5329
p1034(2))
"... Sleep et al. described the consequences of large impacts for the
prebiotic environment. Large
impacts could have repeatedly vaporized not only the entire ocean of the
early Earth, but also
enough rock to create 100 bar [1 bar = atmospheric pressure at sea level] of
rock vapor and
suspended droplets with a temperature of 2000[degrees]C. Smaller impacts
that vaporize only the photic
zone of the oceans were also discussed by Sleep... ... the size of the
impactor useful for
producing polyphosphates [is limited] to about 90-km diameter. Much bigger
blasts in earlier
times (even bigger than ocean blaster) would have destroyed any complex
molecules, including
polyphosphates." (Hendrik Tiedemann, "Killer" Impacts and Life's Origins,
Science, volume 277,
number 5332. September 12, 1997. p1687-1688)
"Scientists who belong to the periodic-impact school suggest that these
events were caused by
occasional disturbances of the great cloud of comets that surrounds the Solar
System. These
disturbances, perhaps caused by passing stars, sent volleys of comets
spiraling inward toward the
Sun, and a few of them slammed into the Earth and the Moon.
Impact events of such magnitude on the Earth would have released
enough energy to
vaporize the oceans and sterilize the plants in a burst of superheated steam.
Kevin Maher and
David Stevenson of the California Institute of Technology have suggested that
life could have in
fact originated several times but that it was destroyed again and again by
these violent, sterilizing
impacts. Perhaps life might have taken a different form, were it not for
these repeated "impact
frustrations"." (Christopher Wills & Jeffrey Bada, The Spark of Life: Darwin
and the Primeval
Soup, Perseus Publishing, 2000, p78)
Yet, scientists have found that the first signs of life are 3.87 billion
years old - so the time that chemical evolution had to produce life is
basically 0 years.
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