DNAunion: Continuing from my personal notes.
Time can no Longer be the Hero
Starting as early as 1907, scientists began using radiometric dating
calculations to push Bishop James Ussher's 1658 estimate of the age of the
Earth – which came out to be about 6,000 years, since the Earth was
supposedly created October 22, 4004 B.C. - back to over 2,000,000,000 years.
"Using the best estimates he could make of the rate of uranium decay to lead,
and the measured
ratio of lead to uranium, [the American chemist Bertram Boltwood] calculated
in 1907 that some
rocks had ages greater than 2 billion years." (Christopher Wills & Jeffrey
Bada, The Spark of Life:
Darwin and the Primeval Soup, Perseus Publishing, 2000, p67-68)
It was not too long before even this enormous lengthening of the Earth's age
was shown to be an underestimate.
"In 1929, in his last contribution to the topic of the age of the Earth,
Rutherford used the estimated
decay rates of two uranium isotopes (uranium 235 and uranium 238) to
calculate the age for the
oldest rocks of at least 3.4 billion years." (Christopher Wills & Jeffrey
Bada, The Spark of Life:
Darwin and the Primeval Soup, Perseus Publishing, 2000, p68)
A later, bit-more-refined estimate yielded about the same age for the oldest
Earth rocks, onto which an unknown amount of time would need to be tacked on
to arrive at an estimated age of the Earth.
"These advances permitted far more accurate estimates of the age of the
Earth. As a result, it
came to be generally accepted during the 1940s that the most ancient Earth
rocks were about 3.5
billion years old. ... However, scientists realized that most Earth rocks
probably did not go back
to the beginning of the planet's history, so that this was still a minimum
estimate." (Christopher
Wills & Jeffrey Bada, The Spark of Life: Darwin and the Primeval Soup,
Perseus Publishing,
2000, p68)
The first definitive age for the Earth (that is, a calculated value that
still prevails today) was made in the early 1950s.
"In 1953, using the uranium/lead dating method and combining data obtained
from meteorites and
the Earth itself, Clair Patterson at the California Institute of Technology
and Friedrich Houthermans at the University in Bern independently announced
that the age of the Earth, and therefore the age of the Solar System itself,
was about 4.5 billion years. By 1956, Patterson had refined his estimates to
yield an age for the Earth of 4.550 [plus/minus] 0.070 billion years. This
value remains essentially unchanged today." (Christopher Wills & Jeffrey
Bada, The Spark of
Life: Darwin and the Primeval Soup, Perseus Publishing, 2000, p69)
Taking another look at the above timeline of estimates, we can see that by
(and through) the 1940s, the Earth was already considered by scientists to
have been at least 3.5 billion years old (having dated rocks of that age),
and that as early as 1953, the estimate had been pushed back to 4.5 billion
years.
At about that time (the early 1950s), scientists thought that life arose only
about half a billion years ago based on the oldest fossils known at that time
- from the so-called "Cambrian explosion", which began about 544 million
years ago.
"In 1950, when Ledyard Stebbin's Variation and Evolution in Plants first
appeared, the known
history of life - the familiar progression from spore-producing to
seed-producing to flowering
plants, from marine invertebrates to fish, amphibians, then reptiles, birds,
and mammals -
extended only to the beginning of the Cambrian Period of the Phanerozoic Eon,
roughly 550
million years ago." (J. William Schopf, Solution to Darwin's Dilemma:
Discovery of the Missing
Precambrian Record of Life, PNAS, vol 97 no 13, June 20 2000, p6947)
Taking into consideration the age estimates of the Earth of the early 1950s,
this now-outdated estimate for life's first appearance allotted well over 3.5
billion years to evolution in order to generate life. Those proposing a
purely-natural origin of life in the early 1950s took advantage of the gaping
multi-billion-year window and asserted that "anything is possible given
enough time".
"Time is in fact the hero of the plot. Given so much time the impossible
becomes possible, the
possible probable, and the probable virtually certain. One has only to wait:
Time itself performs
the miracles." (George Wald, "The Origin of Life", Scientific American,
191:48, May 1954)
However, as time passed, older and older fossils were found and the
appearance of the first living organisms was consequently pushed back to a
much earlier time, thus constricting more and more the amount of time
available for life to have arisen. Now, in fact, the newly-found earliest
signs of life, which seem to have arisen precariously close to or even during
the period of large impacts, clearly indicate that time can no longer be the
"hero".
"If life did exist on our planet as early as 3.8 billion years ago [the
currently oldest signs of life are
dated at 3.87 billion years old, as mentioned elsewhere in the article], a
number of researchers told
Science, it must then have arisen either during or perilously close to a
period when Earth is
thought to have been regularly blasted by the comets, asteroids, and
meteorites---many 100
kilometers or more in diameter--that were swarming around the early solar
system. This period of
heavy bombardment began soon after Earth formed, about 4.5 billion years ago,
and finally tailed
off about 3.9 billion years ago. If the new date [for the first sign of life]
is correct, it would take
away the comfortable 400-million-year window between the end of the
bombardment and the first
appearance of life." (Michael Balter, Looking for Clues to the Mystery of
Life on Earth , Science,
August 16, 1996 v273 n5277 p870(3))
So we can see that from about 1953 to at the most 1996, the window of
opportunity for life to arise dwindled from about 3.7 billion years down to a
"comfortable" 400 million. And now, recent findings suggest that even the
400-million-year window is overstated.
That window of opportunity has two boundaries: the ending boundary, the one
that exists later in time (say at 3.85 billion years ago), and the beginning
boundary, the one that existed before it (say at 4.0 billion years ago). The
later boundary is marked by the best dating of the first appearance of life,
which has been steadily being pushed back to earlier and earlier times as
more fossils and chemical traces are found. The beginning boundary is not
marked by the formation of the Earth, but rather by when it is assumed that
conditions first arose that would allow for the existence of life: for
example, when the Earth had cooled down from accretion, and solid rocks
formed a crust, and the water vapor in the atmosphere rained out and
accumulated on the Earth’'s crust to form oceans, and the heaviest
bombardment by bolides abated. This estimate is more uncertain and harder to
time than the first appearance of life. Nevertheless, it is because of the
use of this beginning boundary that life's appearin
g at 3.8 billion years ago would result in a window less than the previous
one of 400 million years (if the formation of the Earth were used instead, it
would be 4.55 Gya – 3.8 Gya = 0.75 Gya).
"Around 300 to 400 million years after the Earth had accreted to its present
size, the impact
frequency had declined to the point that the planet's surface temperature
started to drop and a thick
crust could solidify. Finally, as the crust cooled below the boiling point
of water, the oceans
began to form. ...
Life as we know it could not have appeared without bodies of water. Because
of Schopf's work, we know that life was well established on the planet by 3.5
billion years ago.
Thus the Earth's oceans must have originated roughly sometime between 4.2
and, at the latest,
3.6 billion years ago.
Even after the Earth's oceans formed, however, the threat of impacts would
still have made the appearance of life very difficult. At least ten large
bolides struck the Moon between 3.8 and 4.1 billion years ago." (Christopher
Wills & Jeffrey Bada, The Spark of Life: Darwin and the
Primeval Soup, Perseus Publishing, 2000, p77)
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