DNAunion: Continuing from my personal notes:
Concerning the earliest fossil findings, it should be remembered that the
cyanobacteria-like organisms that left microfossils in 3.5 billion year old
rock, those studied by and written about by Schopf, were highly developed -
they must have already had DNA, RNA, proteins, ribosomes, cell membranes,
genes, metabolism, etc. And these cellular components and processes must
have been present much earlier because by 3.5 billion years ago, complex life
was already widespread.
"The most ancient [fossils] are 3.5 billion years old, but the cells are
arrayed in long filaments,
mats, and other complex structures - implying a long history of even earlier
evolution that is
missing from the record." (Gretchen Vogel, Going Beyond Appearances to Find
Life's History,
Science, June 25, 1999, v284 n5423, 2112)
"Schopf's fossils have preserved enough details to show that by as early as
3.5 billion years ago,
life had advanced well beyond the very primitive stage. The organisms living
then looked very
much like the common photosynthetic bacteria that live in the oceans today.
And the modern
appearance of the ancient fossils meant that these tiny fossils must already
have had a long
evolutionary history. They were the oldest fossils yet found, but they were
apparently far from
being the oldest living entities." (Christopher Wills & Jeffrey Bada, The
Spark of Life: Darwin
and the Primeval Soup, Perseus Publishing, 2000, p60)
In fact, chemical and isotopic signatures of life dating back to 3.85 billion
years ago have been found.
"Rather than being sterile, the sediments, extending in time beyond 3850 Ma
[3.85 billion years
ago], contain ubiquitous "chemofossils" (Mojzsis et al. 1996; Mojzsis and
Arrhenius 1998) with
isotopic composition suggestive of highly evolved enzyme systems - life seems
to have been
developing for an unknown, but probably considerable length of time before
3850 Ma. ... Life on
Earth instead of (as is often quoted) originating 3.5 to 3.8 billion years
ago, is found to have
developed to a high degree of autotrophic sophistication already before 3850
Ma. Carbon isotope
fractionation mechanisms indicated by the geological record of even the
oldest rocks are matched
today only by the action of enzymes such as the ribose- and
ribulose-phosphate carboxylases that
mediate the first step in carbon fixation. (Mojzsis et al. 1996)." (Stephen
J Mojzsis,
Ramanarayanan Krishnamurthy, and Gustaf Arrhenius, Before RNA and After:
Geophysical and
Geochemical Constraints on Molecular Evolution, Chapter 1 of The RNA World:
Second
Edition,Cold Spring Harbor Laboratory Press, 1999, p26, 29-30)
Taking multiple lines of evidence into consideration, Jeffrey Bada and Chris
Wills estimate life arose between 4 billion and 3.8 billion years ago.
"Bill Schopf has found definitive fossil traces of the presence of life in
3.5-billion-year-old rocks,
and there is some less certain evidence that life was present 3.8 billion
years ago. Some of the
genealogies of the genes that we explored in Chapter 9 have been traced back
approximately as
far. This means that life probably arose between about 4 billion years and
3.8 billion years ago."
(Christopher Wills & Jeffrey Bada, The Spark of Life: Darwin and the Primeval
Soup, Perseus
Publishing, 2000, p258)
The values presented so far all agree if one accepts that life appeared
sometime prior to 3.85 billion years ago. But even more-extreme estimates
exist:
"... the last common ancestor [of all life on Earth] was a hyperthermophile
[an organism that
thrives in extremely hot environments], says John Baross, an evolutionary
microbiologist at the
University of Washington, Seattle. Extrapolating back from the first fossil
evidence of microbes
3.8 billion years ago, he and others estimate this organism lived about 4.3
billion years ago."
(Virginia Morrell, Tracing the Mother of All Cells, Science, May 2 1997, v276
n5313, p700)
Different estimates exist for the amount of time between large impacts, and
hence, theoretically, the available time window for life to have arisen.
Some sources suggest a "large" window of 20 million years (remember, it was
originally thought that nearly 4 billion years was available for life to get
started, and until recently, even 400 million years was considered a
"comfortable window") while others believe impacts were occurring every 6,000
years.
"According to Greenberg (1995), the collision frequency of comets at the end
of the strong bombardment period, some 3.8 Ga [billion years] ago, decreased
to about 1 in 6,000 years. Thus, the time window of his scenario [for the
origin of life] is approximately 6,000 years." (Noam Lahav, Biogenesis:
Theories of Life’s Origins, Oxford University Press, 1999, p160)
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