quote:
That "about 3.85 billion" leaves a lot of leeway.
I read other reports that say that life seems to have begun surprising early, perhaps within 100 million years after the earth was cool enough. Ross gets 0 time, and others get 100 million years. There is enough uncertainty in the timings, that one cannot pin it down as precisely as Ross suggests. Here's a report suggesting that the earth might have already been capable of supporting life as early as 4.2 billion years ago. I am not asserting that as correct. I am saying that it illustrates the large amount of uncertainty in the timings.
Yes, there is apparently some scholarly disagreement and debate about the timing. Some researchers claim evidence for life at more than 3.8 billion years ago, and others a bit less. Ross is going with about 3.8 billion, which is not quite the oldest and not the youngest. Ross' position on this may turn out to be slightly incorrect, but to accuse him of "lying for Jesus" on this point is ridiculous!
Here is a more recent reference:
Kevin D. McKeegan, Anatoliy B. Kudryavtsev and J. William Schopf, "Raman and ion microscopic imagery of graphitic inclusions in apatite from older than 3830 Ma Akilia supracrustal rocks, west Greenland," Geology; July, 2007; v. 35; no. 7; p. 591-594; DOI: 10.1130/G23465A.1 writes:
Three-dimensional molecular-structural images of apatite grains and associated minerals embedded in a banded quartz-pyroxene-magnetite supracrustal rock from Akilia, southern west Greenland, were constructed by using Raman confocal spectroscopy. The rock sample is the same as that from which apatite-hosted isotopically light graphitic inclusions were reported by Mojzsis and colleagues in 1996; the results were challenged in 2005 by Lepland and colleagues who failed to find carbon-bearing inclusions in this and other Akilia samples. Here we demonstrate that inclusions of graphite wholly contained within apatite occur in this rock. The carbon isotopic composition of one such inclusion, its graphitic composition established by Raman spectroscopy, was measured by secondary ion mass spectrometry to be isotopically light (13C = —29 4), in agreement with earlier analyses. Our results are thus consistent with the hypothesis that graphite-containing apatite grains of the older than 3830 Ma Akilia metasediments may represent chemical fossils of early life.
quote:
I am confused by what Ross calls "prebiotic life". You seem to be treating that as if only inorganic chemistry was involved. I am assuming that there could be some significant amount of organic chemistry going on, perhaps even something with many of the characteristics of biology, although not enough of them that we would consider it to be life.
I believe most isotopic fractionation is due to photosynthesis. C3 photosynthesis (trees and most plants) gives a delta-13 of about -25 to -30 per mil (consistent with the 3.83 Ga deposits above), while C4 photosynthesis (e.g. maize) gives a delta-13 of about -10 to -15 per mil.
quote:
"Science week" writes:
Owing to the difficulties in distinguishing between life and nonlife, no one signature of life -- for example, the fractionated isotopic ratio, the molecular carbon composition, or an isolated microfossil -- should be considered unequivocal evidence for traces of past life.
http://scienceweek.com/2005/sw050513-1.htm
It's not clear to me what the Science week author is claiming, and why. Is he claiming that abiotic chemical processes can produce a fractionation as large as the -29 per mil that is seen? If so, I am very skeptical. What is his evidence for this, other than hand-waving? What specific chemical processes is he invoking, and where is the evidence that they can fractionate so much? Or is he claiming that early (non-photosynthetic?) biotic processes could have produced a low fractionation? I would find this more plausible.
quote:
It seems to me that Ross is drawing conclusions that are far too strong, given the uncertanties involved.
Ross is simply following the claims of researchers in the field. I don't think these claims are too strong at all, in light of the large isotopic fractionation seen in the early Greenland rocks. If you disagree, please provide some experimental evidence of abiotic processes that can produce such large fractionations.
™ Version 4.2