Prebiotic Oxygen. A key question in origin-of-life research is the oxidation state of the prebiotic atmosphere (the current best guess is that the origin of life occurred somewhere around 4.0-3.7 bya (billion years ago)). Wells wants you to think that there is good evidence for significant amounts free oxygen in the prebiotic atmosphere (significant amounts of free oxygen make the atmosphere oxidizing and make Miller-Urey-type experiments fail). He spends several pages (14-19) on a pseudo-discussion of the oxygen issue, citing sources from the 1970's and writing that (p. 17) "the controversy has never been resolved", that "Evidence from early rocks has been inconclusive," and concluding that the current geological consensus -- that oxygen was merely a trace gas before approximately 2.5 bya and only began rising after this point -- was due to "Dogma [taking] the place of empirical evidence" (p. 18).
None of this is true (see e.g.
Copley, 2001).
Certain minerals, such as uraninite, cannot form under significant exposure to oxygen. Thick deposits of these rocks are found in rocks older than 2.5 bya years ago, indicating that essentially no oxygen (only trace amounts) was present. On page 17 Wells notes that uraninite deposits have been found in more recent rocks, but neglects to mention to his readers that these only occur under rapid-burial conditions, whereas ancient deposits of uraninite occur in slow deposition conditions, for example in sediments laid down by rivers, so that the minerals were exposed to atmospheric gases for significant periods of time before burial.
'Red beds' are geologic features containing highly oxidized iron (rust) indicative of high amounts of oxygen. Wells (p. 17) notes that red beds are found before 2 bya, but fails to mention that the temporal limit of red beds is just a few hundred million years before 2 bya.
Wells doesn't even mention the evidence that banded iron formations (incompletely oxidized iron indicative of ultralow-oxygen conditions) are very common prior to 2.3 bya and very rare afterwards.
Wells also doesn't mention that early paleosols (fossil soils) from about ~2.5 bya contain unoxidized cerium, impossible in an oxygenic atmosphere (e.g., Murakami et al., 2001).
Finally, Wells doesn't mention to his readers that pyrite, a mineral even more vulnerable to oxidation than uraninite, is found unoxidized in pre-2.5 bya rocks, and with significant evidence of long surface exposure (i.e. grains weathered by water erosion; e.g. Rasmussen and Buick, 1999).
Why does Wells leave out the converging independent lines of geological evidence pointing to an anoxic early (pre ~2.5 bya) atmosphere?
Was the prebiotic atmosphere reducing? Are the Miller-Urey experiments "irrelevant"? The famous Miller-Urey experiments used a strongly reducing atmosphere to produce amino acids. It is important to realize that the original experiment is famous not so much for the exact mixture used, but for the unexpected discovery that such a simple experiment could indeed produce crucial biological compounds; this discovery instigated a huge amount of related research that continues today.
Now, current geochemical opinion is that the prebiotic atmosphere was not so strongly reducing as the original Miller-Urey atmosphere, but opinion varies widely from moderately reducing to neutral. Completely neutral atmospheres would be bad for Miller-Urey-type experiments, but even a weakly reducing atmosphere will produce lower but significant amounts of amino acids. In the approximately two pages of text where Wells actually discusses the reducing atmosphere question (p. 20-22), Wells cites some more 1970's sources and then asserts that the irrelevance of the Miller-Urey experiment has become a "near-consensus among geochemists" (p. 21).
This statement is misleading. What geochemists agree on is that if the early earth's mantle was of the same composition as the modern mantle and if only terrestrial volcanic sources are considered as contributing to the atmosphere, and if the temperature profile of the early atmosphere was the same as modern earth (this is relevant to rates of hydrogen escape) then there will be much less hydrogen compared to Miller's first atmosphere (20% total atm.). Even if this worst-case scenario is accepted, hydrogen will not be completely absent, in fact there is a long list of geochemists that consider hydrogen to have been present (although in lower amounts, roughly 0.1-1% of the total atmosphere). At these levels of H2 there is still significant (although much lower) amino acid production.
Also, many geochemists think that these conditions do not represent the early earth, contrary to the impression given by Wells. For example, on p. 20, Wells mentions terrestrial volcanos emitting neutral gases (H2O, CO2, N2, and only trace H2), but he fails to mention that mid-ocean ridge vents could have been significant sources of reduced gases -- they are important sources of reduced atmospheric gases even today, emitting about 1% methane (Kasting and Brown, 1998) and producing reduced hydrogen and hydrogen sulfide (e.g. Kelley et al., 2001; Perkins, 2001; Von Damm, 2001) and potentially ammonia prebiotically (Brandes et al., 1998; Chyba, 1998). Why does Wells exclude oceanic vents from consideration?
Another strange omission is that Wells completely fails to mention the extraterrestrial evidence, which is the only direct evidence we have of the kinds of chemical reactions that might have occurred in the early solar system. For example he neglects to mention the famous Murchison meteorite, which contains mixtures of organic compounds much like those produced in Miller-Urey style experiments, and which constitutes direct evidence that just the right kind of prebiotic chemistry was occurring at least somewhere in the early solar system, and that some of those products found their way to earth (see e.g. Engel and Macko, 2001 for a recent review).
Wells asserts that since the 1970's, non-reducing atmospheres have become the "near-consensus." The latest article that Wells cites supporting this view, however, is a 1995 nontechnical news article in Science (Cohen, 1995). Why doesn't he quote Kral et al. (1998), who write,
The standard theory for the origin of life postulates that life arose from an abiotically produced soup of organic material (e.g., Miller, 1953; Miller, 1992). The first organism would have therefore been a heterotroph deriving energy from this existing pool of nutrients. This theory for the origin of life is not without competitors (for a review of theories for the origins of life see Davis and McKay, 1996), but has received considerable support from laboratory experiments in which it has been demonstrated that biologically relevant organic materials can be easily synthesized from mildly reducing mixtures of gases (e.g., Chang et al., 1983). The discovery of organics in comets (e.g., Kissel and Kruger, 1987), on Titan (e.g., Sagan et al., 1984), elsewhere in the outer solar system (e.g., Encrenaz, 1986), as well as in the interstellar medium (e.g., Irvine and Knacke, 1989) has further strengthened the notion that organic material was abundant prior to the origin of life.
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Quote by: Keith Hamburger
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