"I don't doubt that you don't doubt it. I do, however, doubt that you have
calibrated your doubtometer recently. :-)"
I'm glad you smiled when you said that, partner.
Alright. Your argument is that only strongly reducing conditions can
produce enough amino acids to make any prebiotic scenario feasible, yet
Miller claims that such conditions never existed. What is his evidence?
"Considerable opinion" is not scientific evidence. If it was, there is also
"considerable opinion" that such conditions did exist, very early, within
the first 500 million years at least. What's more, there is even evidence
to support that opinion.
Here are some contemporary (with your Miller paper) statements regarding
this issue from the other side of the fence [Stephen F. Mason, _Chemical
Evolution_, Oxford:Clarendon Press, 1991, pg. 111-114]:
"The inner planets, formed from volatile-depleted planetesimals, lacked any
significant primitive atmosphere. Gases and volatile elements were occluded
or chemically combined in the planetesimals, as they are in recovered
meteorites, and the thermal processing of the planet released noble gas
atoms and molecular gases to form a secondary atmosphere....Soon after the
accretion of the terrestrial planets, it is probable that their surfaces
were covered by magma oceans, possibly 200 km deep in the case of the Earth,
due to the heat liberated during accretion and core formation....Intense
volcanic activity during crust formation led to the degassing of occluded
and chemically combined volatiles, forming the early secondary atmosphere.
Predominantly reducing gases are obtained by heating up to 1500 K meteoritic
material modelling the terrestrial planetesimal composition (98 per cent of
high-iron chondrite and 2 per cent of C1 chondrite). The main gases
liberated up to about 1000 K are, in order of decreasing abundance, methane,
nitrogen, hydrogen, ammonia and water vapour, with minor amounts of hydrogen
sulphide, carbon monoxide, and carbon dioxide. With full outgassing over
the 1000--1500 K range, nitrogen and ammonia drop to the level of minor
constituents and comparable amounts of methane, carbon monoxide, carbon
dioxide, water vapour, and hydrogen are released (Lewis and Prinn 1984
_Planets and their atmospheres: Origin and evolution_ Academic Press,
London).
"The lifetime of a reducing atmosphere on the terrestrial planets is
expected to be short, owing to the photolysis of the hydrides by the solar
ultraviolet radiation and the escape of hydrogen atoms from the
gravitational field of the planet in its upper atmosphere. The planetary
atmosphere then becomes neutral, composed mainly of carbon dioxide and
nitrogen, as observed for Venus and Mars at present....On the Earth
uniquely, temperature conditions over much of the surface have been
restricted to the liquid range of water over the past 4 billion years or so,
thereby facilitating the fixation of atmospheric carbon dioxide. First, in
the weathering reaction, aqueous bicarbonate attacked silicate minerals to
liberate silica and form carbonate sediments, which date back to about 3.8
million years ago. Second, photosynthetic organisms reduced dissolved
carbon dioxide with hydrogen split from water, liberating the molecular
oxygen produced into the atmosphere at significant partial pressures from
about 2 billion years ago.
"It has been argued that the atmosphere of the Earth was never reducing and
that it has evolved continuously over the whole history of the Earth by the
volcanic emission of gases from the mantle throuhgh the crust. The view
rests on the nineteenth-century uniformitarian principle that the history of
the Earth must be expained only in terms of the geological forces observed
in operation today. The principle components of present-day volcanic gases
are steam and carbon dioxide, which are partially soluble in silicate melts.
At 30 kbar pressure and 1625 C melts of diopside (CaMgSi2O6) dissolve up to
24 per cent of water and 5 per cent of carbon dioxide by weight (Holland
1984 _The chemical evolution of the atmosphere and oceans_ Princeton
University Press). The gradual release of such quantities of water and
carbon dioxide from the mantle of the Earth by volcanic action over 4
billion years suffices to account for the present volume of the oceans and
the mass of the carbonate deposits. But the argon isotope ratios indicate
that the Earth was over 75% degassed by 4 billion years ago. The
accumulation of [Ar-40] in the mantle from the radioactive decay of [K-40]
now gives a mantle ratio of [Ar-40]/[Ar-36] > [10^4], estimated from the
gases entrained in the volcanic lavas at the mid-ocean ridges. The
atmospheric ratio of [Ar-40]/[Ar-36] is only 295.5, corresponding to the
mantle value more than 4 billion years ago when the major degassing of the
Earth must have occurred (Ozima 1987 _Geohistory: Global evolution of the
earth_ Springer, Berlin)."
To summarize Mason, from 4.5 to 4 billion years ago, 75% of the gases
trapped in the crust were released, including large amounts of methane,
ammonia, hydrogen, nitrogen and water, exactly the kind of reducing
components "considerable opinion" claims were never present. (Evidence:
same gases in same percentages are obtained when planetesimal meteorites are
heated.) Then, by 4 billion years ago, the outgassing had all but been
exhausted, allowing for a switch in gas release from reducing to neutral.
This information supports both Henderson-Sellers and Chang, but does not
support your implied conclusion that the atmosphere was always neutral from
Day One. In fact, atmospheric argon ratio supports the idea that by 3.8
billion years ago nearly all the gases that you ever be released had been,
which means that from then until now continued outgassing has largely been
negligable. It also means that reducing gases would have been a major
constituent of the primeval atmosphere until outgassing ceased, at which
point they would quickly disappear. In any event, this establishes that for
the first 500 to 700 million years, there was probably enough reducing gases
to make all the amino acids needed for later proteinoid formation.
"If you check the Miller paper cited above you'll find that the yields drop
way down for mildly reducing atmospheres. Typically less than 0.1% total
yield, some times less than 0.001%."
Funny you should mention that. In 1983 Miller co-authored a paper in the
_Journal of Molecular Evolution_ with G Schlesinger entitled "Prebiotic
synthesis in atmospheres containing CH4, CO, and CO2. I. Amino acids"
[19(5):376-82]. Here is the complete abstract:
"The prebiotic synthesis of organic compounds using a spark discharge on
various simulated primitive earth atmospheres at 25 degrees C has been
studied. Methane mixtures contained H2 + CH4 + H2O + N2 + NH3 with H2/CH4
molar ratios from 0 to 4 and pNH3 = 0.1 torr. A similar set of experiments
without added NH3 was performed. The yields of amino acids (1.2 to 4.7%
based on the carbon) are approximately independent of the H2/CH4 ratio and
whether NH3 was present, and a wide variety of amino acids are obtained.
Mixtures of H2 + CO + H2O + N2 and H2 + CO2 + H2O + N2, with and without
added NH3, all gave about 2% yields of amino acids at H2/CO and H2/CO2
ratios of 2 to 4. For a H2/CO2 ratio of 0, the yield of amino acids is
extremely low (10(-3)%). Glycine is almost the only amino acid produced from
CO and CO2 model atmospheres. These results show that the maximum yield is
about the same for the three carbon sources at high H2/carbon ratios, but
that CH4 is superior at low H2/carbon ratios. In addition, CH4 gives a much
greater variety of amino acids than either CO or CO2. If it is assumed that
an abundance of amino acids more complex than glycine was required for the
origin of life, then these results indicate the requirement for CH4 in the
primitive atmosphere."
So this earlier research actually establishes that neutral atmospheres can
produce as much amino acids as a reducing atmosphere can as long as there is
enough hydrogen present. As Mason points out, hydrogen was a constituent of
the atmosphere, sometimes a major one, for at least 500 million years.
During this time large amounts of various amino acids could be made, the
primeval oceans could have protected them from UV destruction [Cleaves HJ,
Miller SL "Oceanic protection of prebiotic organic compounds from UV
radiation" _Proc Natl Acad Sci USA_ 1998;95(13):7260-3 ] and evaporating
pools could have concentrated the amino acids to levels necessary for
thermal copolymerization to occur. Then, when the neutral gases replaced
the reducing gases amino acid production could continue, but more
importantly hydrogen cyanide, formaldehyde and ammonia production would
continue as well [Schlesinger G, Miller SL "Prebiotic synthesis in
atmospheres containing CH4, CO, and CO2. II. Hydrogen cyanide, formaldehyde
and ammonia" _J Mol Evol_ 1983;19(5):383-90]. What makes this important is
that these compounds can form purines and sugars (necessary for making
nucleic acids) as well as some amino acids. And when you also throw in the
fact that the components of coenzyme A (which is used to activate amino
acids prior to peptide synthesis) can probably be made abiotically [Keefe
AD, Newton GL, Miller SL "A possible prebiotic synthesis of pantetheine, a
precursor to coenzyme A" _Nature_ 1995;373(6516):683-5] you have a situation
where proteinoid microspheres, utilizing their native catalytic capability
plus the environmental amino acids, formaldehyde, ammonia and coenzyme A,
could start making their own amino acids and proteinoids. This is all
that's need for natural selection to begin promoting systems that are more
efficient than others. This would then lead to the first true (primitive
cell).
All of which makes me wonder whether your Miller paper is really critical of
abiogenesis, or is simply discussing the issues?
Kevin L. O'Brien