Download The prebiotic geochemistry of formaldehyde_Review

Precambrian Research 164 (2008) 111–118
Contents lists available at ScienceDirect
Precambrian Research
journal homepage: www.elsevier.com/locate/precamres
Review
The prebiotic geochemistry of formaldehyde
H. James Cleaves II ∗
Geophysical Laboratory, The Carnegie Institution for Science, 5251 Broad Branch Road NW, Washington, DC 20015, United States
a r t i c l e
i n f o
Article history:
Received 5 September 2007
Received in revised form 2 April 2008
Accepted 3 April 2008
Keywords:
Formaldehyde
Formose
Prebiotic chemistry
Origin of life
Hexamethylenetetramine
Cyanide
Hydrogen sulfide
Geochemistry
a b s t r a c t
Formaldehyde (HCHO), the simplest aldehyde, is an intermediate oxidation state one carbon molecule
that exists transiently but prominently in the abiological carbon cycle, and is ubiquitous in the cosmos. Its
potential prebiotic importance is suggested by the fact that it readily undergoes a variety of addition and
redox reactions to give products of biological significance including sugars and amino acids. It is especially
important with respect to the origin of an RNA or pre-RNA world, since HCHO may be a precursor to ribose
and other sugars. HCHO is introduced to the environment by a number of processes including atmospheric
and aqueous phase synthesis as well as extraterrestrial delivery, balanced by various destructive processes
such as photolysis and redox equilibration in hydrothermal environments. While the Strecker synthesis of
amino acids can occur at very low dilution, even best case scenarios for HCHO steady-state concentrations
in the primitive oceans are too low for the formation of sugars to occur. Concentration mechanisms
would thus be necessary. As HCHO is volatile, direct evaporation is not possible, but other geochemical
mechanisms such as eutectic freezing and conversion to non-volatile derivatives by reaction with other
species present in the primitive environment, followed by evaporation, could have concentrated HCHO
sufficiently to allow for sugar synthesis.
© 2008 Elsevier B.V. All rights reserved.
Contents
1.
2.
3.
4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Prebiotic sources of HCHO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Prebiotic sinks for HCHO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Prebiotic solution chemistry of HCHO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.
Oligomerization chemistry—polyoxymethylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.
Oligomerization chemistry—the formose reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.
Reactions with amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.
Reactions with sulfur species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.
Mineral interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.
Reaction with HCN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.
Concentration mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Formaldehyde (HCHO) is a one carbon molecule intermediary
along the redox continuum between CO2 and CH4 , at the same
oxidation state (0) as graphite. HCHO appears to be an abundant
∗ Tel.: +1 202 478 8957; fax: +1 202 478 8464.
E-mail address: [email protected].
0301-9268/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.precamres.2008.04.002
111
112
112
113
113
113
114
114
115
115
115
116
116
116
interstellar molecule and is suggested to be a major constituent of
cometary ices (Schutte et al., 1993; Fomenkova et al., 1994; Biver
et al., 2002). It is readily produced in prebiotic simulation experiments from a variety of gas mixtures and energy sources (Bossard et
al., 1982; Schlesinger and Miller, 1983; Stribling and Miller, 1987).
It has been suggested for some time that HCHO may have played
an important role in the synthesis of organic molecules relevant
to the origin of life. HCHO is likely a significant precursor for
the prebiotic synthesis of glycine (Miller, 1953), and HCHO reacts
112
H.J. Cleaves II / Precambrian Research 164 (2008) 111–118
readily with glycine to form a number of other amino acid products (Choughuley et al., 1975) which have been detected in both
meteorites and prebiotic simulation experiments (Wolman et al.,
1972). It has long been known that HCHO can react to form sugars under basic conditions (Butlerow, 1861), a reaction that has
potential importance for the origin of an “RNA World” (Gilbert,
1986; Gesteland and Atkins, 1993) or early nucleic acids based
on alternative sugars in a potential “pre-RNA World” (Joyce et al.,
1987; Eschenmoser, 2004). It has been argued based on kinetic
and thermodynamic principles that HCHO may in fact be the only
one carbon C-, H- and O-containing molecule capable of generating
complex organic compounds for the origin of life (Weber, 2002).
The chemistry of HCHO in the primitive environment would
have depended on a number of factors, including the atmospheric
composition, energy flux through the atmosphere, temperature and
pH of the oceans, the availability and nature of minerals, dry land
surfaces and microenvironments. Unfortunately there is a great
deal of uncertainty in most of these considerations.
Estimates of the composition of the early atmosphere range
from fairly reducing (Tian et al., 2005) to neutral (Chyba and Sagan,
1992). The flux of energy was probably dominated by UV radiation, along with significantly lesser amounts of electrical energy
and energy from radioactive decay (Chyba and Sagan, 1992). The
early Earth’s surface temperature may have been extremely cold,
resulting in a generally ice-covered planet (Bada et al., 1994;
Price, 2007), extremely warm (Knauth, 2005), or perhaps with
regions of both cold and hot temperature similar to today’s climate
(Henderson–Sellers and Henderson–Sellers, 1988; Valley et al.,
2002; Vincent et al., 2005). The oceanic composition may have been
more carbonic than today (Morse and Mackenzie, 1998), slightly
more saline (Morse and Mackenzie, 1998), appears to have been
suffused with large amounts of ferrous iron (Holland, 1973; Walker
and Brimblecombe, 1985; Isley and Abbott, 1999), and likely had a
pH somewhere between 5 and 11 (Kempe and Degens, 1985; Morse
and Mackenzie, 1998; Kempe and Kazmierczak, 2002), although it
has been argued that ocean pH may have been maintained fairly
close to the modern value (∼8) by mineral buffering (Sillén, 1967).
It is debated whether there was any dry land at the time of the origin of life, as widescale crustal recycling to form continents may
not yet have begun (Hofmann, 1997; Godderis and Veizer, 2000;
Wilde et al., 2001), nevertheless island arcs may have been present
(Barley et al., 1979). All of these considerations are important to the
discussion that follows, and certainly some combinations of these
are incompatible with certain proposals for the origin of life, and
more specifically, HCHO’s role in it.
2. Prebiotic sources of HCHO
The most important source of HCHO on the primitive Earth
was probably gas phase atmospheric photochemical synthesis by
the photo-reduction of CO2 with H2 O (among other mechanisms)
(Reaction (1)):
CO2 + H2 O ⇔ HCHO + O2
(1)
Pinto et al. (1980) estimated that HCHO could be produced in
yields of up to 1011 mol/year, reaching a steady-state oceanic concentration of 10−3 M in 107 years. The significance of this time
interval is discussed below. Holland (1984) on the other hand estimated a steady-state oceanic concentration of HCHO of 4 × 10−4 M.
It has been estimated that if all of the carbon in the Earth’s surface
reservoirs (crust, atmosphere and oceans) were dissolved in oceans
of the present size, a concentration of ∼1 M would be obtained
(Schwartz, 1981), but this extreme is meant solely for purposes
of comparison. Electric discharges acting on a variety of gas mix-
tures also produce HCHO in good yield (Miller, 1953; Stribling
and Miller, 1987), though there would have been considerably less
energy available from electric discharges than from UV sources
(Chyba and Sagan, 1992). Photochemical production from atmospheric CH4 and/or CO could also have been significant sources of
HCHO (Hubbard et al., 1971; Ferris and Chen, 1975; Bar-Nun and
Hartman, 1978; Schwartz, 1983; Wen et al., 1989). The production
and rainout rates of HCHO are highly sensitive to energy source and
atmospheric composition (Chang, 1993).
HCHO could also potentially be produced under hydrothermal
vent conditions (Ferris, 1994), for example by the reduction of aqueous formate, CO or CO2 , or by the oxidation of methanol or CH4 ,
although HCHO and methanol do not appear to be stable redox
states under hydrothermal conditions (Osada et al., 2004; Seewald
et al., 2006). Little HCHO is observed in modern fluids, possibly for
this reason.
One poorly surveyed topic is the photo-reduction of dissolved
CO2 species with reducing agents such as Fe2+ in the photic zone of
the primitive oceans. Fe2+ concentrations in the primitive oceans
are estimated to have been as high as 10−4 M (Holland, 1973). Photoreduction of dissolved CO2 species in this manner has been claimed
(Åkermark et al., 1980), and given the high UV flux, large amounts
of dissolved carbonate species, and possibly transition metals in
addition to water as reducing equivalents, this might have been a
significant source of reduced carbon species.
It has also been suggested that extraterrestrial materials such
as comets may have delivered significant quantities of HCHO to
the surface of the Earth (Chyba et al., 1990). Indeed depending on
the composition of the atmosphere and the nature of the extraterrestrial input this could have represented the single largest source
of reduced organic compounds to the surface of the Earth (Chyba
and Sagan, 1992). Of course upon reaching the primitive surface,
this material would have been subject to the same environmental
chemistry as endogenously synthesized organic material.
3. Prebiotic sinks for HCHO
HCHO decomposes thermally in the gas phase above 300 ◦ C to
give CO and H2 (Bone and Smith, 1905) (Reaction (2)):
HCHO(g) ⇔ CO(g) + H2 (g)
(2)
Between 150 ◦ C and 300 ◦ C methanol is also observed as a
product (Calvert and Steacie, 1951). Relatively long wavelength
photochemical destruction was also likely a significant sink for
HCHO (see below) (Calvert and Steacie, 1951). Sekine (2002) found
that MnO2 catalyzes the oxidation of HCHO at room temperature to
CO2 under atmospheric conditions. Löb (1906) found that electric
discharges acting on HCHO in the presence of water vapor give CO,
CO2 , H2 and CH4 as products, which is essentially the reverse of the
reaction demonstrated by Miller (1953) mentioned above.
The present mean lifetime for the cycling of the entire ocean
through the mid-ocean ridge hydrothermal vents is ∼107 years
(Edmond et al., 1982). This cycling likely occurred much more
rapidly on the primitive Earth (by perhaps a factor of 5) than it
does today based on heat flux estimates (Turcotte, 1980). Small prebiotic precursor species (HCHO, HCN, NH3 , etc.) would have been
redox equilibrated under hydrothermal vent conditions (Lazcano
and Miller, 1994). As mentioned earlier, HCHO appears to be thermally unstable with respect to CO2 , CO, and CH4 , thus hydrothermal
vents are more likely to be net destroyers of HCHO than net producers.
There is reason to believe that there was a considerable flux of
atmospherically synthesized and rained-out H2 O2 to the primitive
oceans, particularly if the atmosphere was non-reducing (Kasting
H.J. Cleaves II / Precambrian Research 164 (2008) 111–118
113
4.1. Oligomerization chemistry—polyoxymethylene
Fig. 1. Cyclic HCHO oligomers, trioxane (left) and tetraoxane (right).
and Brown, 1998). Reaction of HCHO with dissolved peroxide,
which can also be formed photochemically, gives formate and H2
(Walker, 1964) (Reaction (3)):
2HCHO(aq) + 2OH− + H2 O2 (aq)
⇔ 2HCOO− (aq) + 2H2 O + H2 (g)
(3)
4. Prebiotic solution chemistry of HCHO
An excellent monograph has been written on the chemistry of
HCHO (Walker, 1964). In aqueous solution HCHO is mostly present
as the monohydrate, methylene glycol (Reaction (4)). Unhydrated
HCHO is only present to a degree of ∼0.1% in aqueous solution.
HCHO + H2 O ⇔ CH2 (OH)2
(4)
One important consequence of this is that while HCHO
absorbs considerable energy in the range of 250–350 nm, resulting in photolysis, the hydrate is does not absorb light at these
wavelengths (Ferris and Chen, 1975), thus dissolution in water
could be an important photo-protection mechanism for HCHO.
Nevertheless, as discussed below, there are several major photochemical transformations of HCHO which occur in aqueous
solution.
A series of low molecular weight polymers, polyoxymethylene
glycols (POMs) (of formula HO(CH2 O)n H) as well as the cyclic
oligomers trioxane and tetraoxane (Fig. 1), form readily in neutral
concentrated aqueous HCHO solutions.
As the chain length of POMs grows, their solubility decreases,
and their solubility is temperature dependant, being more soluble at higher temperatures. It seems unlikely that bulk ocean
concentrations of HCHO and its polymers could have been high
enough to form precipitates. For example, a 5 wt% (∼1.3 M) aqueous HCHO solution contains only ∼14% POM dimer, 3% trimer,
and 0.5% tetramer. Polymerization equilibria would be much
lower for the prebiotic oceanic concentrations of HCHO estimated
above.
4.2. Oligomerization chemistry—the formose reaction
HCHO can be oligomerized under basic conditions into sugars
via the so-called formose reaction (Butlerow, 1861) (Fig. 2), which
has been observed to occur as dilute as 0.01 M (Reid and Orgel,
1967; Schwartz and De Graaf, 1993), although there are accounts of
its reaction as dilute as 10−3 M (Gabel and Ponnamperuma, 1967).
As mentioned earlier, these concentrations may have been difficult
to achieve in the bulk early oceans.
HCHO reacts with acetaldehyde at much lower concentrations
to give acrolein among other products (Cleaves, 2003). There must
be limiting reaction conditions of concentration, pH and temperature under which this reaction will not occur, though these are
not well surveyed. The formose reaction is generally carried out
under extremely basic conditions, but reaction has been observed
at near neutral pH in the presence of minerals (Cairns-Smith
et al., 1972; Schwartz and De Graaf, 1993). It has been pointed
out that the reaction conditions which allow oligomerization of
HCHO also facilitate the products’ decomposition into tars (Reid
and Orgel, 1967; Shapiro, 1988), furans and other low molecular
Fig. 2. Possible mechanism for the formose reaction and observed products.
114
H.J. Cleaves II / Precambrian Research 164 (2008) 111–118
Fig. 5. Products of the reaction of H2 S with HCHO. Formthionals (top) and trithiane
(bottom).
Fig. 3. Branched polyols detected in photochemical formose reactions.
weight degradation products (De Bruijn et al., 1986; Cooper et al.,
2001).
The reaction of HCHO into formose is one limiting reaction
pathway for the accumulation of HCHO in primitive waters, which
would depend on the rate of HCHO production and delivery to
the primitive oceans (Pinto et al., 1980; Schwartz, 1983). If HCHO
became sufficiently concentrated, side reactions may have established the steady-state concentration regardless of the production
rate. Given the observed photochemical synthesis of pentaerythritol (Fig. 3, left hand side) (Schwartz and De Graaf, 1993), and its
likely reaction mechanism, it seems possible that HCHO is converted to CH3 CHO by UV light, and thus it appears unlikely that
bulk oceanic concentrations of HCHO could have been much higher
than ∼10−3 M.
HCHO would also have been prone to photoreaction in the primitive oceans. For example, HCHO photochemically reacts to give
mostly pentaerythritol (Schwartz and De Graaf, 1993), among other
non-sugar products (Fig. 3) (Shigemasa et al., 1977).
4.3. Reactions with amines
Another possible limitation to oceanic HCHO concentrations is
the conversion of HCHO into hexamethylenetetramine (HMT or 1,
3, 5, 7-tetraazatricyclo [3.3.1.13,7 ] decane) (Fig. 4) by reaction with
ammonia (NH3 ) (Butlerov, 1860; Walker, 1964), as noted by Gabel
and Ponnamperuma (1967).
The concentration of NH3 in the primitive oceans would have
been governed by the reduction rate of atmospheric N2 (Schwartz,
1983; Schrauzer et al., 1983; Brandes et al., 1998) and the overall redox balance of the primitive ocean–atmosphere system
(Summers and Chang, 1993). NH3 is also easily photo-oxidized
(Ferris and Nicodem, 1972), thus its survival would have depended
on the pH of the early oceans (except under extremely basic conditions, most NH3 would end up dissolved in the oceans) as well as
by the availability of reduced species such as Fe2+ (Summers and
Chang, 1993; Walker and Brimblecombe, 1985). Reasonable bulk
ocean concentrations of NH3 have been estimated in the range of
10−3 to 2 × 10−6 M (Bada and Miller, 1968; Summers, 1999). The
Keq to form HMT is extremely high—estimated to be ∼1010 (Walker,
1964). This species may be chemically inert (Wolman et al., 1971),
although there are dissenting opinions (Fox and Windsor, 1970).
HMT has been shown to be produced by UV irradiation of interstellar ice analogs, along with a variety of amino acid products
(Bernstein et al., 2002). The prebiotic chemistry of HMT is worthy of
further investigation. It is noteworthy that HMT is non-volatile and
thus could serve as a means of concentrating volatile NH3 and HCHO
by evaporation. Since HMT is more stable at higher pH, changes
in pH that occur as various minerals precipitate during the evaporation of seawater (Lazar et al., 1983) might then liberate these
reagents in more concentrated form for further reactions. Interestingly, sulfuric acid dissociates HMT to (NH4 )2 SO4 and HCHO, and
is a catalyst for POM formation (Walker, 1964). Thus, sulfate–HMT
interactions during evaporation may have allowed concentration
and reaction of HCHO.
The reaction of fairly dilute aqueous HCHO with NH3 (∼0.003 M
each) at high temperature (100–250 ◦ C) has been shown to produce
amino acids, amines and amino alcohols (Aubrey et al. (submitted
for publication)), and the reaction of somewhat more concentrated
HCHO (0.02 M) with NH3 (0.04 M) has been shown to produce
traces of amino acids at lower temperatures (40 ◦ C) (Weber, 1998).
Thus for at least several simple amino acids, NH3 and HCHO may
be sufficient for synthesis to occur.
HCHO reacts rapidly and reversibly with urea to form methylol
ureas (Walker, 1964) (Reactions (5) and (6)):
HCHO + H2 NCONH2 ⇔ HOCH2 NHCONH2
(5)
HCHO + HOCH2 NHCONH2 ⇔ HOCH2 NHCONHCH2 OH
(6)
These polymers have found widespread commercial use, and
in fact the liberation of HCHO from such polymers is a serious
health concern (Lemière et al., 1995). In a prebiotic context these
reversible reactions could analogously serve as a method of concentrating HCHO in a non-volatile form in evaporating environments,
later liberating it for subsequent reaction to form other more stable
products.
4.4. Reactions with sulfur species
HCHO may be lost to reduction via reaction with species such
as Fe2+ and H2 S. In reducing environments, HCHO may be merely
an intermediate in the redox equilibration of other more stable
compounds (Seewald et al., 2006). HCHO and H2 S react rapidly
to produce trithiane and other low molecular weight oligomers
(Walker, 1964) (Fig. 5), which have been detected in hydrothermal
vent effluent (Simoneit (1992)), as well as in hydrothermal vent
simulations (Cole et al., 1994) (Reactions (7–9)).
H2 S + H2 C(OH)2 ⇔ HOCH2 SH + H2 O
(7)
2HOCH2 SH ⇔ HSCH2 OCH2 SH
(8)
HOCH2 SH + HSCH2 OCH2 SH ⇔ HSCH2 SCH2 OCH2 SH + H2 O
(9)
It is generally believed that the early oceans were sulfatic
(SO4 2− -containing) rather than sulfidic (S2− -containing) (Walker
and Brimblecombe, 1985; Grotzinger and Kasting, 1993), but high
concentrations of H2 S may have been present in hydrothermal
vents or other deep anoxic environments. Aqueous HCHO reacts
with SO2 to give methylolsulfonic acid (Walker, 1964) (Reaction
(10)):
Fig. 4. Hexamethylenetetramine (HMT), a product of the condensation of HCHO
with NH3 .
H2 CO + H2 O + SO2 ⇔ HOCH2 SO3 H
(10)
H.J. Cleaves II / Precambrian Research 164 (2008) 111–118
The salts of this compound are non-volatile, but decompose
in dilute acid, liberating HCHO. It is interesting to speculate that
cycling of these compounds in a drying or eutectic environment
might allow for the concentration of HCHO to allow for synthesis
of non-volatile compounds such as sugars. It is worth noting that
the Murchison meteorite contains large amounts of alkane sulfonic
acids (Cooper et al., 1992), and perhaps this is one mechanism by
which these compounds were formed. It has been suggested that
sulfite (SO3 2− ) may have been the principal sulfur species in the
primitive oceans, but the primitive sulfur cycle was likely somewhat complex (Kasting et al., 1989).
4.5. Mineral interactions
Another sink for HCHO may have been mineral adsorption. There
have been few studies of the adsorption of HCHO to minerals,
though the ones that do exist suggest that HCHO–clay adsorption equilibria are not especially high. This is generally also true
for low molecular weight POMs (Parfitt and Greenland, 1970). The
adsorption equilibria may be more significant for clays such as illite
and kaolinite (De and Chandra, 1978; Chandra and De, 1983). Clay
adsorption may have lead to significant concentration effects which
may have facilitated reactions of HCHO such as oligomerization to
formose products and redox reactions to methanol, formate and
CO2 .
Certain minerals, in particular borates (Ricardo et al., 2004), stabilize several intermediates in the formose reaction pathway. There
may be yet other minerals that alter reaction pathways (Gabel and
Ponnamperuma, 1967; Reid and Orgel, 1967; Cairns-Smith et al.,
1972), for example both apatite and calcite were found to be catalysts for the formose reaction (Schwartz and De Graaf, 1993). These
effects have generally been observed using high concentrations of
HCHO (0.01 M), it remains unknown whether they can alter the
reactions of more dilute HCHO.
4.6. Reaction with HCN
Aqueous HCHO reacts readily with HCN to give glycolonitrile
(Henry, 1890) (Reaction (11)).
HCHO + HCN ⇔ HOCH2 CN
(11)
Here HCHO represents the combined concentrations of HCHO
and its hydrate. The Keq for the reaction is exceptionally high
(Schlesinger and Miller, 1973), thus any atmospheric composition
which results in the production of the two compounds essentially results in the production of glycolic acid (Arrhenius et al.,
1994), after hydrolysis of the nitrile. A “best case” concentration
of HCN in the primitive ocean has been estimated at 3.5 × 10−5 M
(Stribling and Miller, 1987). Schwartz and Goverde (1982) found
that the addition of HCHO or glycolonitrile to HCN actually accelerates the formation of HCN tetramer, diaminomaleonitrile (DAMN).
Eschenmoser and co-workers found that HCHO reacts readily with
DAMN to form a crystallizable product (Koch et al., 2007), a fact
which noted some years earlier (Gluesenkamp, 1958). The ratio
of HCN/HCHO production rates is again highly dependant on the
atmospheric composition and nature of the energy source (Chang,
1993).
If the concentration and reaction of HCHO on the primitive Earth
were difficult because of all of the potential competing geochemical sinks, sugars may have been derived from meteoritic input.
Polyols such as glycerol and ribitol have been identified in the
Murchison meteorite, although actual sugars (of formula (CH2 O)n )
higher than dihydroxyacetone have not been identified (Cooper et
al., 2001). This can be explained partially by the relative instability
of sugars over the ∼4 billion years since the Murchison parent body
115
formed and these reactions presumably occurred: the initial synthesis to form sugars may have occurred relatively rapidly on the
parent body, while only those compounds stable enough to survive
until the present day are still detectable. The detection of these
compounds does however present an interesting paradox for prebiotic chemistry. If the polyols and their precursors were generated
by the same aqueous phase chemistry which produced the amino
and hydroxy acids, as well as the purines and pyrimidines which
have been detected in the Murchison meteorite (Pizzarello, 2004),
apparently the inhibition of HCN chemistry by HCHO and vice versa
(Schlesinger and Miller, 1973; Arrhenius et al., 1994) is not a genuine
problem. The ratio of glycolic acid to glycine in Murchison suggests
that NH3 concentrations were fairly high in the parent body (Peltzer
et al., 1984). These quantities also suggest that the reactions which
form HMT and glycolonitrile are not limiting to either purine or
sugar synthesis, and that slow kinetic effects may be more important than the initial rapid equilibrium obtained. This is certainly a
question worthy of further investigation.
Cannizzaro reactions may also be important loss channels for
HCHO. If HCHO concentrations are high enough, HCHO disproportionates significantly to form formate and methanol (Walker, 1964),
which reduces the concentration of HCHO available to form sugars. These reactions are acid-, base-, and metal-catalyzed (Walker,
1964). Since the reaction reportedly shows third order kinetics, it
is not clear whether it would occur significantly in dilute solution.
4.7. Concentration mechanisms
A serious problem with the formose reaction as it relates to the
origin of life is the concentration of formaldehyde, likely very dilute
in the primitive seas, to high enough values that reactions to form
sugars and higher products could occur. While it seems unlikely
that this could have occurred significantly in the open oceans, it is
also unlikely that HCHO could have been sufficiently concentrated
on benthic clays or in evaporitic basins due to its extreme dilution
(Lahav and Chang, 1976), and general low affinity for mineral surfaces in the first case, and in the second case due to its volatility.
Incorporation into low molecular weight non-volatile compounds
such as HMT and glycolonitrile could be a mechanism for the concentration of HCHO in evaporating environments.
Eutectic freezing has been shown to be an effective method for
concentrating dilute prebiotic reactants to form purines, pyrimidines, and amino acids (Sanchez et al., 1966; Levy et al., 2000;
Miyakawa et al., 2002; Cleaves et al., 2006), as well as the oligomerization of HCN which is further accelerated by the addition of HCHO
(Schwartz and Goverde, 1982). Whether HCHO can also be concentrated in a eutectic sufficiently to form sugars is unknown, but
seems plausible, and is currently under investigation in our laboratory. One tremendous advantage of this is that sugars are generally
much more stable at low temperatures (Larralde et al., 1995). It is
possible that the eutectic freezing and thawing of precursor icegrain organics such as HCHO in carbonaceous chondrites may have
allowed for the synthesis of sugars on these bodies.
In contrast to many of the investigations which have been conducted in prebiotic chemistry, the oceanic concentrations of most
precursor compounds were likely quite low (Stribling and Miller,
1987). It is necessary to consider the geochemical processes which
might have concentrated these compounds sufficiently to allow
them to react further. It is also worth considering what happens
to dilute solutions of HCN, HCHO and aldehydes, urea, nitrate,
nitrite and ferrous iron as they are evaporated in the presence of
UV and visible light over mineral surfaces, or as they are frozen
from dilute solution under visible and/or UV irradiation. Double
layer hydroxide minerals have been shown to be catalysts for the
self-condensation of HCN into non-volatile products (Boclair et al.,
116
H.J. Cleaves II / Precambrian Research 164 (2008) 111–118
Fig. 6. Geochemical cycling of HCHO. HCHO is produced and destroyed in the atmosphere (1), and introduced by extraterrestrial delivery to the atmosphere and oceans (2).
HCHO is also delivered to the oceans by rainout from atmospheric sources (as well as generated and destroyed photochemically in surface waters) (3) and destroyed during
passage through hydrothermal vents (4). Aqueous phase reactions of HCHO may generate amino acids via the Strecker synthesis (5) as well as other non-volatile products
via reaction with other dissolves species such as HCN and NH3 (6). HCHO cannot be concentrated by evaporation as it is volatile (7). Non-volatile species generated in (6)
may be concentrated by evaporation and subsequently liberate HCHO in concentrations sufficient for the synthesis of sugars via the formose reaction (8). HCHO may also be
concentrated in eutectics, for example in sea ice (9), allowing the formose reaction to occur.
2001) as well as catalysts for aldol reactions (Arrhenius et al., 1994).
In this regard the frozen clay model proposed by Lahav and Chang
(1976) also warrants careful reconsideration.
The equilibrium constant for the dimerization of HCHO to give
glycolaldehyde is not well known, but is of considerable interest,
since while HCHO is volatile, glycolaldehyde is not, and could thus
be concentrated by evaporation. A rudimentary calculation using
the free energy of the aldol reaction of HCHO given by Weber (2002)
suggests the equilibrium constant is ∼40, but of course this does not
take into account the various kinetic factors which may limit the
robustness of the reaction. This simple reaction is worthy of further
investigation from the standpoint of prebiotic chemistry, especially
given the apparent ubiquity of glycolaldehyde in interstellar space
(Hollis et al., 2000).
5. Conclusions
Sugars are important for several models for the origin of life
(Gilbert, 1986; Weber, 1997, 2001). Sugars are easily made by the
reaction of HCHO under certain conditions (fairly high concentration, basic to neutral pH). The attainment of such conditions of pH
and concentration, is likely only possible in certain geological environments such as eutectics (occurring only in locations subject to
at least occasional freezing) and in evaporative basins (after HCHO
is rendered non-volatile by reaction with other aqueous species).
Fig. 6 shows plausible environments where sugar synthesis could
occur in the context of HCHO geochemical cycling.
Given present uncertainties regarding primitive Earth conditions, the possibility that such specialized conditions existed
cannot be ruled out. The synthesis of sugars in such environments
from even low initial HCHO concentrations under mild conditions
might be extremely facile, and warrants further investigation, as
does sugar synthesis in the presence of likely congeners such as
HCN, NH3 , and inorganic compounds such as borates and sulfites,
and the impact of various radiation types on this chemistry.
In contrast, submarine hydrothermal environments, generally
characterized by low pH (with significant exceptions), low organic
species concentrations, and often very high temperatures, are not
highly compatible with formose chemistry, though as mentioned
earlier some synthesis of amino acids might be mediated by HCHO
provided sufficient concentrations. The instability of HCHO under
hydrothermal conditions also argues against high concentrations
of HCHO in these environments (Osada et al., 2004; Seewald et al.,
2006), but the detection of trithianes in hydrothermal vent fluids
may be a compelling anomaly (Simoneit, 1992), although it remains
controversial whether much of the organic material detected in
these systems has a truly abiological source rather than being
reworked biological matter.
The importance of HCHO rests mainly on its role in sugar synthesis which still generally needs to be placed in the context of
nucleic acid synthesis, otherwise it is hard to understand its potential importance on the primitive Earth. It should be borne in mind
that sugar synthesis is merely one step on the way to nucleoside,
nucleotide and nucleic acid synthesis, which likely also require
certain specialized environmental conditions (Fuller et al., 1972;
Cleaves and Chalmers, 2004).
Acknowledgements
The author would like to thank Mr. Patrick Griffin, Professor
Robert Hazen, Dr. Caroline Jonsson, and Mr. Christopher Jonsson
for helpful comments during the preparation of this manuscript,
and Dr. Angèle Ricolleau for assistance with figure preparation.
References
Åkermark, B., Eklund-Westlin, U., Baeckström, P., Löf, R., 1980. Photochemical, metalpromoted reduction of carbon dioxide and formaldehyde in aqueous solution.
Acta Chem. Scand. B34, 27–30.
Arrhenius, T., Arrhenius, G., Paplawsky, W., 1994. Archean geochemistry of formaldehyde and cyanide and the oligomerization of cyanohydrin. Orig. Life Evol. Biosph.
24, 1–17.
H.J. Cleaves II / Precambrian Research 164 (2008) 111–118
Aubrey, A., Cleaves, H., Bada, J., submitted for publication. Organic Synthesis in Submarine Hydrothermal Systems I: Amino Acids.
Bada, J., Miller, S., 1968. Ammonium ion concentration in the primitive ocean. Science
159, 423–425.
Bada, J., Bigham, C., Miller, S., 1994. Impact melting of frozen oceans on the early
Earth: implications for the origin of life. Proc. Natl. Acad. Sci. USA 91, 1248–1250.
Barley, M., Dunlop, J., Glover, J., Groves, D., 1979. Sedimentary evidence for an
archaean shallow-water volcanic-sedimentary facies, Eastern Pilbara Block,
Western Australia. Earth Planet. Sci. Lett. 43, 74–84.
Bar-Nun, A., Hartman, H., 1978. Synthesis of organic compounds from carbon monoxide and water by UV photolysis. Orig. Life 9, 93–101.
Bernstein, M., Dworkin, J., Sandford, S., Cooper, G., Allamandola, L., 2002. Racemic
amino acids from the ultraviolet photolysis of interstellar ice analogues. Nature
416, 401–403.
Biver, N., Bockelée-Morvan, D., Crovisier, J., Colom, P., Henry, F., Moreno, R., Paubert,
G., Despois, D., Lis, D., 2002. Chemical composition diversity among 24 comets
observed at radio wavelengths. Earth Moon Planets 90, 323–333.
Boclair, J., Braterman, P., Brister, B., Jiang, J., Lou, S., Wang, Z., Yarberry, F., 2001.
Cyanide self-addition, controlled adsorption, and other processes at layered
double hydroxides. Orig. Life Evol. Biosph. 31, 53–69.
Bone, W., Smith, H., 1905. The thermal decomposition of formaldehyde and acetaldehyde. Chem. Soc. 87, 910–916.
Bossard, A., Raulin, F., Mourey, D., Toupance, G., 1982. Organic synthesis from reducing models of the atmosphere of the primitive earth with UV light and electric
discharges. J. Mol. Evol. 18, 173–178.
Brandes, J., Boctor, N., Cody, G., Cooper, B., Hazen, R., Yoder, H., 1998. Abiotic nitrogen
reduction on the early Earth. Nature 395, 365–367.
Butlerov, A., 1860. Über ein neues methylenderivat. Ann. Chem. 115, 322–327.
Butlerow, A., 1861. Formation synthétique d’une substance sucrée. C. R. Acad. Sci. 53,
145–147.
Cairns-Smith, A., Ingram, P., Walker, G., 1972. Formose production by minerals: possible relevance to the origin of life. J. Theor. Biol. 35, 601–604.
Calvert, A., Steacie, E., 1951. The vapor phase photolysis of formaldehyde at wavelength 3130. J. Chem. Phys. 19, 1976–1982.
Chandra, K., De, S., 1983. Adsorption of formaldehyde by clay minerals in presence
of urea and ammonium sulfate in aqueous system. Indian J. Agric. Chem. 16,
239–245.
Chang, S., 1993. Prebiotic synthesis in planetary environments. In: Greenberg, J.M.,
Mendoza-Gomez, C.X., Pirronello, V. (Eds.), The Chemistry of Life’s Origins.
Kluwer, Boston.
Choughuley, A., Subbaraman, A., Kazi, Z., Chadha, M., 1975. Transformation of some
hydroxy amino acids to other amino acids. Orig. Life 6, 527–535.
Chyba, C., Sagan, C., 1992. Endogenous production, exogenous delivery and impactshock synthesis of organic molecules: an inventory for the origins of life. Nature
355, 125–132.
Chyba, C., Thomas, P., Brookshaw, L., Sagan, C., 1990. Cometary delivery of organic
molecules to the early Earth. Science 249, 366–373.
Cleaves, H., 2003. The prebiotic synthesis of acrolein. Monatsh. Chem. 134, 585–
593.
Cleaves, H., Chalmers, J., 2004. Extremophiles may be irrelevant to the origin of life.
Astrobiology 4, 1–9.
Cleaves, H., Nelson, K., Miller, S., 2006. The prebiotic synthesis of pyrimidines in
frozen solution. Naturwissenschaften 93, 228–231.
Cole, W., Kaschke, M., Sherringham, J., Curry, G., Turner, D., Russell, M., 1994. Can
amino acids be synthesized by H2 S in anoxic lakes? Mar. Chem. 45, 243–256.
Cooper, G., Onwo, W., Cronin, J., 1992. Alkyl phosphonic acids and sulfonic acids in
the Murchison meteorite. Geochim. Cosmochim. Acta 56, 4109–4115.
Cooper, G., Kimmich, N., Belisle, W., Sarinana, J., Brabham, K., Garrel, L., 2001. Carbonaceous meteorites as a source of sugar-related organic compounds for the
early Earth. Nature 414, 879–883.
De, S., Chandra, K., 1978. Adsorption of formaldehyde by Indian clay minerals. Sci.
Cult. 44, 462–464.
De Bruijn, J., Kieboom, A., Van Bekkum, H., 1986. Reactions of monosaccharides in
aqueous alkaline solutions. Sugar Technol. Rev. 13, 21–52.
Edmond, J., von Damm, K., McDuff, R., Measures, C., 1982. Chemistry of hot springs
on the east Pacific rise and their effluent dispersal. Nature 297, 187–191.
Eschenmoser, A., 2004. The TNA-family of nucleic acid systems: properties and
prospects. Orig. Life Evol. Biosph. 34, 277–306.
Ferris, J., 1994. The potential for prebiotic synthesis in hydrothermal systems. Orig.
Life Evol. Biosph. 24, 363–381.
Ferris, J., Chen, C., 1975. Chemical evolution. XXVI. Photochemistry of methane, nitrogen, and water mixtures as a model for the atmosphere of the primitive earth.
J. Am. Chem. Soc. 97, 2962–2967.
Ferris, J., Nicodem, D., 1972. Ammonia photolysis and the role of ammonia in chemical evolution. Nature 238, 268–269.
Fomenkova, M., Chang, S., Mukhin, L., 1994. Carbonaceous components in the comet
Halley dust. Geochim. Cosmochim. Acta 58, 4503–4512.
Fox, S., Windsor, C., 1970. Synthesis of amino acids by the heating of formaldehyde
and ammonia. Science 170, 984–986.
Fuller, W., Sanchez, R., Orgel, L., 1972. Studies in prebiotic synthesis VII. J. Mol. Evol.
1, 249–257.
Gabel, N., Ponnamperuma, C., 1967. Model for origin of monosaccharides. Nature
216, 453–455.
Gesteland, R., Atkins, J., 1993. The RNA world: the nature of modern RNA suggests a
prebiotic RNA world (Monograph/Cold Spring Harbor Laboratory, No 24).
117
Gilbert, W., 1986. The RNA world. Nature 319, 618.
Gluesenkamp, E., 1958. Amine-formaldehyde resins. US patent 2848438.
Godderis, Y., Veizer, J., 2000. Tectonic control of chemical and isotopic composition
of ancient oceans; the impact of continental growth. Am. J. Sci. 300, 434–461.
Grotzinger, J., Kasting, J., 1993. New Constraints on precambrian ocean composition.
J. Geol. 101, 235–243.
Henderson–Sellers, A., Henderson–Sellers, B., 1988. Equable climate in the early
Archaean. Nature 336, 117–118.
Henry, L., 1890. Sur le nitrile gycolique et la synthèse directe de l’acide glycolique.
Comptes Rendus 110, 759–760.
Hofmann, A., 1997. Early evolution of continents. Science 275, 498–499.
Holland, H., 1973. The oceans; a possible source of iron in iron-formations. Econ.
Geol. 68, 1169–1172.
Holland, H., 1984. The chemical evolution of the atmosphere and oceans. Princeton
University Press, Princeton.
Hollis, J., Lovas, F., Jewell, P., 2000. Interstellar glycolaldehyde: the first sugar. Astrophys. J. 540, L107–L110.
Hubbard, J., Hardy, J., Horowitz, N., 1971. Photocatalytic production of organic compounds from CO and H2 O in a simulated martian atmosphere. Proc. Natl. Acad.
Sci. USA 68, 574–578.
Isley, A., Abbott, D., 1999. Plume-related mafic volcanism and the deposition of
banded iron formation. J. Geophys. Res. 104, 461–477.
Joyce, G., Schwartz, A., Miller, S., Orgel, L., 1987. The case for an ancestral genetic
system involving simple analogues of the nucleotides. Proc. Natl. Acad. Sci. USA
84, 4398–4402.
Kasting, J., Brown, L., 1998. Setting the stage: the early atmosphere as a source of
biogenic compounds. In: Brack, A. (Ed.), The Molecular Origins of Life: Assembling the Pieces of the Puzzle. Cambridge University Press, New York, pp. 35–
56.
Kasting, J., Zahnle, K., Pinto, J., Young, A., 1989. Sulfur, ultraviolet radiation, and the
early evolution of life. Orig. Life 19, 95–108.
Kempe, S., Degens, E., 1985. An early soda ocean? Chem. Geol. 53, 95–108.
Kempe, S., Kazmierczak, J., 2002. Biogenesis and early life on Earth and Europe:
favored by an alkaline ocean? Astrobiology 2, 123–130.
Knauth, L., 2005. Temperature and salinity history of the Precambrian ocean:
implications for the course of microbial evolution. Palaeogeogr. Palaeoclimatol.
Palaeoecol. 219, 53–69.
Koch, K., Schweizer, W., Eschenmoser, A., 2007. Reactions of the HCN-tetramer with
aldehydes. Chem. Biodiv. 4, 541–553.
Lahav, N., Chang, S., 1976. The possible role of solid surface area in condensation
reactions during chemical evolution: reevaluation. J. Mol. Evol. 8, 357–380.
Larralde, R., Robertson, M., Miller, S., 1995. Rates of decomposition of ribose and
other sugars: implications for chemical evolution. Proc. Natl. Acad. Sci. USA 92,
8158–8160.
Lazar, B., Starinsky, A., Katz, A., Sass, E., Ben-Yaakov, S., 1983. The carbonate system
in hypersaline solutions: alkalinity and CaCO3 solubility of evaporated seawater.
Limnol. Oceanogr. 28, 978–986.
Lazcano, A., Miller, S., 1994. How long did it take for life to begin and evolve to
cyanobacteria? J. Mol. Evol. 39, 546–554.
Lemière, C., Desjardins, A., Cloutier, Y., Drolet, D., Perrault, G., Cartier, A., Malo, J.,
1995. Occupational asthma due to formaldehyde resin dust with and without
reaction to formaldehyde gas. Eur. Respir. J. 8, 861–865.
Levy, M., Miller, S., Brinton, K., Bada, J., 2000. Prebiotic synthesis of adenine and
amino acids under Europa-like conditions. Icarus 145, 609–613.
Löb, W., 1906. Studien über die chemische Wirkung der stillen elektrischen Entladung. Z. Elektrochem. 15, 282–312.
Miller, S., 1953. A production of amino acids under possible primitive Earth conditions. Science 117, 528–529.
Miyakawa, S., Cleaves, H., Miller, S., 2002. The cold origin of life. B. Implications
based on pyrimidines and purines produced from frozen ammonium cyanide
solutions. Orig. Life Evol. Biosph. 32, 209–218.
Morse, J., Mackenzie, F., 1998. Hadean ocean carbonate geochemistry. Aquat.
Geochem. 4, 301–319.
Osada, M., Watanabe, M., Sue, K., Adschiri, T., Arai, K., 2004. Water density dependence of formaldehyde reaction in supercritical water. J. Supercrit. Fluids 28,
219–224.
Parfitt, R., Greenland, D., 1970. The adsorption of poly(ethylene glycols) on clay
minerals. Clay Miner. 8, 305–315.
Peltzer, E., Bada, J., Schlesinger, G., Miller, S., 1984. The chemical conditions on the
parent body of the Murchison meteorite: some conclusions based on amino,
hydroxy and dicarboxylic acids. Adv. Space Res. 4, 69–74.
Pinto, J., Gladstone, G., Yung, Y., 1980. Photochemical production of formaldehyde in
Earth’s primitive atmosphere. Science 210, 183–185.
Pizzarello, S., 2004. Chemical evolution and meteorites: an update. Orig. Life Evol.
Biosph. 34, 25–34.
Price, P.B., 2007. Microbial life in glacial and implications for a cold origin of life.
FEMS Microbiol. Ecol. 59, 217–231.
Reid, C., Orgel, L., 1967. Synthesis of sugars in potentially prebiotic conditions. Nature
216, 455.
Ricardo, A., Carrigan, M., Olcott, A., Benner, S., 2004. Borate minerals stabilize ribose.
Science 303, 196.
Sanchez, R., Ferris, J., Orgel, L., 1966. Conditions for purine synthesis: did prebiotic
synthesis occur at low temperatures? Science 153, 72–73.
Schlesinger, G., Miller, S., 1973. Equilibrium and kinetics of glycolonitrile formation
in aqueous solution. J. Am. Chem. Soc. 95, 3729–3735.
118
H.J. Cleaves II / Precambrian Research 164 (2008) 111–118
Schlesinger, G., Miller, S., 1983. Prebiotic synthesis in atmospheres containing CH4 ,
CO, and CO2 . II. Hydrogen cyanide, formaldehyde and ammonia. J. Mol. Evol. 19,
383–390.
Schrauzer, G., Strampach, N., Hui, L., Palmer, M., Salehi, J., 1983. Nitrogen photoreduction on desert sands under sterile conditions. Proc. Natl. Acad. Sci. USA 80,
3873–3876.
Schutte, W., Allamandola, L., Sandford, S., 1993. An experimental study of the
organic molecules produced in cometary and interstellar ice analogs by thermal
formaldehyde reactions. Icarus 104, 118–137.
Schwartz, A., 1981. Chemical evolution—the genesis of the first organic compounds. Elsevier Oceanography Series (Amsterdam) 31 (Mar. Org. Chem.), pp. 7–
30.
Schwartz, A., 1983. Chemical evolution: the first stages. Naturwissenschaften 70,
373–377.
Schwartz, A., De Graaf, R., 1993. The prebiotic synthesis of carbohydrates: a reassessment. J. Mol. Evol. 36, 101–106.
Schwartz, A., Goverde, M., 1982. Acceleration of HCN oligomerization by formaldehyde and related compounds: implications for prebiotic syntheses. J Mol. Evol.
18, 351–353.
Seewald, J.S., Zolotov, M., McCollom, T., 2006. Experimental investigation of single
carbon compounds under hydrothermal conditions. Geochim. Cosmochim. Acta
70, 446–460.
Sekine, Y., 2002. Oxidative decomposition of formaldehyde by metal oxides at room
temperature. Atmos. Environ. 36, 5543–5547.
Shapiro, R., 1988. Prebiotic ribose synthesis: a critical analysis. Orig. Life Evol. Biosph.
18, 71–85.
Shigemasa, Y., Matsuda, Y., Sakazawa, C., Matsuura, T., 1977. Formose reactions II.
The photochemical formose reaction. Bull. Chem. Soc. Jpn. 50, 222–226.
Sillén, L., 1967. The ocean as a chemical system. Science 156, 1189–1197.
Simoneit, B., 1992. Aqueous organic geochemistry at high temperature/high pressure. Orig. Life Evol. Biosph. 22, 43–65.
Stribling, R., Miller, S., 1987. Energy yields for hydrogen cyanide and formaldehyde
syntheses: the hydrogen cyanide and amino acid concentrations in the primitive
ocean. Orig. Life Evol. Biosph. 17, 261–273.
Summers, D., 1999. Sources and sinks for ammonia and nitrite on the early Earth
and the reaction of nitrite with ammonia. Orig. Life Evol. Biosph. 29, 33–46.
Summers, D., Chang, S., 1993. Prebiotic ammonia from reduction of nitrite by iron
(II) on the early Earth. Nature 365, 630–633.
Tian, F., Toon, O., Pavlov, A., De Sterck, H., 2005. A hydrogen-rich early Earth atmosphere. Science 308, 1014–1017.
Turcotte, D., 1980. On the thermal evolution of the Earth. Earth Planet. Sci. Lett. 48,
53–58.
Valley, J., Peck, W., King, E., Wilde, S., 2002. A cool early Earth. Geology 30, 351–
354.
Vincent, W., Mueller, D., Van Hove, P., Howard-Williams, C., 2005. Glacial periods
on early earth and implications for the evolution of life. In: Origins. Springer,
Netherlands, pp. 483–501.
Walker, J., 1964. Formaldehyde, 3rd ed. Rheinhold, New York.
Walker, J., Brimblecombe, P., 1985. Iron and sulfur in the pre-biologic ocean. Precambrian Res. 28, 205–222.
Weber, A., 1997. Energy from redox disproportionation of sugar carbon drives biotic
and abiotic synthesis. J. Mol. Evol. 44, 354–360.
Weber, A., 1998. Prebiotic amino acid thioester synthesis: thiol-dependent amino
acid synthesis from formose substrates (formaldehyde and glycolaldehyde) and
ammonia. Orig. Life Evol. Biosph. 28, 259–270.
Weber, A., 2001. The sugar model: catalysis by amines and amino acid products. Orig.
Life Evol. Biosph. 31, 71–86.
Weber, A., 2002. Chemical constraints governing the origin of metabolism: the thermodynamic landscape of carbon group transformations under mild aqueous
conditions. Orig. Life Evol. Biosph. 32, 333–357.
Wen, J., Pinto, J., Yung, Y., 1989. Photochemistry of CO and H2 O: analysis of laboratory
experiments and applications to the prebiotic Earth’s atmosphere. J. Geophys.
Res. 94, 14957–14970.
Wilde, S., Valley, J., Peck, W., Graham, C., 2001. Evidence from detrital zircons for the
existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409,
175–178.
Wolman, Y., Miller, S., Ibanez, J., Oró, J., 1971. Synthesis of amino acids by the heating
of formaldehyde and ammonia. Formaldehyde and ammonia as precursors to
prebiotic amino acids. Comments Sci. 174, 1039–1040.
Wolman, Y., Haverland, W., Miller, S., 1972. Nonprotein amino acids from spark discharges and their comparison with the Murchison meteorite amino acids. Proc.
Natl. Acad. Sci. USA 69, 809–811.