PLEASE NOTE:
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CCNet ESSAY, 1 December 2000
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"In this essay I suggest that, whereas (1) there can be
little doubt
that life abounds throughout the universe and is not confined to
Earth, and (2) under special circumstances interplanetary
transport of
microbes in impact ejecta may be possible, in view of the
evidence to
date the panspermia hypothesis constitutes a philosophical notion
rather
than a scientific theory."
-- Andrew Glikson, 30 November 2000
TERRESTRIAL VS. EXTRATERRESTRIAL BIOGENESIS: WHEN THE TOTAL IS
GREATER THAN
THE SUM OF THE PARTS
By Andrew Glikson <geospectral@spirit.com.au>
Australian National University.
Canberra, ACT 0200
Suggestions of cometary origin and/or transport of microbes and
of universal
seeding of life from cometary dust, colloquially known as
panspermia (eg.
Hoyle and Wichramasinghe, 1980), constitute an extraordinary
claim.
According to Carl Sagan, "Extraordinary claims require
extraordinary
evidence". In this essay I suggest that, whereas (1) there
can be little
doubt that life abounds throughout the universe and is not
confined to
Earth, and (2) under special circumstances interplanetary
transport of
microbes in impact ejecta may be possible, in view of the
evidence to date
the panspermia hypothesis constitutes a philosophical notion
rather than a
scientific theory.
Principal questions relevant to panspermia include (1) the often
overlooked
fundamental distinction between "pre-biotic" organic
molecules (mainly amino
acids) and complex information-rich biomolecules (RNA, DNA,
proteins,
enzymes) of which life and the biosphere consist, and (2) the
need to
establish criteria for discrimination between organic compounds
derived from
endogenic volcanic and hydrothermal sources and from
extraterrestrial
sources. As elaborated by Davies (1998), whereas the raw
materials and
building blocks of life - volatiles and amino acids - are
everywhere - the
biosphere is founded on yet little understood universal quantum
information
laws.
Different versions of panspermia are related to early accretional
stages of
Earth, when incorporation of dust particles under temperatures in
excess of
about 1000K would have resulted in late accretion of volatile and
organic
components (Delsemme, 2000), and/or later bombardment by comets
and/or
settling cometary dust. By about or earlier than 4.2 Ga, water
must have
been present in the Earth's mantle and crust, as attested by the
occurrence
of 4.23 Ga (billion years) old zircons in the Gascoyne Province,
Western
Australia (Froude, 1983) - implying partial melting processes
which require
the presence of volatiles in the Earth mantle and crust.
Such additions
could have been particularly important during the "late
heavy bombardment"
of the terrestrial planets (LHB - 3.95-3.8 Ga, Ryder,
1990). The suggestion
that cometary and cosmic dust may have contributed significantly
to the
volatile inventory of the terrestrial atmosphere and hydrosphere
during the
early meteoritic bombardment (Chyba, 1987; Chyba and Sagan,
1996), and to a
lesser extent during later cometary impacts, requires
discrimination between
volatile and organic components introduced from terrestrial
endogenic
sources and extraterrestrial sources.
As attested by the composition of fluid inclusions in
mantle-derived
minerals and by indigenous volcanic and hydrothermal fluids,
organic
compounds are capable of being synthesised from original
terrestrial
components. Comparisons between the volatile fraction of
comets (H-56%;
O-31%; C-10%; N-2.7%; S-0.3%) and interstellar frost (H-55%;
O-30%; C-13%;
N-1%; S-0.8%) (Delsemme 2000) on the one hand, with volatile
fractions of
mantle-derived terrestrial volcanics and the organic molecules
inventory of
hydrothermal fluids on the other hand, indicate existence of
similar
components, albeit in different proportions. That in many
instances
terrestrial organic compounds are unlikely to represent recycled
cometary-introduced fractions follows, for example, from the
young
mid-oceanic loci of volcanic and hydrothermal systems where
synthesis of
organic molecules takes place, for example at Kilauea, Hawaii
(1918
eruption: H2O - 30%; H2 - 0.35%; CO2 - 40%; CO - 1.2%; SO2 - 28%;
S2 -
0.04%; HCL - 0.034% and 1965 eruption: H2O - 67.1; CO2 - 8.5%;
SO2 - 24.3%).
The indigenous origin of the carbon is attested by the CO2-rich
composition
of mantle-derived fluid inclusions in olivine (Roedder and
Emslie, 1970).
Theoretical considerations and laboratory experiments, assuming
different
atmosphere and hydrosphere chemistry are consistent with the
synthesis of
organic molecules in the early terrestrial environment, forming
the basis of
extensive literature (eg. Haldane, 1929; Oparin, 1938, 1957;
Miller, 1953;
Dyson, 1985; Russell and Hall, 1997; Shock, 2000; Henley,
2000). Assumed
starting conditions vary from reducing atmosphere rich in
methane, ammonia,
hydrogen and water, synthesised into amino acids by electric
currents
(Miller, 1953), to CO2-dominated atmosphere and interaction
between alkaline
fluids and low-pH surface water, resulting in precipitation of
colloidal FeS
membranes (Russell and Hall, 1997). These authors write:
"The earliest
truly replicating cells probably required only twenty or so
elements (da
Silva and Williams, 1991), all of them available at submarine hot
springs.
and as many fundamental organic molecular components (Wald, 1964;
Eck and
Dayhoff., 1968).
Central to the question are criteria capable of discriminating
between
mantle-derived and cometary-derived volatiles and organic
molecules.
Delsemme (2000) pointed to the potential significance of
deuterium level in
the hydrosphere vis-a-vis cometary import, a distinction which
requires
detailed comparisons between fluid inclusions in mantle-derived
minerals and
extraterrestrial volatiles. The 3H isotopic anomalies associated
with
meteoritic impact fallout deposits (Farley et al., 1998) may
provide further
clues in this regard, in so far as fluid inclusions in early
Archaean
sediments may be amenable for this test. The distinct
chiralic structure of
amino acids found in carbonaceous chondrites and in K-T impact
boundary
deposits at Steve Klint, Denmark, is probably the most promising
criterion
for discrimination between terrestrial and extraterrestrial
organic
components. Extraterrestrial organic compounds include amino
isobutayric
acid (AIB)(CH3)2CNH2COOH) and racemic isovaline
(CH3CH2(CH3)CNH2COOH) (Zhao
and Bada, 1989), which are rare to unknown in terrestrial
environments. The
only reports of these components in terrestrial environments is
in some
fungal peptides. The isovaline in these peptides is either in the
form of
the D-enantiomer or L-enantiomer, while in the K/T boundary
sediments the
isovaline is racemic (J. Bada, pers. comm., 1999). Had
large-scale
introduction of AIB and isovaline occurred early in terrestrial
history, the
question is whether they would have been preserved as such or
undergo
structural modification in the terrestrial environment?
Alternatively, this
evidence may militate against large-scale introduction of
cometary organic
molecules into the atmosphere and hydrosphere.
However, even if a significant contribution was made to the
terrestrial
atmosphere and hydrosphere by cometary organic molecules, this
would hardly
explain the origin of the biosphere, defined in the Encyclopedia
Britannica
as the "zone of life, the total mass of living
organisms". As highlighted
by Paul Davies (1998) the fundamental question inherent in the
origin of
life hinges on the qualitative "quantum leap" from
organic molecules (amino
acids, purines, pyrimidines) to complex information-rich
biomolecules
(peptides, nucleic acids, proteins, enzymes). Whereas the
information
contained by the former can be expressed by algorithms, the
latter are
qualitatively unique, with a probability of forming by chance at
1:10^120.
Since organic molecules are ubiquitous on Earth and in space,
whether the
earliest biomolecules formed by synthesis of terrestrial or/and
cometary
contributions may only be relevant in so far as, hypothetically,
cometary
molecules were favoured by biological processes. Such a surmise
would be
inconsistent with the structural symmetry (chirality) of amino
acids
mentioned above. A corollary of the panspermia notions would be a
suggestion
that, in so far as some of the earliest iron tools may have been
made by
smelting Fe-Ni meteorites, technological civilisation was
imported from
outer space ... as in Von Daniken-type UFO ideas. There is more
to the
origin and evolution of complex information systems, culminating
in life and
civilisation, than the raw materials of which their products
consist. The
total is greater than the sum of its parts!
References
Basaltic Volcanism on the Terrestrial Planets, 1981 (Kaula, W.M.,
Ed).
Pergamon Press, 1286 pp.
Chyba, C.F.and Sagan, C., 1996. Comets as the source of
prebiotic organic
molecules for the early Earth. In: Comets and the Origin and
Evolution
of Life (P.J. Thomas, C.F. Chyba and C.P. McKay, Eds.),
pp.147-174.
Springer Verlag, New York.
Davies, P., 1998. The Fifth Miracle. Penguin Books.
Delsemme, A.H., 2000. Cometary origin of the biosphere - 1999
Kuiper prize
lecture. Icarus 146, p.313-325.
de Silva, J.J.R.F. and Williams, R.J.P., 1991. The
Biological Chemistry of
the Elements. Clarendon Press, oxford.
Dyson, F., 1985. Origins of Life. Cambridge
University Press, Cambridge.
Eck, R.V. and Dayhoff, M.O., 1968. Evolution of the
structure of ferredoxin
based on living relics of primitive amino acid sequences.
Science
152, 363-366.
Farley, K.A., Montanari, A., Shoemaker, E.M., and Shoemaker,
C.S., 1998.
Geochemical evidence for a comet shower in the Late Eocene.
Science 280,
1250-1253.
Froude, D.O., Ireland, T.R., Kinny, P.K., Williams, I.S.,
Compston, W.,
Williams, I.R., Myers, J.S., 1983. Ion Microprobe identification
of
4,100-4,200 Myr-old terrestrial zircons. Nature 304,
616-618.
Haldane, J.B.S., 1929. The origin of life.
Rationalist Annual 3, 148-153.
Henley, R.W., 2000. Chemical, and physical context for life
in terrestrial
hydrothermal systems: chemical reactors for the early development
of life
and hydrothermal ecosystems. Proceedings CIBA Foundation
Meeting, 2000,
61-82.
Hoyle, F. and Wichramasinghe, N.C., 1980. Comets and
the Origin of Life
(C. Ponnaperuma, Ed.), Reidel, Dordrecht, 227 pp.
Miller, S.L., 1953. A production of amino acids under
possible primitive
Earth conditions. Science 117, 528-529.
Oparin, A.I., 1938. The origin of Life. Dover, New
York.
Oparin, A.I., 1957. The Origin of Life on Earth.
Oliver and Boyd,
Edinburgh.
Roedder, P.L. and Emslie, R.F., 1970. Olivine-liquid
equilibrium. Contrib.
Miner. Petrol. 29, 275-289.
Russell, M.J. and Hall, A.J., 1997. The emergence of life
from iron
monosulphide bubbles at a submarine hydrothermal redox and pH
front.
J. Geol. Soc. London 154, 377-402.
Ryder, G., 1990. Lunar samples, lunar accretion and the early
bombardment of
the Moon. Eos 71, 313-322.
Shock, E.L., 2000. Hydrothermal systems as environments for the
emergence of
life. Proceedings CIBA Foundation Meeting, 2000, 40-60.
Wald, G., 1964. The origin of life. Proceeding of the
national Academy of
Science USA 52, 595- 611.
Zhao, M. and Bada, J. 1989, Nature 339, 463-464,
Copyright 2000, Andrew Glikson
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