CCNet ESSAY, 1 December 2000
"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 <firstname.lastname@example.org>
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
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
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
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!
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
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,
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,
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,
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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