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CCNet-Essay, 13 March 2000
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THE VIOLENT HABITAT OF EARLY TERRESTRIAL LIFE
By Andrew Glikson <geospectral@spirit.com.au>
Research School of Earth Science, Institute of Advanced Studies,
Australian National University, Canberra, A.C.T. 0200
Confirmation of the biological nature of 3.45 billion years-old
stromatolites in the Pilbara, Western Australia, tells a story of
survival amid volcanic and meteoritic impact events
The phenomenon of life is that which defies definition. Crystals
multiply, flames oxidise, emit carbon dioxide, grow, dance and
die.
Computers mimic every vital function - the brain children of a
species
which learnt to program the electromagnetic waves. Uniquely
living
systems metabolise and mutate and are capable of evolving by
natural
selection. Life transcends the limits of space, scale and time.
Living
bacteria are found several kilometres deep beneath the surface,
the size
of the smallest known bacteria is measured in few tens of
nanometres,
the time of the beginning is not as yet known. The transition
between
inanimate matter and proteins, the building blocks of life,
constitutes
the quantum jump not yet bridged by science. In this light, the
recent
confirmation of the biological origin of 3.45*10^9 years-old
microbial
stromatolites in the Pilbara, Western Australia, allows an
assessment of
the environment in which early life forms evolved. Photo
synthesising
bacteria occupying shallow water colonies needed to reach a
balance
between exposure to solar radiation and protection from the
ultraviolet
and cosmic radiation under ozone-less skies. Stromatolites could
only
survive during brief intermissions in volcanic activity and
crustal
subsidence, intermittently perturbed by meteoritic impacts -
telling a
remarkable story of survival against the greatest odds.
Historical perspective
Intrinsic to Charles Darwin's (1809-1882) theory of evolution is
the
inquiry into where and when did earliest life forms emerge, an
unanswered question to this day. The rise of the uniformitarian
paradigm
of James Hutton (1726-1797) and Charles Lyell (1797-1875),
regarding the
present as the key to the past, saw a rift between the early
'Plutonist'
and 'Neptunist' schools of thought. The first implied magmatic
origin
and progressive metamorphic obliteration of the geological
record,
whereas the second pointed to continuous sedimentation in the
seas,
including evidence provided by fossils. The uniformitarian
paradigm
arose against a background of biblical notions such as Noahs
flood, as
well as the catastrophic school of thought of Cuvier (1769-1832).
Nowadays, Lyell's uniformitarian principle is challenged by
astronomical, lunar and terrestrial evidence for high incidence
of
impact by near-Earth asteroids (NEA) and comets, with
implications to
the survival of early habitats.
The advent of isotopic age determination resulted in the
surprising
realisation that, in places, some of the oldest continental
crustal
fragments have escaped high grade metamorphism and deformation,
containing detailed records of early surface processes, including
primitive life forms. The quest for the oldest fossils, pioneered
by
William Dawson (1820-1899), Charles Walcott (1850-1927), Charles
Seward
(1863-1941), Stanley Tyler (1906-1963), Elso Barghoorn
(1915-1984),
Preston Cloud (1912-1991), Vasil Timofeev (1916-1982), Alexander
Oparin
(1894-1980), and Martin Glaessner (1906-1989), has been recently
reviewed by Schopf (1999) and Walter (1999). The most important
breakthrough took place when Cryptozoon - micro fossils-bearing
stromatolites - was discovered by Tyler and Cloud in the 2.0*10^9
years-old Gunflint chert on an island in Lake Superior.
The physical limits of life
Both present-day and ancient bacteria tell a story of
extraordinary
endurance under extreme physical conditions, including
temperatures up
to about 150oC, the breakdown limit of DNA, such as around hot
springs
and submarine sulphide-rich "black smoker" fumaroles.
Bacteria are found
in drill holes several kilometre-deep beneath the surface, under
pressures greater than 1 kilobar, and in frozen lakes beneath
thousands
metres of Antarctic ice. Recently nanometre-scale tubular living
cells,
"nanobes", were described from both Precambrian
sediments (Glikson and
Taylor, 2000) and fractures in deep-seated drilled sandstones
(Uwins et
al., 2000). Micron to sub micron-scale tubes found within iron
and
magnesium-rich minerals (Bischoff and Coenraads, 1994) testify to
the
interface between living bacteria and crystal lattices.
Prior to the evolution of the ozone layer from photo
synthetically
released oxygen, fatal ultraviolet and cosmic radiation all but
precluded the exposure of life on land surfaces. A low oxygen
Archaean
atmosphere is attested, for example, by pebbles of pyrite in
2.8*10^9
years-old conglomerates in the Witwatersrand (Transvaal) and the
Pilbara, as sulphides do not survive oxidation in present-day
waters.
The radiation hazard and repeated volcanic and meteoritic impact
events
(Chyba, 1993), suggest the evolution of photo-synthesising
bacteria was
likely predated by that of chemotropic bacteria, deriving energy
by
reduction of volcanic CO2 to CH4 and SO3 to H2S, as around
present-day
submarine hot springs. These biologic induced fractionations,
where
lighter isotopes are preferentially partitioned into released
gases, are
reflected by low 13/12C and low 34/32S values. Nearer to the
surface,
photo-synthesising bacteria had to strike a balance between their
need
for solar energy and protection in shallow water from fatal
ultraviolet
and cosmic radiation. The multiply domed geometry of stromatolite
colonies allows maximum protection for a majority of bacterial
cells.
What is the geological nature of the terrains where early life
emerged
and survived?
Early Pilbara environments
In the Pilbara region of Western Australia, as well as in parts
of the
eastern Transvaal and Zimbabwe, volcanic and sedimentary rocks
older
than 3.4*10^9 years contain a wealth of well preserved primary
textural
and compositional features, which allowed detailed information on
early
submarine and to a lesser extent surface environments. The
Pilbara
craton has been documented by the pioneering work of Arthur
Hickman and
his colleagues of the Geological Survey of Western Australia
(Hickman,
1983).
The sediments which host the stromatolitic colonies are underlain
and
overlain by thick successions of subaqueous mafic (Mg and
Fe-rich)
volcanic lavas. The evidence suggests that the extrusions were
associated with rapid subsidence of the sea floor, maintaining
sub-aqueous conditions despite the great accumulated thicknesses.
Volcanic units include Mg-rich 'ultramafic' volcanic lavas -
so-called
'komatiites' after the type locality on the Komati River,
Barberton
Mountain, Transvaal - with bladed crystal textures and globular
to
tubular structures "lava pillows" formed by rapid
cooling. Primary
igneous minerals and geochemical patterns allow an insight into
the
composition of the early mantle and fractionation history of the
magmas.
Intercalated with the lava flows are silica-rich 'dacite'
pyroclastic
units and derived detrital sediments, which in places formed
submarine
topography or exposed islands.
Quiescent stable intervals between volcanic eruptions are
represented by
colloidal silica deposits of chert and/or interbanded
silica-ferric iron
units - banded iron formations. Thin units of carbonate and
barite
(barium sulphate) occur, showing bladed crystal growth typical of
evaporitic deposition in hypersaline waters. Disruptions of
depositional
environments by tectonic movements, uplift and denudation are
represented by unconformable erosional surfaces. In some
instances
vertical movements resulted in the emergence at the surface of
granitic
bodies, locally preserved as buried islands or small continental
nuclei
- mapped in the Strelley area, central Pilbara (Buick et al.,
1995). The
fallout from distant meteoritic impacts is recorded by altered
glass
spherule layers, condensed from impact-generated silicate vapour
('microkrystites'), originally discovered along the
Cretaceous-Tertiary
extinction boundary in the Apenines (Alvarez, 1980). Similar
impact
fallout deposits are observed in the Pilbara in 3.45*10^9 year
old
sediments (Lowe and Byerly, 1986), in the Barberton Mountains in
3.24*10^9 sediments (Lowe et al., 1989) and in the Hamersley
region of
Western Australia in 2.63, 2.56 and 2.49*10^9 years-old sediments
(Simonson, 1972; Simonson and Hassler, 1997).
Minimum estimates of the impact incidence by asteroids and comets
during
the Archaean indicate more that 150 impacts forming craters
larger than
100 km, including some 20 craters larger 300 km in diameter
(Glikson,
1996, 1999), some of which are observed from impact fallout
deposits in
South Africa (Lowe et al., 1989). These episodes would have
annihilated
life over large areas, through a thermal flash, solar clouding
effects
and acid rain. Remaining bacteria cells must have found new
habitats in
suitable shallow seas, lagoons or lakes. Considering the
combination of
volcanic and impact factors, perhaps it is not surprising
stromatolites
are only rarely found in the many kilometre-thick Archaean
sequences.
Archaean ecosystems
Intercalated with Pilbara chert, carbonate and barite units are
undulating to dome-structured, commonly silicified, laminated
carbonate
sediments, the result of activity by a myriad of prokaryotic
(nucleus-free) filamentous blue-green microbes. Although
decimated from
about 600 million years ago by grazing marine creatures, similar
dome-shaped eukaryotic (single celled nuclei-bearing)
stromatolite
colonies occupy estuaries (Shark Bay, Hamelin Pool) and lagoons
along
the West Australian coast. Eukaryotes may have only emerged about
2.0*10^9 years ago, with first manifestations of algal sea weed
(Grypania) at Gunflint island, Lake Superior. Following the
discovery of
3.45*10^9 years-old stromatolites in the Pilbara by John Dunlop
and
Roger Buick (Walter et al., 1980; Buick et al., 1981) and Don
Lowe
(Lowe, 1980), doubts lingered regarding their biological origin.
Lowe
(1994) reinterpreted these structures in terms of deformed
laminated
sediments. At that stage, the only confident identification of
Archaean
life hinged on micro fossils, such as 3.45*10^9 years-old
filamentous
bacteria in cherts intercalated with high-Mg 'komatiite'
volcanics in
the Marble Bar area in the Pilbara (Schopf, 1993) and ovoid forms
in the
Barberton Mountains (Muir, 1978).
Some twenty years passed before outcrops of cone-shaped carbonate
stromatolites, found by Alec Trendall, Arthur Hickman and Kath
Grey, all
of the Geological Survey of Western Australia, offered new
convincing
evidence of biogenicity. Morphological analysis leaves little
doubt of a
biological origin (Hoffman et al., 1999). The stromatolites show
similarities to living "pinnacle mat" stromatolites and
to fossil
Proterozoic Jacutophyton and Thayssagetes, although the latter do
not
display axial zone elongation. In view of their intra-formational
position, branching forms, occurrence of inter-cone detrital
deposits,
and current-elongation patterns, the individual cone structures
are
unlikely to have formed by later deformation.
The biological significance of banded iron formations remains an
enigma.
The restriction of this type of sediments to geological systems
older
that about 1600*10^6 years, about the same time as oxidised 'red
bed'
sandstones appeared, hints at a relation with the increasing
atmospheric
oxygen levels. Conceivably, iron oxidising bacteria used the
little free
oxygen which existed prior to this time to oxidise ferrous into
the
ferric iron of the banded ironstones. In the absence of micro
fossils in
banded iron formations the possibility remains unconfirmed. The
advent
of shallow water bacterial ecosystems is likely to have postdated
that
of better protected chemotropic bacteria, such as those likely to
have
been associated with the Cu-Zn sulphide at Sulphur Springs,
central
Pilbara (Vearncombe et al., 1996). In these environments,
evidence of
alternations between oxygenated waters and reducing conditions is
furnished by the intercalation of barite (BaSO4)-rich
evaporitic
sediments and the sulphide-rich deposits.
Terrestrial versus extraterrestrial origins
No reasons have ever been given why the Archaean microbes are
anything
but original Earthlings. In the fifties Fred Hoyle and his
student
Chandra Wickramasinghe invoked the spectral signatures of amino
acid
molecules in interstellar dust and cometary tails as evidence for
inter-galactic biological seeding, or 'panspermia'. More recently
Paul
Davies, physicist-philosopher, considered inter-planetary
transport of
bacterial spores aboard meteorites (1998, The Fifth Miracle,
Penguin
Press). The reality of sub-micron-scale microbe-like forms
claimed to
occur in a Mars-derived Antarctic meteorite ALH84001 has been
questioned, among other due to the high temperature origin of the
carbonate of which the putative fossils are made.
The panspermia hypothesis has to contend with major objections.
As the
oldest signatures of life occur in 3.8*10^9 years-old rocks,
importation
of bacteria to Earth would have taken place during the so-called
Late
Heavy Bombardment (4.2-3.8*10^9 years ago), represented by mare
basins
on the Moon, when life anywhere in the solar system would have
been
under enormous stress. It has never been explained how proteins
can
escape prolonged cosmic radiation without fatal consequences.
Bacteria
older than the 2.0*10^9 years-old Gunflint chert, Minnesota, are
not
known to have cell walls or form spores, and may not have been
capable
of space transport. Viruses, which can occur in a frozen
crystallised
state, contain DNA or RNA but never contain both, and are thus
incapable
of reproduction except as parasites within a living host. Despite
intensive studies, the essential molecules of life - DNA, RNA,
ATP and
ADP (adenosine tri- and di-phosphate) are not found in
meteorites, whose
carbon isotopic composition is heavier than in life remnants
represented
by kerogen. Amino acids found in carbon-rich chondritic
meteorites -
isobutaric acid and racemic isovaline, believed to have been
shock-modified during deep space impacts, are exceedingly rare on
Earth
- a key observation militating against 'panspermia'.
On the origin of intelligence
The cone-shaped and branching algal columns display remarkable
regularities. No one yet understands how billions of individual
cells
communicated without a central nervous system, how each bacterium
knew
where to position itself relative to its neighbours to ensure the
perfectly formed geometrical patterns, or were they merely
subjected to
environmental controls? At 3.45*10^9 years ago the intelligence
that
underlies life is already in evidence. In a sense it does not
matter
where life originated, for wherever it was the enigma remains,
how
combinations of carbon, oxygen, hydrogen, nitrogen and sulphur
evolve
all the way into a brain and into technological civilisations!
Taylor (1999) believes technological civilisations may be unique
in the
Universe, which contrasts with estimates arising from the
so-called
Drake Equation - N = R**pNe*l*i*cL (N - probable number of
intelligent
civilisations in the Milky Way galaxy capable of radio
communications; R
- rate of star formation; *p - fraction of stars with
planetary
systems; Ne - fraction of planets favourable for life; *l -
fraction of
planets on which life does develop; *i - fraction of planets with
intelligent creatures; *c - fraction of planets on which
technical
civilisations develop; L - longevity of technical civilisation).
On this
basis between 10 000 and one billion planets with technological
civilisation exist at present, depending among other on the L
factor and
thereby optimism (Shklovskii and Sagan, 1977). To me any
restrictive
view based on Earthly experience is anathema, the classic
"worm in the
apple" situation - the worm believes it is the only worm in
the only
apple in the entire universe. The chance of amino acids combining
at
random into a protein molecule - the basic molecule of life
- is 1 in
10^130 - a larger number than the number of planets in the
Universe -
10^20. Life must be written into the laws of nature,
arising wherever
conditions and sufficient time allow. Intelligence is a
subjective value
judgement, reflecting species-specific arrogance - there is as
much
intelligence in a bee dance as in the Swan Lake ballet. Perhaps
we are
not meant to know the answers to the deepest questions.
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Copyright 2000, Andrew Glikson
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