CCNet-Essay, 13 March 2000
THE VIOLENT HABITAT OF EARLY TERRESTRIAL LIFE
By Andrew Glikson <email@example.com>
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.
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
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,
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.
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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