Exobiology in the Solar System & The Search for Life on Mars - ESA

morphological and biochemical signatures of extraterrestrial life: utility of terrestrial analogues/I.4
that precursor floras had been extant prior to ~3.5 Gyr when <strong>the</strong> preserved rock record
becomes scant and <strong>in</strong>creas<strong>in</strong>gly metamorphosed. In this context, <strong>the</strong> observation of
cell-like carbonaceous structures <strong>in</strong> <strong>the</strong> 3.8 Gyr-old metasediments from Isua, West-
Greenland, attracted considerable attention. Described as Isuasphaera isua (Pflug,
1978), <strong>the</strong> biogenicity of this morphotype (Fig. I.4.3.1.2/1) has been violently
disputed, specifically on grounds of <strong>the</strong> improbability of survival of delicate cell
structures dur<strong>in</strong>g <strong>the</strong> amphibolite-grade metamorphism of <strong>the</strong> host rock.
Meanwhile, however, <strong>the</strong>re is ample evidence that fossils <strong>in</strong> general and microfossils
<strong>in</strong> particular may – <strong>in</strong> variable degree – withstand obliteration <strong>in</strong> rocks subjected
to medium-grade metamorphism. <strong>The</strong>re<strong>for</strong>e, we cannot a priori exclude microbial
aff<strong>in</strong>ities <strong>for</strong> selected cell-like microstructures from <strong>the</strong> Isua metasediments, and
notably <strong>for</strong> Isuasphaera-type micromorphs that show a strik<strong>in</strong>g resemblance to a
possible counterpart of recognised biogenicity <strong>in</strong> <strong>the</strong> younger (Proterozoic) record
described as Huroniospora sp. In spite of <strong>the</strong> uncerta<strong>in</strong>ty surround<strong>in</strong>g a large number
of morphotypes described from Isua, and of occasional convergences with purely
m<strong>in</strong>eralogical features, <strong>the</strong>re is a reasonable chance that <strong>the</strong> microstructure <strong>in</strong>ventory
as a whole <strong>in</strong>cludes at least some elements of a structurally degenerated microfossil
assemblage such as might result from metamorphic impairment of a Warrawoonatype
microflora. As detailed <strong>in</strong> Section I.4.3.2, <strong>the</strong> existence <strong>in</strong> Isua times of microbial
ecosystems would not only be consistent with, but also conditional <strong>for</strong>, <strong>the</strong> actually
observed carbon content and carbon isotope geochemistry of <strong>the</strong> Isua suite.
I.4.3.2 Biogeochemical Evidence
Apart from morphological relics, organisms also leave a chemical record of <strong>the</strong>ir
<strong>for</strong>mer existence. While <strong>the</strong> bulk of <strong>the</strong> body material degrades after <strong>the</strong> death of <strong>the</strong>
organism (with <strong>the</strong> dom<strong>in</strong>ant carbon component rem<strong>in</strong>eralised as CO 2), a m<strong>in</strong>iscule
fraction of <strong>the</strong> organic substance (between 10 -3 and 10 -4 ) usually escapes decomposition
by burial <strong>in</strong> newly-<strong>for</strong>med sediments. Here, <strong>the</strong> primary biopolymers undergo
a large-scale reconstitution (‘humification’ with subsequent trans<strong>for</strong>mation <strong>in</strong>to
<strong>in</strong>organic carbon polymers), result<strong>in</strong>g f<strong>in</strong>ally <strong>in</strong> <strong>the</strong> <strong>for</strong>mation of kerogen, a
chemically <strong>in</strong>ert (acid-<strong>in</strong>soluble) polycondensed aggregate of aliphatic and aromatic
hydrocarbons that represents <strong>the</strong> end-product of <strong>the</strong> diagenetic alteration of primary
biogenic matter <strong>in</strong> sediments (Durand, 1980). Represent<strong>in</strong>g <strong>the</strong> residuum of liv<strong>in</strong>g
substances, kerogenous materials and <strong>the</strong>ir graphitic derivatives constitute per se
first-order proxies of past biological activity.
Moreover, <strong>the</strong> isotopic composition of <strong>the</strong> kerogenous carbon constituents gives
fur<strong>the</strong>r testimony to <strong>the</strong>ir biogenicity as it reflects 13 C/ 12 C fractionations of a magnitude
and direction that are typically obta<strong>in</strong>ed <strong>in</strong> <strong>the</strong> common assimilatory (photosyn<strong>the</strong>tic)
pathways, notably <strong>the</strong> ribulose bisphosphate (RuBP) carboxylase reaction of <strong>the</strong>
Calv<strong>in</strong> Cycle that operates <strong>in</strong> C3 photosyn<strong>the</strong>sis. Particularly if seen <strong>in</strong> conjunction
with <strong>the</strong> isotope record of coexist<strong>in</strong>g carbonate, <strong>the</strong> 13 C/ 12 C compositions of
sedimentary organic carbon convey a remarkably consistent signal of biological
carbon fixation that derives, <strong>for</strong> <strong>the</strong> most part, from a k<strong>in</strong>etic isotope effect imposed on
<strong>the</strong> first CO 2-fix<strong>in</strong>g enzymatic carboxylation reaction of <strong>the</strong> photosyn<strong>the</strong>tic pathway.
A third category of biochemical evidence of ancient life is represented by quasiprist<strong>in</strong>e
organic molecules (mostly pigments or s<strong>in</strong>gle discrete hydrocarbon cha<strong>in</strong>s)
that have preserved <strong>the</strong>ir identities over <strong>the</strong> entire pathway of kerogen evolution from
dead organic matter to <strong>the</strong> polymerised aggregates of solid hydrocarbons that mark
<strong>the</strong> end of <strong>the</strong> maturation series of primary biogenic substances. <strong>The</strong> record of such
‘biomarker’ molecules or ‘chemofossils’, respectively (Egl<strong>in</strong>ton & Calv<strong>in</strong>, 1967),
goes far back <strong>in</strong>to Precambrian times, ga<strong>in</strong><strong>in</strong>g special significance <strong>in</strong> connection with
<strong>the</strong> geologically oldest (Proterozoic) petroleum occurrences (Summons & Powell,
1992). Current work on molecular fossils from <strong>the</strong> kerogen fraction of Precambrian
rocks (Hoer<strong>in</strong>g & Navale, 1987) holds <strong>the</strong> promise of considerable potential <strong>for</strong> <strong>the</strong>
fur<strong>the</strong>r elucidation of <strong>the</strong> early history of life.
In <strong>the</strong> follow<strong>in</strong>g, <strong>the</strong> above categories of evidence that document <strong>the</strong> biochemical
record of <strong>for</strong>mer life are characterised <strong>in</strong> more detail.
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