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The oldest biofossils
The geologic source of the 3.556 billion-year-old samples proved questionable,
but Schopf and others reexamined the 3.465 billion-year-old filaments for their
authenticity with Laser-Raman spectroscopy, focusing in particular on the compositional
aspects of the fossilized materials (Nature, v. 416, 2002, p. 73-76).
The new data reasserted the biogenicity, or biological origin, of the Apex chert
filaments. Other investigators, however, do not agree with these conclusions.
They contend that Schopf's fossils are likely artifacts, based on reinterpretations
of the Warrawoona structures — after new geologic mapping, more extensive
optical and electron microscopy, stable isotopic analyses, and Raman spectroscopy
of other carbonaceous materials.
Martin Brasier at the University of Oxford and others suggest that the locality
of the Apex chert microfossils is not, as previously suggested, a bedded conglomerate,
but rather a metalliferous hydrothermal vein that formed at temperatures of
250 to 350 C — more than 100 C hotter than even the most optimistic view
of the upper limit for life (Nature, v. 416, 2002, p. 76-81). They further
argue that abiotic synthesis may be responsible for the reduced carbon compounds
analyzed in the chert and their light isotopic signatures. Jill Pasteris and
Brigitte Wopenka of Washington University take further issue with the Raman
analysis of the Apex fossils (Nature, v. 420, 2002, p. 476-477). They
note that this technique can in no way unambiguously identify biogenic carbon,
citing numerous examples of clearly non-biogenic carbon compounds that yield
spectra indistinguishable from Schopf's spectra. In their reply, Schopf and
colleagues state that Raman spectra can hint at biogenicity, but add that these
data should always be interpreted in connection with other lines of evidence
— evidence, which, however, is also being reevaluated (Nature, v.
420, 2002, p. 477).
The oldest chemofossils
In May of last year, Chris Fedo and Martin Whitehouse reported evidence that
directly contradicts the origin of the Greenland rocks that harbor the oldest
chemofossils (Science, v. 296, 2002, p. 1448-1452). What Mojzsis and
colleagues described as a metamorphosed sedimentary banded iron formation, Fedo
and Whitehouse interpreted as an ultramafic igneous rock. This reinterpretation
is largely based on trace element comparisons between this and various igneous
rocks and on a different reading of the regional geologic history. The new view,
if accurate, precludes that the isotopically light, graphitic carbon analyzed
by Mojzsis' group represents a biological signature. Whether or not the rocks
in question are sedimentary in origin remains unresolved, and, consequently,
so does the validity of the chemofossils at about 3.8 billion years (Palin,
Science, v. 298, 2002, p. 961).
Sunlight-independent microbial
metabolism
Solar radiation provides energy to photosynthetic life, driving the progression
of otherwise thermodynamically unfavorable reactions. A few studies notwithstanding,
there is a consensus, however, that the earliest organisms on Earth were chemotrophs
and not phototrophs. In addition, approximately the first billion years of life
predated oxygenic photosynthesis. Hence, finding extant communities that live
completely independent from photosynthesis is of great interest. Such communities
and their environments would provide significant clues not only for understanding
life's origin on Earth, but also for elucidating extraterrestrial ecosystems
{emdash} most directly, subsurface abodes for microbes on Mars and Europa.
In January 2002, Francis Chapelle and others described a hydrogen-based microbial
community 200 meters down in the subsurface hydrothermal groundwater of Lidy
Hot Springs in Idaho (Nature, v. 415, 2002, p. 312-315). Based on three
different culturing-independent molecular techniques, they determined that more
than 95 percent of the microbial population belonged to the domain Archaea (which
together with the domain Bacteria defines the prokaryotes); no eukaryotes and
only 1 to 5 percent Bacteria were present. Further, phylogenetic analyses revealed
that more than 90 percent of the archaeal sequences were most closely related
to methanogens — strict anaerobes that use carbon dioxide to oxidize hydrogen,
forming methane and water. Geochemical analyses showed that neither organic
carbon nor oxygen, which are common products of photosynthesis, figured in the
microbial metabolism at this site. Chapelle and colleagues argue that hot water-rock
interactions produced the hydrogen, which serves as the primary energy for these
subsurface communities.
Hydrogen is also a critical source of energy and reducing power in other continental
and marine hydrothermal ecosystems. A recent review on this topic by Anna-Louise
Reysenbach and Everett Shock (Science, v. 296, 2002, p. 1077-1082) reminds
us that methanogenesis and sulfur-reduction — two hydrogen-consuming processes
— are common metabolic strategies of primary producers deep in the global
phylogenetic tree of life. Many high temperature Archaea and Bacteria, most
of which thrive in the dark subsurface, are fueled by thermodynamically favorable,
but kinetically inhibited redox reactions. Reysenbach and Shock also note that
both genomes and rocks are "records of evolutionary changes;" once
encoded and annotated, they may reveal how Earth became habitable first to chemosynthetic,
most likely thermophilic, single-celled life — later to photosynthesizers,
eukaryotes, and ultimately the explosion of multicellular organisms.
Expanding the field
Although the timing and process of life's origin remain obscured and the extent
to which life inhabits Earth's subsurface and extraterrestrial planets remains
unbounded, the sandbox in which a geomicrobiologist plays is rapidly expanding.
Rovers will scour the martian surface next year looking for clues to life. Ocean
and continental drilling programs are aggressively incorporating — in fact
rallying around — geomicrobiology. This year, the National Science Foundation
will fund its first research proposals through the Biogeosciences Program. New
journals are targeting the biology-geology interface; Geobiology will
publish its inaugural volume later this year. In short, the field of geomicrobiology
is more dynamic than ever, and the questions being pursued hit at the core of
life on Earth … and elsewhere.
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