Rocking the Cradle of Life

Categories: Feature Stories Geology

Finding the Prebiotic Boundary

The Apex Chert microfossils (above) formed in association with hot fluids near a volcanic structure.
Credit: UCLA

To find what many consider to be the earliest fossil evidence for life on Earth, one starting point might be to book a flight to the Pilbara region in Western Australia.

On arrival in this gold-mining district, the searcher finds strange rock layers, many shaped like egg-cartons. Billions of years ago, these colonies may have formed as microbial reef deposits. Under closer examination, a few of these rocks reveal microscopic segmented shapes that hint at their complex and possibly biological origin. Given that many of these rock layers date back 3.45 billion years, their detailed study has attracted interest in the search for life on early earth.

Called stromatolites (from the greek for "stoney carpet") such sediments may be the remains of microbial mats that grew in stages: first seeking nutrients and then incorporating minerals into their rock layers. Examples of living stromatolites can be seen today at Hamelin Pool, Shark’s Bay Western Australia. It is also suggested that on early earth, stromatolites may have formed chemically in underwater, hot vents where minerals precipitated in sculpted layers. This explanation does not require biology.

Questions about the origin of these sediments thus center on the fine details of how they arose. Are these true fossils or some volcanic remant? Did they originate from chemical or biological starting materials, a key distinction that fuels a spirited debate and attention in a research field called paleobiology.

The mystery underlying Earth’s early fossil record is complicated not just because these rocks are very old, but also because on Earth, biology itself was very young. Pre-Cambrian organisms lacked hard parts that could be preserved once the hosts died and decayed. Little from the first organisms could be preserved. The early diversity of life also was limited, so hospitable locations for fauna to take root were more narrowly defined.

Other than the interest of paleobiologists studying when life started, this controversy highlights deeper questions for astrobiologists: How to detect life? Can shape (or morphology) be used to identify simple, primitive life forms ? What kinds of evidence would compel a definitive conclusion about some future martian fossil, particularly if a candidate rock had preserved just the incomplete biological outline that only vaguely resembled a microbe once seen on Earth?

Astrobiology Magazine had the opportunity to talk with researcher Nicola McLoughlin of Oxford University’s Department of Earth Sciences about her work on whether the earliest putative microfossils give a useful starting date for posing the big question: When did life begin?

Astrobiology Magazine (AM): What about the Western Australian finds first interested you?

Modern reef colonies, Sharks Bay, Western Australia

Nicola McLoughlin (NM):
The shear age and relatively good preservation of the Warrawoona Group, the unrivalled exposure of stromatolites, the challenge and opportunity to understanding the co-evolution of the early bio/litho/hydro/atmospheres encoded in these rock. My stromatolite research is focussed on the Strelley Pool and North Pool Cherts. I have also spent a lot of field time mapping the (infamous) Apex Chert.

AM: Do you work with Martin Brasier on whether these are chemical [and not biological] in origin?

The Knoll Criterion for Life Detection

One astrobiology ‘null’ hypothesis is called the Knoll criterion, and it can be applied on another world like Mars or to the Earth’s fossil record.

Named after Harvard paleontologist Andrew Knoll, the methodology is cited as one example of not just how a shape might be similar to something biological, but whether a presumption is given to another explanation in the absence of biology. "You do your exploration," said Knoll, "and if, in the course of that exploration, you find a signal that is (a) not easily accounted for by physics and chemistry or (b) reminiscent of signals that are closely associated with biology on Earth, then you get excited. What will happen then, I can guarantee you, is that 100 enterprising scientists will go into the lab and see how, if at all, they can simulate what you see – without using biology."

NM: Yes. In studying putative Archean microfossils and stromatolites our group adopts the approach that a biological origin should not be accepted until all plausible abiogenic explanations for their origin have been examined and can be falsified.

Herein I the term stromatolite in the non-genetic sense: as attached, laminated, lithified sedimentary growth structures that accrete away from a point or limited surface of initiation (Semikathov et al., 1979). This definition describes the fundamental morphological and textural characteristics of a stromatolite, whilst encompassing multiple or indeterminate origins.

The biogenicity of Archean stromatolites is much debated due to their relative simple macro-morphology, the diagenetic destruction of organic microfossils and microfabrics (in the case of the Warrawoona group by pervasive syn- and post-depositional silicification–or conversion to silica). These challenges are coupled with an increasing realisation of the extent of seafloor chemical precipitation during the early Precambrian and the morphologically diverse precipitates that can result.

My ongoing research on the Strelley Pool Chert indicates extensive, primary chemical precipitation that produced the large horizons of crystal fan arrays. The conical stromatolites are intimately related with the crystal fans, often inheriting the topography of underlying crystal fans, raising the possibility that the stromatolites formed by similar processes.

Our null hypothesis is that, abiogenic crystallisation can produce complex undulose bedforms by the interaction of multidirectional seafloor currents with cohesive sediments – or "crystalline pavements" if you like. In an attempt to better understand these processes we are conducting experimental work to constrain the stromatoloid macro-morphologies and microfabrics that can be produced by abiogenic chemical precipitation.

Whilst the possibility remains that chemical precipitation of the Strelley Pool Chert stromatolites was microbially mediated, the lack of convincing relict microbial mat fabrics currently renders this hypothesis unproven . In addition to petrography and morphological modelling, we are also analysing stromatolite laminae for elevated levels of elements known to be concentrated by microbial mats.

AM: Can you broadly describe the mechanism of hydrothermally-heated graphite formation as an alternative to the microbial explanations?

NM: This mechanism has been suggested for the origin of microfossil like structures in black, kerogenous, hydrothermal cherts, principally the Apex "microfossils" (Schopf structures), and possibly also the Dresser/North Pole "microfossils" and " microfossils" found in cherts of the Mnt Ada Basalt.

Canadian stromatolites, Great Slave.

It is envisaged that processes such as Fischer-Tropsch (FTT) synthesis could produce simple carbon compounds deep in the Archean crust.

[Note: These chemical reactions produce long chain hydrocarbons terminated with an alcohol group. If these can react, surfactants or detergent-like molecules form, which would effectively precede the first primitive cell walls.. In detail, FTT is a also process where carbon dioxide [CO2] is converted to methane and short chain hydrocarbons by reaction with hydrogen [H2] possibly derived from serpentinisation reactions, this process is catalysed by magnetite and Ni-Fe alloys common in Archean ultramafic rocks. Significantly this process can produce carbon [C] compounds with [carbon-13 isotopes] d13C values of -20 to -54 %o previously taken as an indicator of biogenicity.]

Circulating hydrothermal fluids could carry these carbon [C] compounds up the dyke system, possibly entraining further re-mobilised carbon [C] on its way, agglomerating as it moves and being trapped by the cherts crystallising in the dyke/vent system. Further morphological modification of these organic rich artefacts or pseudofossils may then occur by later re-crystallisation.

In this way hydrothermal cherts with complex fracture-fill fabrics could contain clots of carbon [C] compounds with "microfossil" morphologies. No such "microfossils" structures are reported from stromatolitic horizons in the Warrawoona Group, but it has been suggested that if exhaled from a vent these pseudofossils could be trapped within an accreting stromatolite, perhaps also concetrating into amorphous organic rich layers.

AM: Do you have a favored result for any of the more undisputed claims to finding the earliest fossil forms of life? What is the best date now for the beginning of this microbial fossil record?

NM: Probably the oldest, most convincing biogenic stromatolites I have seen are from the 2.7 billion year [giga-year, or giga-annum, Ga] Fortesque group, with good micro-textural preservation, significant morphological complexity and diversity. The challenge is to bridge the gap between these stromatolites and the Warrawoona structures. Further systematic study of stromatolites from the Steep Rock Group 2.9-2.7 billion year old [Ga] Northen Canada and other horizons from Africa and India, may help to resolve, where the biotic-prebiotic boundary can be drawn (good review by Hofmann 2000).

In terms of the micro-fossil record there are two intriguing horizons: the structures recently reported by Furnes et al. (2004) in 3.5 billion year old [Ga] basaltic pillow lavas from South Africa and the 3.2Ga structures reported by Rasmussen et al. (2002) from the Warrawoona Group. These two horizons are quite different from the examples discussed in Qu 3 and I’d like to see further supporting petrography and geochemistry before accepting their biogenicity. Here in Oxford we are studying a new remarkably well preserved assemblage of microtubular structures from the Warrawoona Group that are morphologically similar to the Furnes et al. material. We hope soon to report the criteria used to investigate the biogenicity of these structures and on the basis of their context and mineralogy draw comparisons with results from recent Mars missions.

AM: Is it likely in your opinion that the early Archaean stromatolites were purple and green sulfur bacteria, and not photosynthesizing to produce oxygen?

NM: This is a highly plausible hypothesis given that molecular phylogenetic work by Carrine Blank and others, suggests that Archean microbes were restricted to "hot, deep" geothermal environments. Indeed, there is growing field evidence for a large hydrothermal influence on the depositional environment of the North Pole and more controversially the Strelley Pool Chert stromatolites. These are the types of environments in which chemosynthetic metabolisms may have been viable, but also, where pre-biotic synthesis of simple organic compounds and precipitation of morphologically complex chemical deposits may have occurred. Furthermore, my numerical modelling of the macromorphology of the Strelley Pool Chert stromatolite’s suggests that they are not strongly phototrophic structures, and thus we are searching for geochemical and micro-textural evidence to investigate a possible chemosynthetic origin.

AM: What kinds of experimental or field techniques are now being applied to help resolve disputes about these examples?

NM: The experimental synthesis of microfossils artefacts using sodium silicate gel and metal salts, has been used to gain insights into the abiogenic mechanisms that can generate pseudofossils (Garzia Ruiz 2003). We are conducting analogous experiments using a range of media to investigate the formation of synthetic stromatolites. There is much work to be done coupling this laboratory modelling with numerical modelling of stromatolite form and detailed field analysis of stromatolites. The recent application of complexity analysis to the investigation of stromatolite morphology (Corsetti et al.) is exciting but requires further verification.

Close-up of famous martian meteorite shapes in Allen Hills meteorite [ALH84001], found at Allen Hills, Antarctica.
Image Credit: NASA

Micro-laser Raman is an analytical technique beiing employed by several groups to identify organic remains in Archean cherts , and has proven to be a useful geo-thermometer but a poor indicator of biogenicity. High resolution analysis of coupled isotope systematics including carbon [C], oxygen [O], iron [Fe] and sulfur [S] using techniques such as nanno-sims and HRTEM, are being explored to identify microbial processing in both putative microfossils and stromatolites.

AM: Many have criticized a criterion centered around ‘appearance’ of bacteria-like rods as indicating either biogenic or abiogenic origins, whether in stromatolites, fossils or even meteors. Are there diagnostics or techniques one can imagine to help clarify this tenuous relation between structure and origin?

The martian spherules, ‘blueberries’ have become important to clues on an alien landscape. Credit: NASA/JPL

NM: The morphology of simple coccoid and rod shaped structures should not be taken alone as an indicator of biogenicity. Analysis of putative microfossil morphology should always be integrated with finescale petrographic and geochemical investigations.

Our group is undertaking quantitative morphological analysis of microfossil and macrofossil (i.e. stromatolite) morphology and find that putative biological and abiological structures often
occupy overlapping regions of morphospace. The true pessimist might conclude, that morphology is a poor indicator of biogenicity and that the simplest, most primitive life forms are easily mimicked by abiology. The challenge is to identify the types and scales of morphology unique to life – most progress will probably be made by correlating morphological and geochemical biogenicity criteria.

AM: What kinds of topics do you hope your research takes up in future work?

NM: Stromatolites are the most abundant macro-fossils in the Precambrian yet we have a very limited understanding of their morphogenesis and have only partially exploited stromatolite morphology as an indicator of environmental and evolutionary change. There is much work to be done at the interface of sedimentology, paleoobiology, numerical and laboratory modelling to better understand the morphology of stromatolites. More generally I am interested by the co-evolution of the Precambrian atmos/lithos/hydro/bio-spheres in particular the events of the Late Pre-Cambrian, the ediacara fauna and precursor events to the Cambrian explosion.

Related Web Pages

Ancient Fossils – or just plain rocks?
Earliest Life or Rare Dirt?
Raman Reveals Relics
Questioning the Evidence for Earth’s Oldest Fossils
The Invasion of the Deep-Sea Microbes
J. William Schopf’s Cradle of Life
Barberton Mountain Land rocks (3.4 Gy, stromatolites) from northern South Africa
Isua (3.8 GY Greenland)
Warrawoona Group