Is it time to stop worrying over whether the ancient structures called stromatolites are of microbial origin? ‘Yes’ is the answer to emerge from field and lab work on a 3,430-million-year-old marine ecosystem.
Writing in these pages earlier this year1, Don E. Canfield wrote of an early Earth teeming with a variety of microorganisms. Stromatolites were mentioned as among the possible evidence of microbial life existing 3.5 billion years ago, but only as “probably” of biological origin. What is it about stromatolites that engenders caution when interpreting them as fossils? In their paper on page 714 of this issue2, Allwood and colleagues provide good reasons to suppose that such reservations really aren't necessary.
Part of the caution in interpreting stromatolites stems from their nature. They are laminated sedimentary structures, with shapes that range from simple domes to elaborately branched columns, and they range from the millimetre scale to more than 10 metres in size. Modern analogues are known where photosynthetic microorganisms — principally cyanobacteria — are responsible for forming them. Only rarely are microfossils found in ancient examples, but many researchers consider stromatolites to be the products of microbe–sediment interaction, and so to be fossils3.
Confidence in the interpretation of stromatolites as biogenic structures was dealt a serious blow with the proposal that stromatolite structure could theoretically result from abiotic processes in sediment accumulation4. To make matters more bewildering, the very definition of stromatolite is contentious3,5. So there has been a long-standing debate about whether these laminated sedimentary structures are indeed indicators of ancient microbial life5 — a debate that has intensified with the increased attention being paid to searches for evidence of life on other planets, and so to the earliest record of life on Earth6.
This is the background for Allwood and colleagues' report2. They investigated the relationship of stromatolite shape to depositional environment across more than 10 km of nearly continuous outcrop of the 3,430-million-year-old Strelley Pool Chert in Western Australia (Fig. 1). Rather than wade through long philosophical and semantic arguments (theirs are brief and to the point), Allwood et al. wade through the evidence. They conclude that the stromatolites are biogenic in origin, and were formed on a ‘carbonate platform’ — a broad, relatively flat expanse of carbonate sediments — that was subject to the influence of tides but generally remained submerged. Sea-level rise (transgression) was a major feature of this environment, but small fluctuations — transgressions and regressions (sea-level fall) — also occurred. At times, regression and restricted circulation with the open sea took place, leading to evaporation of water and the formation of evaporite minerals.
Allwood et al.2 recognize seven distinct stromatolite facies (sedimentary rocks with distinguishing characteristics that correspond to the conditions of deposition). They identify distinctive stromatolite shapes in each facies, and the supplementary material for their paper presents a dazzling array of images and information. This is not the first report on stromatolites from this formation7,8 and it probably won't be the last.
What is so striking about their scenario is that it is so ‘normal’ — in the sense that transgressive, tidally influenced carbonate platforms with stromatolites are common in the geological record9,10. Stromatolites are also known to form in similar environments today. Given this context, and combined with other features, Allwood et al.2 argue that the most likely interpretation is that the stromatolites are biogenic. Among those features are the seven distinct stromatolite types and their facies (see Fig. 1 of the paper2 on page 716); the geometries of the stromatolites; and the fine details of the sedimentary textures of the stromatolites themselves and the regions between them (the ‘interspace’ areas).
The sediment-grain compositions and textures of the stromatolites cannot be explained exclusively by mechanical processes. For example, the laminae of the ‘complex cone’ structures are not of the equal thickness that would indicate abiotic precipitation. And the sedimentology of the cones differs from that of interspace areas: sediment in the interspace area has a vertical gradation in grain size, indicating the operation of mechanical processes, whereas grains in the laminae are not graded. Rather, these grains evidently accumulated on the steep slopes of the cones8, suggesting that there was some mechanism for trapping and binding them, like that found in microbially influenced sediment accretion. Allwood et al.2 also point out that there are no known abiotic processes that could produce the facies-related stromatolites persistently and sometimes simultaneously on the carbonate platform.
No mechanical processes have been identified that form coniform stromatolites11. But we do know of modern coniform stromato-lites that are built by microorganisms. Growth experiments12 on the microbes that build the modern cones indicate that the cone shape results from gliding by a photosynthetic filamentous microorganism (a cyanobacterium). Horizontally gliding filaments interfere with one another, form a clump, and other gliding filaments encounter the clump. These are deflected upwards towards the light and a cone results. Such structures could become fossilized by trapping and binding of sediment and/or by precipitation of mineral matter while the cones are growing. Coniform stromatolites make up about 44% of ten types of stromatolite recognized from 35 geological units ranging from 3.5 billion to 2.5 billion years in age13.
Hydrothermal environments have been favoured as the most likely places where life existed on the early Earth14. But the setting for the stromatolites of the Strelley Pool Chert indicates that microorganisms were adapted to and thriving in shallow marine environments, and that it is not necessary to invoke the presence of hydrothermal activity. The same conclusion applies to the microbial mats from the 3,416-million-year-old Buck Reef Chert in South Africa15.
So, microbial ecosystems evidently developed in environments that we know (carbonate platforms) and produced structures that we recognize (stromatolites; Fig. 2). Why do we have to demand evidence that is more absolute? The prudent use of well-understood ancient analogues and modern examples to interpret stromatolites is a powerful scientific tool5. Actualism has served palaeontology well.
The late American comedian Rodney Dangerfield16 had a perpetual complaint: “I don't get no respect.” The status of stromatolites as indicators of early life on Earth has suffered from a similar attitude. The work of Allwood et al.2 will surely help to change that.
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Comparative study of two relatives, MISS and Stromatolites: example from the Proterozoic Kunihar Formation, Simla Group, Lesser Himalaya
Arabian Journal of Geosciences (2016)