Stromatolites are considered Earth’s oldest fossils, but more importantly, many scientists allude stromatolites to cyanobacteria and the emergence of life. However, debates have risen on whether the oldest of stromatolites are all made biologically. In some cases, stromatolites can be formed inorganically, and as a result, the definition of a stromatolite is still disagreed upon.
The safest way to prevent incorrect assumption of biological activity is to assume that all stromatolites are inorganic unless proven otherwise. Using this assumption, a stromatolite is nothing more than a laminated structure with synoptic relief. One must reveal a combination of evidence such as microfossils, biomarkers, stable isotopes, and the physical and chemical structure of the stromatolite in order to determine whether a stromatolite is biogenic.
Archaean
Stromatolites dating back to 3.5 billion years ago have been found in the Warrawoona Group in Northwest Australia. In addition, microstructures resembling bacteria have been found in the stromatolites. However, these structures contain no biomarkers, and thus the question of whether these microstructures and stromatolites are biogenic has still remained unresolved.
According to Knoll, microbial mat processes are not the only process that can give rise to these structures. Similar features can be formed geologically without the influences of microbial mats, thus making the stromatolites of inorganic origin. As a result, bacteria-like microfossils are not strong enough evidence to conclude the biogenic origins of stromatolites, and consequently, we can only accept the Warrawoona stromatolites as biogenic if we can rule out alternative physical explanations.
Proterozoic
Whereas the biological origins of the Archaean Warrawoona stromatolites remain uncertain, stromatolites from the Proterozoic reveal an abundant of microfossils, suggesting a biogenic origin. However, just like the Warrawoona microfossils, paleontologists must prove that their stromatolites are biogenic using a combination of microfossils, biomarkers, and stable isotopes.
The Gunflint Formation—dated 2.1-1.9 billion years old—contains tiny fossils in finger-like stromatolites of black chert; the most common fossils were iron-coated tubes. The idea that 2.1-1.9 billion-year-old bacteria were iron-loving made sense due to the fact that the oceans were rich in iron.
Furthermore, these tiny fossils contained biomarkers, signifying a biogenic origin. Similar microfossils have been found globally in sedimentary rock that date back to 2.1-1.8 billion years, and therefore demonstrates that Gunflint’s microfossils were not a local anomaly but a persistent feature worldwide.
However, some paleontologists still debate whether these fossils actually record the microbial mat communities that built their stromatolitic structures or that the bacteria simply fell onto the accreting silica surfaces. Knoll believes that there is not enough dense interweaving of filaments in the Gunflint stromatolites that are found in other known biogenic stromatolites.
Conclusion
The strongest evidence for biogenic stromatolites is a combination of microfossils, biomarkers, stable isotopes, and structure of the stromatolite itself. Furthermore, the biogenic origins of a stromatolite cannot be proven with just one form of evidence. For instance, microfossils alone cannot prove a stromatolite is biogenic due to the fact that structures resembling these fossils can be made geologically. Consequently, further research must be undertaken to fully understand what are perhaps to be the first organic structures made on Earth some 3.5 billion years ago.
References
Knoll, A. H. 2003. Life on a Young Planet. Princeton University Press: New Jersey.
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