The quest for signs of life in the Earth's oldest rocks shows no signs of diminishing. Wen-Long Zang is concerned with information coming from metamorphosed rocks, where there is growing evidence for the preservation of single-celled animals in metamorphosed chert. Newly published is his discovery of spinose acritarchs in the Harris Greenstone Domain in South Australia. This is dated as Late Archaean, around 2500 million years, and part of the first supercontinent Kenorland.
The significance of his report is that these particular fossils are eukaryotes, not the bacterial cells that we normally associate with the Archaean. Previous to this finding, eukaryotes were suspected in the Archaean because researchers had detected biomarkers, but the "oldest confirmed macroscopic eukaryotic algae were reported from the 2100-1900 Ma Neganee Iron Formation, Michigan." This led many to link the evolution of eukaryotes with the rise of oxygen in the Earth's atmosphere (the Great Oxidation Event is dated about 2.4 Ga). However, there have been several significant and related finds: Archaean oil, eukaryotic biomarkers in the Archaean, and reinterpretation of evidences relating to lack of atmospheric oxygen prior to 2.4 Ga. Dutkiewicz et al (2006) write: "The presence of abundant biomarkers for cyanobacteria and eukaryotes derived from and trapped in rocks deposited before the Great Oxidation Event is consistent with an earlier evolution of oxygenic photosynthesis than previously thought and suggests that some aquatic settings had become sufficiently oxygenated for sterol biosynthesis by this time."
The picture emerging of the Late Archaean is one that includes prokaryotes and eukaryotes, photosynthesis, an oxygenated atmosphere and lots of biological activity. This is a big contrast from the picture even 10 years ago. The significance for our thinking about origins is that the eons of time demanded by Darwinian processes are not available. Eukaryotic life appears to be irreducibly complex, yet these life forms appear as body fossils in the Late Archaean and their origin is still one of the greatest mysteries of biology. This constitutes evidence that Darwinism is not a good way to gain understanding, and also evidence that an information-rich approach is the only viable way forward.
Deposition and deformation of late Archaean sediments and preservation of microfossils in the Harris Greenstone Domain, Gawler Craton, South Australia
Precambrian Research, June 2007, 156(1-2): 107-124.
Abstract: Late Archaean sediments, felsic, mafic and ultramafic volcanics in the Harris Greenstone Domain, central Gawler Craton, South Australia are interpreted to have been deposited at ~2520 Ma in back arc settings and were metamorphosed at ~2440 Ma. Sedimentary evidence suggests that the metasediments in the Domain might have been deposited in fluvial/estuarine to deltaic-shelf environments and deformed by intermediate amphibolite facies metamorphism accompanying the Sleafordian Orogeny at ~2440 Ma. Microfossils are preserved in metachert layers that had been boudinaged into lenses. These microfossils, including spinose acritarchs, are organic-walled as evidenced by Laser-Raman microscopy. Fossil-bearing off-cuts were then treated by HF and similar microfossils, degraded organic matter and acid-resistant minerals, such as zircon, were exposed in situ on the etched surface. Their morphologically complex forms and detailed wall structures suggest that these microfossils are of primary biological origin. The microfossils in foliated metachert lenses, either from original deposition or from post-depositional quartz veins, are older than the ~2440 Ma age of the Sleafordian Orogeny, which provides a minimum age for the host rocks. In either case, these spinose acritarchs are the oldest protists known to date.
Dutkiewicz, A., et al. Biomarkers from Huronian oil-bearing fluid inclusions: An uncontaminated record of life before the Great Oxidation Event. Geology: 2006, Vol. 34, No. 6, pp. 437-440.
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