Eukaryote conquest of the continents

NW end of a classic example of a mesa form of ...

Suilven, a spectacular outlier of Torridonian terrestrial sandstones resting on a buried landscape of Archaean gneisses near Lochinver, Sutherland. Image via Wikipedia

Geologists often assume that the continents were first colonised by plants, insects then vertebrates beginning in the Ordovician Period with preservation of spores very like those of the liverworts, which incidentally can only be removed from gravel driveways by the use of acetic acid, glyphosate, pycloram and flamethrowers having no lasting effect. The most intractable of all organisms found on the land surface today are prokaryotic (nucleus-free cells) cyanobacteria whose biofilms cement desert varnish (see Desert varnish, May 2008 in Subjects: GIS and Remote Sensing). Cyanobacteria have long been suspected to have been the first life forms to adopt a terrestrial habit, and their cells have been discovered in the now-famous Neoproterozoic lagerstätten in the Doushantuo Formation of China (see The earliest lichens, May 2005 in Subjects: Geobiology, palaeontology, and evolution) The oldest un-metamorphosed sediments in Britain, the Torridonian redbeds that form the magnificent scenery of north-western Scotland, now push back the date of the earliest eukaryotic (cells with nuclei) terrestrial life, of which we are one form, half a billion years before the Doushanto cyanobacteria (Strother, P.K. et al. 2011. Earth’s earliest non-marine eukaryotes. Nature, v. 473, p. 505-509). The Torridonian is one of the thickest (~12 km) terrestrial sequences on the planet, and spans a time range of around 200 Ma (1.2 to 1 Ga). It is a repository of almost the entire range of humid continental sedimentary environments: colluvial fan; bajada; alluvial; deltaic and lacustrine build-ups. Grey lake-bed mudstones and phosphate nodules in the Torridonian yield small organic fossils lumped in the sack-term acritarchs. Similar bodies, whose affinities are diverse and generally obscure, have been reported from marine sediments as old as 3.2 Ga. The fascination of those from the Torridonian, other than their terrestrial association, is that some include aggregates of spherical cells with tantalising suggestions of central nuclei and, as a whole assemblage, exhibit a range of morphologies far beyond that of nucleus-free prokaryotes and the signature of cytoskeletal filaments that form a ‘scaffold’ for eukaryote cells. Worth noting is that one of the authors is Martin Brasier of Oxford University, whose meticulous bio-morphological skills in microscopy has made him one of the foremost critics of speculation on Precambrian  microfossils (see Doubt cast on earliest bacterial fossils April 2003 in Subjects: Geobiology, palaeontology, and evolution). The authors opine that the ecological diversity of freshwater and land systems, and the physico-chemical stress associated with repeated wetting and desiccation compared with the marine domain may have been instrumental in origination of the Eucarya, which should give the Torridonian a scientific reputation that extends beyond these shores.

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