Did land life emerge earlier than thought?Science thrives on passionate debates, and the Earth scien

Did land life emerge earlier than thought?Science thrives on passionate debates, and the Earth sciences are no exception. Rocks are reinterpreted in each generation, sometimes correctly, others not. Since the 1990’s, a paleosols specialist (the study of ancient fossilised soils) at the University of Oregon in Eugene has argued for a reinterpretation of some of the most crucial rocks in the early history of life: The sandstones of the Ediacara hills in South Australia. The Ediacaran was officially recognised as an era in 2004 by the international commission on stratigraphy. It covers the period when the last snowball Earth glaciations of the proposed Cryogenian era ended (see our previous post at http://tinyurl.com/bvvysmj), involved an atmospheric oxidation event and saw the transition towards the Cambrian and its explosion of hard shelled organisms. Palaeontologically, it was marked by the radiation of the first large complex multicellular organisms, which are still described by many scientists as ‘enigmatic’. These were discovered by Australian geolegend Reg Sprigg, and later found in widespread locations around the globe.These fossils have been interpreted in many ways, as lichens, algae, invertebrates, giant foraminifera, microbial mats and others. They are poorly understood, and many prefer to refer to them as a biota rather than a fauna, since they do not feel confident placing them in any of life’s main groups (such as animals, fungi or plants). Some have even posited that they were a form of life unrelated to any existing today. The general consensus is that they were marine lifeforms, who lived in the sandy beds of warm shallow seas, and that those specimens found in terrestrial rocks were washed ashore by waves.In an article published in nature, Gregory Retallack puts forward the controversial idea that some of the rocks are paleosols, and that some of the Ediacaran fauna were land dwellers. This would make terrestrial life much older than the currently accepted date in the Ordovician, about a hundred million years later. This Ordovician date was once just as contentious as the hypothesis he advances. He posits that the relevant biota were mostly terrestrial, included fungi and lichens (an algae/fungi symbiosis), and adds a variety of robust evidence to support his claim that they originated in soils in a cold tundra or desert environment. If this turns out to be true, then studies of past environments are based on flawed assumptions and the history of life on Earth will be rewritten.Proving these rocks to be paleosols is no easy matter. Without root marks from land plants he had to turn to more subtle evidence. He notes the oxidation pattern of the rocks, that seem to have been weathered in situ during deposition since there is no sign of the outside in weathering that exposed rocks suffer. He also cites sand replacing gypsum crystals, the weathering pattern of clay minerals, mud cracks and carbonate nodules as evidence of terrestrial origin. His strongest evidence comes from microscopic chemical patterns of enrichment and leaching of nutrients within the rocks and stable isotope ratios that are typical of paleosols.Few agree with this contention, but the paper was robust enough to be accepted by Nature. Detractors think all these features could have come about in marine conditions, or resulted from later surface weathering. They point out the claim is based on evidence from rocks rather than the morphology of fossils, some of which resemble early invertebrates. They add that some specimens have been found on surfaces bearing typical wave formed ripple marks. They also comment that the same organisms have been discovered in clearly marine rocks elsewhere.This story illustrates well how geological knowledge grows: Rocks are constantly being reinterpreted, often in ways that are incompatible. This leads both sides to seek new evidence in the rocks to support their story, sometimes resulting in unexpected discoveries. It’s said that the greatest discoveries start with the phrase 'that’s funny’. As technology develops, our ability to interpret grows, along with our understanding. It often doesn’t matter who turns out right, the process itself is the reality of science at work, and it provides the impetus to learn whatever we can of the complex and subtle truths of nature.Rellatak will now travel to the UK, Newfoundland in Canada, Russia and China to seek paleosols in the other Ediacaran biota sites. The saying goes that extraordinary claims require extraordinary evidence, and these claims are indeed extraordinary. The jury is still out and the next exciting episode is on the way.LozImage credit for Dickinsonia fossil from the Vendian epoch in the Ediacara Hills of South Australia: Berkeley University.Nature papers:http://www.nature.com/news/life-on-land-1.12001http://www.nature.com/news/controversial-claim-puts-life-on-land-65-million-years-early-1.12017http://www.nature.com/nature/journal/v493/n7430/full/nature11777.htmlhttp://www.nature.com/nature/journal/v493/n7430/full/nature11765.htmlhttp://whyevolutionistrue.wordpress.com/2012/12/20/the-enigmatic-ediacaran-biota-just-got-more-enigmatic-or-did-it/http://www.sciencenews.org/view/generic/id/347062/description/Early_life_forms_may_have_been_terrestrial -- source link

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