life development progress

Discussion in 'Biology & Genetics' started by paddoboy, Jan 17, 2017.

  1. paddoboy Valued Senior Member

    Rock samples suggest oxygen levels during 'Lomagundi Event' were high enough to support life development progress
    January 17, 2017 by Bob Yirka report

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    Credit: NASA
    (—A small team of researchers in the U.S. has found evidence in rock samples that suggests that oxygen levels during the Lomagundi Event were high enough to support the advancement of life on Earth. In their paper published in Proceedings of the National Academy of Sciences, the team describes how they analyzed rocks that had formed during the event period and in so doing found evidence of higher than expected oxygen levels.

    Scientists believe that life first emerged on planet Earth almost 4 billion years ago—but it took almost another 3 billion years for eukaryotes (a cell-based organism with genetic material) to come about. But there is also evidence that the Earth experienced a time when oxygen levels rose, called the Lomagundi Event—it occurred approximately 2.3 to 2.1 billion years ago. Scientists would like to know whether the degree of oxygen enrichment during that time period was enough to support higher life form development, and if so, why eukaryotes didn't arise during that time instead of a billion years later.

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  3. paddoboy Valued Senior Member

    Oxygen is essential for eukaryotic life. The geologic record of early Earth contains abundant evidence of low oxygen levels, and accordingly, a lack of eukaryote fossils. The rise of oxygen to near-modern levels at the end of the Proterozoic Era is thus often cited as the trigger for the evolutionary radiation of complex life forms at this same time. Here we present selenium geochemical data that indicate an expansion of suboxic (>0.4 μM O2) habitats in the shallow oceans between 2.32 and 2.1 Ga––more than one billion years before eukaryotes become abundant in the fossil record. These environments could have harbored the earliest stages of eukaryotic evolution, but may have been too transient for substantial diversification to occur.

    Selenium isotopes record extensive marine suboxia during the Great Oxidation Event

    It has been proposed that an “oxygen overshoot” occurred during the early Paleoproterozoic Great Oxidation Event (GOE) in association with the extreme positive carbon isotopic excursion known as the Lomagundi Event. Moreover, it has also been suggested that environmental oxygen levels then crashed to very low levels during the subsequent extremely negative Shunga–Francevillian carbon isotopic anomaly. These redox fluctuations could have profoundly influenced the course of eukaryotic evolution, as eukaryotes have several metabolic processes that are obligately aerobic. Here we investigate the magnitude of these proposed oxygen perturbations using selenium (Se) geochemistry, which is sensitive to redox transitions across suboxic conditions. We find that δ8/78Se values in offshore shales show a positive excursion from 2.32 Ga until 2.1 Ga (mean +1.03 ± 0.67‰). Selenium abundances and Se/TOC (total organic carbon) ratios similarly show a peak during this interval. Together these data suggest that during the GOE there was pervasive suboxia in near-shore environments, allowing nonquantitative Se reduction to drive the residual Se oxyanions isotopically heavy. This implies O2 levels of >0.4 μM in these settings. Unlike in the late Neoproterozoic and Phanerozoic, when negative δ8/78Se values are observed in offshore environments, only a single formation, evidently the shallowest, shows evidence of negative δ8/78Se. This suggests that there was no upwelling of Se oxyanions from an oxic deep-ocean reservoir, which is consistent with previous estimates that the deep ocean remained anoxic throughout the GOE. The abrupt decline in δ8/78Se and Se/TOC values during the subsequent Shunga–Francevillian anomaly indicates a widespread decrease in surface oxygenation.
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