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[BCG07-01] Biological evolution related with decrease of atmospheric oxygen contents: Crisis evolution hypothesis
Keywords:Evolution of life, Proterozoic, Biodiversity, Eukaryotes, Fluctuation of atmospheric oxygen contents
The secular change of pO2 content through the Proterozoic is enigmatic. We compiled redox sensitive element (RSE) contents of black shales, iodine contents of carbonate rocks and Cr isotopes of carbonate rocks to estimate pO2 contents through the Proterozoic. The RSE contents of pyrites may be also proxies for the pO2 contents, but species of iron-bearing minerals are not used to estimate pO2 contents because species of iron-minerals can discriminate anoxic/euxinic events from oxic periods but cannot differentiate oxic events/periods from anoxic/euxinic events/periods. Mercury (Hg) isotopes have a potential to quantitatively estimate pO2 contents over 10-5 PAL in pO2 contents. Previous studies showed that the pO2 contents estimated from the abundances of RSEs such as Mo increased gradually or stepwise through the Proterozoic (e.g. Scott et al., 2008), but their abundances were actually fluctuated. In fact, other proxies also show that the pO2 contents were fluctuated through the Proterozoic, suggesting higher pO2 contents around 2.2, 1.5 and 0.8 Ga whereas lower around 1.7 and 1.2 Ga, respectively. On the other hand, molecular clock studies and paleontology suggest that appearance of the Eukaryotes, multicellular Eukaryotes (algae), and Alveolata/Rhizaria around 2.1-1.9 Ga, 1.7, and 1.2 Ga, respectively (Strassert et al., 2021; Baludikay et al., 2016; Javaux & Lepot, 2018).
Coevolution of organisms and the Earth is widely discussed (e.g. Williams & Da Silva, 2003), and coevolution of Eukaryotes and increase of pO2is often assumed in the Proterozoic (e.g. Condie & Sloan, 1998). However, the appearance of eukaryotes, namely symbiosis of aerobic bacteria within anaerobic archaea, occurred possibly in the high pO2 period whereas other evolutions occurred possibly in the low pO2 periods. Because their precursors, namely anaerobic Archaea and aerobic Eukaryotes, need anoxic and oxic environments, respectively, it seems inconsistent between the evolutions and environments. We would like to propose a new hypothesis, “The Crisis Evolution Hypothesis,” that the discrepancy became a driving force of evolution of the Eukaryotes, namely symbiosis, in the Proterozoic.
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