Rise of animals on Earth delayed by insufficient oxygen
Geologists of different universities have found that insufficient levels of atmospheric oxygen during the mid-Proterozoic (~ 1.8 billion to 800 million years ago) delayed the evolutionary rise of animals on Earth. They studied chromium isotope data from shallow marine Proterozoic sediments and compared them to younger Phanerozoic sediments deposited in similar environments. Their findings suggest that chromium oxidation in the Proterozoic was very limited and that oxygen levels were at most 0.1 % of present atmospheric levels. Previous estimates of atmospheric concentrations have varied greatly, but it was generally accepted that oxygen existed at Earth’s surface during the Proterozoic. The late appearance and diversification of metazoans was therefore long thought to be limited by genetic advancements and ecological innovation, but now it appears that their evolution was instead delayed by low levels of oxygen.
Journal reference: Planavsky, N. J., Reinhard, C. T., Wang, X., Thomson, D., McGoldrick, P., Rainbird, R. H., … & Lyons, T. W. (2014). Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals. Science, 346(6209), 635-638.
Image: Cirrus clouds above Warsaw, Poland. Source: Przemyslaw Idzkiewicz, Wikimedia Commons.