Bacteria took ~1 billion years to emerge on Earth, but why did it take another ~3 billion years for animals to appear? Scientists analysed isotopes of Chromium in rocks as a measure O2 levels in the atmosphere of the early Earth. Reduced-Cr(III) indicates low O2 levels, oxidised-Cr(IV) indicates high O2 levels. Atmospheric O2 levels dramatically … Continue reading Low oxygen delayed emergence of animals on Earth