ERA PERIODS EPOOCH MAJOR EVENTS       PRE-CAMBRIAN ERA -Origin of Earth around 4.5 billion years ago -Rich in Hydrogen, Helium, Carbon dioxide, Nitrogen but Oxygen was absent -Origin of life in the form of Anaerobic respiration and unicellular bacteria. -Around 2500 million years ago Blue-Green Algae originated. -Formation of Aravalli Mountain Range.   … Continue reading Geological Timescale of Earth-Tabular Form