Animal life is extraordinarily diverse and complex, having colonized almost all environments on Earth—from hostile hydrothermal vents in the deep sea to the skies across our continents. But the planet was not always teeming with complex animal life. For the first 3.7 billion years after it origina ted, life was small, simple and largely confined to the oceans. This microbe-dominated world was a tumultuous place, with several major swings in its climate. ...read more read less