Extreme Environments: Earth (2008)
The Earth is roughly four and a half billion years old, and life started to emerge shortly after the first billion. Our planet’s atmosphere has a high proportion of one of the world’s most reactive elements – oxygen. It is the product of green plants and is essential for the survival of all animal life. Tropical forests help replenish oxygen in our atmosphere, taking the carbon dioxide from the air and converting it to oxygen with the help of the sun and water.