Miracle Planet
Episode Four

 Extinction and Rebirth

 Life on Earth has experienced five great extinctions. The worst occurred about 250 million years ago during the Permian period. Our ancestors at that time were mammal-like reptiles. The super-plume theory attempts to explain this extinction with a gigantic vertical flow of magma in Earth's mantle. About 300 million years ago, under the ocean trenches around the supercontinent Pangea, the end parts of the ocean plates dropped toward the core, creating a reactionary upward magmatic current that caused violent volcanic eruptions. A huge basaltic plateau in Siberia is believed to be evidence of this super-plume. According to the theory, carbon dioxide from volcanoes warmed Earth, causing methane-hydrate long frozen under the ocean floor to melt. Enormous amounts of methane were released into the air, accelerating global warming. The methane also depleted atmospheric oxygen, devastating plants. The oxygen depletion continued for about 100 million years. Reptiles adapted best by developing a unique air-sac system. Mammals were also forced to devise a way to adapt, so they evolved to nurture babies in a womb in order to send them adequate oxygen. Mammals also devised a diaphragm for efficient respiration. This caused them to lose the lower half of their ribs, but as a result they could lie on the ground while twisting their waist. Now they were capable of breast-feeding.