LIFE ON THE EDGE: SURVIVING EARTH'S GREATEST EXTINCTIONS

The sun shone twenty percent less brightly. The atmosphere lost its greenhouse gases. Temperatures plummeted. Ice progressed from the poles. Covered the mid-latitudes. Reached the equator. The oceans froze solid. The planet became a snowball. For three hundred million years. Even the photosynthetic bacteria perished. Light could not reach the water. Endless darkness was a death sentence.