About two and one-half billion years ago, life on Earth was still in its infancy. Complex organisms such as plants and animals had not yet appeared, but the planet was teeming with microscopic bacteria which thrived in the temperate and nutrient-rich environment. Greenhouse methane lingered in the atmosphere and trapped the sun’s warmth, creating a climate very accommodating to the stew of microbes life that made their home on primitive Earth. But a billion years of bacterial evolutionary progress was soon stunted by a catastrophic global event. Geologists find no signs of a great meteor impact nor a volcanic eruption, but they have uncovered the unmistakable geologic scars of rapid worldwide climate change. Average temperatures, which were previously comparable to our present climate, plummeted to minus 50 degrees Celsius and brought the planet into its first major ice age. This environmental shift triggered a massive die-off which threatened to extinguish all life on Earth, and paleoclimatologists have good reason to believe that this world-changing event was unwittingly caused by some of the planet’s own humble residents: bacteria.
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