Early life on Earth

The evolution of life on Earth from the first organisms to the emergence of the earliest human ancestors.

~4.4 billion years ago: Earth's surface began developing a solid crust and oceans; ancient zircon crystals provide evidence for relatively early liquid water.
~2.4 billion-2.1 billion years ago: Great Oxidation Event; Photosynthetic cyanobacteria began releasing large amounts of oxygen (O₂). Initially, the oxygen reacted with dissolved iron and other compounds, but once these were saturated, oxygen accumulated in the atmosphere. This transformed Earth's atmosphere, caused the extinction of many anaerobic organisms, enabled the formation of the protective ozone layer, and paved the way for complex aerobic life.

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