Scientists say the oxygen you just breathed in was once the deadliest poison on the planet, released by tiny microbes that accidentally wiped out most of the life around them in what geologists call t

Key Points:

• Around 2.5 billion years ago, oxygen produced by cyanobacteria acted as a poison, causing the Great Oxidation Event, which led to a mass extinction of anaerobic life forms dominating Earth at the time.
• Cyanobacteria developed photosynthesis that split water molecules, releasing oxygen as a byproduct, but oxygen initially reacted with dissolved iron in oceans, delaying atmospheric oxygen accumulation by nearly a billion years.
• Oxygen is highly reactive and toxic to anaerobic organisms, damaging DNA and enzymes, leading to a planetary catastrophe including a prolonged ice age; only some microbes evolved mechanisms to neutralize and eventually use oxygen for respiration.
• The evolution of oxygen-handling enzymes allowed certain bacteria to harness oxygen's energy potential, leading to more complex life forms; mitochondria in human cells originated from such bacteria engulfed by ancestral cells.
• The current oxygen-rich atmosphere is a continuous product of photosynthesis by cyanobacteria and plants, not a natural equilibrium, representing a persistent "industrial waste" that has shaped Earth's biosphere for over two billion years.