The amount of oxygen in Earth’s atmosphere increased mainly because early microbes and later plants evolved the ability to perform oxygen‑producing photosynthesis, and a portion of the organic matter they made kept getting buried instead of recombining with oxygen.

Big picture: where the oxygen came from

  • The early atmosphere had almost no free oxygen; it was dominated by gases like carbon dioxide, nitrogen, and methane.
  • Certain bacteria (cyanobacteria) evolved oxygenic photosynthesis, using water and CO₂ to make organic matter and releasing O₂ as a waste product.
  • Over hundreds of millions of years, this O₂ started to build up once sinks (things that consumed oxygen) became saturated or less effective.

A simple overall reaction for oxygenic photosynthesis is:
CO2+H2O→“CH2O”+O2\text{CO}{2}+\text{H}{2}\text{O}\rightarrow \text{“CH}{2}\text{O”}+\text{O}{2}CO2​+H2​O→“CH2​O”+O2​
Here “CH₂O” represents organic matter. Free oxygen only accumulates if some of that organic matter is removed from the system (for example, buried in sediments) instead of being oxidized back to CO₂.

Step 1: The Great Oxidation Event

  • Around 2.4–2.1 billion years ago, Earth experienced the Great Oxidation Event (GOE), a major step change from almost no oxygen to low but detectable O₂ in the atmosphere.
  • Cyanobacteria in the oceans pumped out oxygen that initially reacted with dissolved iron and volcanic gases, forming huge deposits of banded iron formations.
  • Once these oxygen “sinks” were overwhelmed, oxygen began to leak into the atmosphere, displacing methane and triggering climate changes, including one of Earth’s earliest ice ages.

What actually made O₂ accumulate?

  • Continuous oxygenic photosynthesis by cyanobacteria provided a persistent source of O₂.
  • Burial of reduced materials (organic carbon, sulfides, and iron) in sediments prevented them from consuming oxygen again, effectively “leaving” O₂ in the air.
  • Changes in volcanic gas chemistry and tectonics likely reduced the flux of oxygen‑consuming gases over time, helping O₂ levels rise.

You can think of it like slowly filling a leaky bathtub: once the leaks (sinks) shrink enough and the tap (photosynthesis) keeps running, the water level (oxygen) finally rises.

Step 2: Later oxygen booms and plants on land

  • After the GOE, oxygen didn’t immediately jump to modern levels; it stayed relatively low for over a billion years, with “whiffs” and smaller rises.
  • Between about 700–550 million years ago, oxygen in the oceans and atmosphere increased again (“oxygen boom”), reaching roughly one‑fifth of today’s level by around 600 million years ago.
  • This rise is linked to more efficient burial of organic carbon and may have paved the way for the Cambrian explosion of complex animal life.

Role of land plants and trees

  • A major later jump in oxygen appears more than 300 million years ago and is associated with the rise and spread of large vascular land plants and trees.
  • These plants produced tough, decay‑resistant tissues (like lignin), which, when buried in swamps and sediments, locked away carbon for long periods.
  • The result was a long interval of very high O₂, probably above today’s 21%, which may have supported giant insects and other large arthropods.

In effect, forests turned into massive “oxygen factories,” especially when some of the dead wood became coal instead of decomposing completely.

Why oxygen didn’t just stay high

  • Oxygen levels have not risen smoothly; they’ve fluctuated, with peaks (for example, in the late Paleozoic) and declines (for example, into the Jurassic).
  • Shifts in:
    • Plate tectonics and mountain building (which affect weathering and nutrient supply)
    • Volcanism (which supplies oxygen‑consuming gases)
    • Ecosystems and organic carbon burial
      all altered the balance between O₂ sources and sinks.
  • Over long timescales, Earth’s system tends to stabilize atmospheric oxygen near levels that are high enough for complex life but low enough to avoid runaway fires and other instabilities.

Today’s atmosphere

  • Modern atmospheric oxygen is about 21% by volume.
  • It is maintained by the same basic mechanisms as in the past: oxygenic photosynthesis (by plants, algae, and cyanobacteria) balanced against respiration, decay, and oxidation of rocks and sediments.

So, the amount of oxygen in the atmosphere increased because photosynthetic organisms kept producing O₂ while geological and biological processes progressively reduced how fast that O₂ was removed, allowing it to accumulate over billions of years.

Information gathered from public forums or data available on the internet and portrayed here.