The first irrefutable examples of life on Earth arose around 2.7 billion years ago. Some scientists claim life developed as long ago as 3.5 billion years. This is difficult to study and even more difficult to prove or disprove because rocks on Earth are weathered and recycled into the Earth’s crust. Rocks from so long ago are very difficult to find and only a few have been discovered.
The early Earth’s atmosphere had a very low concentration of oxygen compared to today. 2.4 billion years ago, the oxygen concentration in the atmosphere was less than one part per billion and the iron concentration in the ocean was much higher than today. Starting 3 billion years ago and lasting for at least a billion years, soluble iron (Fe2+) in the ocean collected any freely available oxygen, and formed ferric iron (Fe3+) which is a solid, also known as rust. Eventually most of the iron in the oceans was depleted and the oxygen level in the atmosphere and in the water began to slowly increase. After about a billion years, the oxygen level had reached a few percent of the total atmospheric pressure.
Then about 500 million years ago, there was a relatively rapid increase in the atmospheric oxygen content. This began when an ancient relative of cyanobacteria evolved the ability to use sunlight and water for photosynthesis, creating oxygen as a by-product. Over the next few hundred million years, the oxygen content in the atmosphere rose to nearly its current value of 18%.
Until this development, all life on Earth must have existed either under at least several centimeters of water or underground. The ozone layer, which blocks most of the damaging ultraviolet light from the Sun, did not exist, so any organism on the surface of the Earth would have been killed by the ultraviolet light. As the oxygen content of the atmosphere increased, the ozone layer formed and began to shield the surface of the Earth from the harmful ultraviolet light. This allowed life forms to evolve to survive on the surface of the oceans and on land, and also allowed organisms who metabolize oxygen (like us!) to develop.