On ancient Earth, nitrogen, water vapor, carbon dioxide, and methane were probably the most abundant gasses in the atmosphere.

People made hypothesis that the early life forms did not need oxygen to live

The first organisms resembled the first archaea that live today in extreme environments, such as polar ice caps, hot springs, and the mud of ocean bottoms.

Although Redi and Pastuer showed that living things do not spontaneously arise on today's Earth, scientists reason that the first life forms probably did arise on nonliving materials.

Scientists hypothesize that small chemical units of life formed gradually over millions of years in Earth's waters. Some of these chemical units joined to form the large chemical building blocks found in cells. Eventually, some of these large chemicals joined together and became the forerunners of the first cells.

A fossil is a trace of an ancient organism that has been preserved in a rock or another substance

At some point much later, some of the cells may have developed the ability to make their own food

As the autotrophs thrived, oxygen accumulated in Earth's atmosphere.

Over hundreds of millions of years, the amount of oxygen increased to its current level

Such experiments can only test hypothesis about how life forms could have arisen.