Exactly when the first life on Earth - the ancestors of modern bacteria - began is a subject of debate, but evidence suggests it could be as much as 3.5 billion years ago.
Early bacterial life introduced oxygen to the atmosphere. As the first free oxygen was released through photosynthesis by cyanobacteria, it was initially soaked up by iron dissolved in the oceans and formed red coloured iron oxide, which settled to the ocean floor. Over time, distinctive sedimentary rocks called banded iron formations were created by these iron oxide deposits. Once the iron in the oceans was used up, the iron oxide stopped being deposited and oxygen was able to start building up in the atmosphere about 2.4 billion years ago.
Image: Stromatolites in Shark Bay, Western Australia. Stromatolites, which are formed by microscopic bacteria, are rare on Earth today but were much more common in the ancient Earth's seas. (credit: L Newman & A Flowers/SPL)
Volcanoes may have played an important role in the emergence of life.
Dr Iain Stewart explains how volcanic activity on the early Earth may have played an important role in the emergence of life about four billion years ago. He visits hot springs in Rotorua, New Zealand, to meet Dr Bruce Mountain, who explains the theory.
Early life affected the structure of the planet.
Presenter Aubrey Manning and geologist Professor Maarten De Wit investigate iron rich rocks known as banded iron formations, which formed when stromatolites, some of the earliest known life forms on Earth, produced oxygen that combined with iron dissolved in the oceans.
Aubrey Manning visits the stromatolites of Shark Bay, Australia.
Aubrey Manning talks to Professor Maarten De Wit about the stromatolites in Shark Bay, Australia. Stromatolites are mounds in shallow seas formed by millions of bacteria and are among the most ancient forms of life on Earth.
The famous Miller-Urey experiment investigates the first steps towards life.
In the 1950s, Stanley Miller and Harold Urey, scientists at the University of Chicago, exposed a sealed flask full of the inorganic chemicals that may have been present in the atmosphere of the early Earth to an electrical current. Amino acids, the organic chemical precursors to proteins, were generated. They showed that it was theoretically possible for lightning strikes in an atmosphere full of their inorganic ingredients to generate some of the organic precursors to life. Many scientists now think that the early Earth may have had an atmosphere with a different composition to the one that Miller and Urey experimented with.
Stromatolites pump oxygen into the early atmosphere.
Dr Iain Stewart explains how stromatolites, one of the earliest forms of life, first released oxygen over three billion years ago when they turned sunlight into energy. Oxygen was initially soaked up by iron in the seas but eventually entered the atmosphere.
The Great Oxygenation Event (GOE), also called the Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust, Oxygen Revolution, or Great Oxidation, was the biologically induced appearance of dioxygen (O2) in Earth's atmosphere. Geological, isotopic, and chemical evidence suggest that this major environmental change happened around 2.3 billion years ago (2.3 Ga).
Cyanobacteria, which appeared about 200 million years before the GOE, began producing oxygen by photosynthesis. Before the GOE, any free oxygen they produced was chemically captured by dissolved iron or organic matter. The GOE was the point when these oxygen sinks became saturated and could not capture all of the oxygen that was produced by cyanobacterial photosynthesis. After the GOE, the excess free oxygen started to accumulate in the atmosphere.
Free oxygen is toxic to obligate anaerobic organisms, and the rising concentrations may have wiped out most of the Earth's anaerobic inhabitants at the time. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth's history. But research has shown that microbial mats of oxygen-producing microbes will produce a thin layer, one or two millimeters thick, of oxygenated water in an otherwise anoxic environment even under thick ice, and before oxygen started accumulating in the atmosphere, organisms living on these mats would already be adapted to being exposed to oxygen. Additionally, the free oxygen reacted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the Huronian glaciation, possibly the longest snowball Earth episode in the Earth's history.
Eventually, aerobic organisms began to evolve, consuming oxygen and bringing about an equilibrium in its availability. Free oxygen has been an important constituent of the atmosphere ever since.