Oxygen in the Earth's atmosphere is necessary for complex forms of life, which use it during aerobic respiration to make energy.
The levels of oxygen dramatically rose in the atmosphere around 2.4 billion years ago, but why it happened then has been debated. Some scientists think that 2.4 billion years ago is when organisms called cyanobacteria first evolved, which could perform oxygen-producing (oxygenic) photosynthesis.
Other scientist think that cyanobacteria evolved long before 2.4 billion years ago but something prevented oxygen from accumulating in the air.
Cyanobacteria perform a relatively sophisticated form of oxygenic photosynthesis -- the same type of photosynthesis that all plants do today. It has therefore been suggested that simpler forms of oxygenic photosynthesis could have existed earlier, before cyanobacteria, leading to low levels of oxygen being available to life.
Now, a research team led by Imperial College London have found that oxygenic photosynthesis arose at least one billion years before cyanobacteria evolved. Their results, published in the journal Geo-biology, show that oxygenic photosynthesis could have evolved very early in Earth's 4.5-billion-year history.
If oxygenic photosynthesis evolved early, it could mean it is a relatively simple process to evolve. The probability of complex life emerging in a distant exoplanet may then be quite high.
It is difficult for scientists to figure out when the first oxygen-producers evolved using the rock record on Earth. The older the rocks, the rarer they are, and the harder it is to prove conclusively that any fossil microbes found in these ancient rocks used or produced any amount of oxygen.
Instead, the team investigated the evolution of two of the main proteins involved in oxygenic photosynthesis. In the first stage of photosynthesis, cyanobacteria use light energy to split water into protons, electrons and oxygen with the help of a protein complex called Photo-system II. Photo-system II is made up of two proteins called D1 and D2. Originally, the two proteins were the same, but although they have very similar structures, their underlying genetic sequences are now different.
Now, the team are trying to recreate what the photo-system looked like before D1 and D2 evolved in the first place. Using the known variation in photo-system genetic codes across all species alive today, they are trying to piece together the ancestral photo-system genetic code.