Oxygen in the Earth’s atmosphere is fundamental for complex types of life, which utilize it amid vigorous breath to make vitality.
The levels of oxygen drastically ascended in the climate around 2.4 billion years ago, yet why it happened then has been discussed. A few researchers believe that 2.4 billion years back is when creatures called cyanobacteria originally evolved, which could perform oxygen-delivering (oxygenic) photosynthesis.
Other scientists think that cyanobacteria evolved long before 2.4 billion years ago but something prevented oxygen from accumulating in the air.
In a new study by the Imperial College London, oxygenic photosynthesis arose at least one billion years before cyanobacteria evolved. Their results show that oxygenic photosynthesis could have evolved very early in Earth’s 4.5-billion-year history.
According to scientists, the discovery could change ideas of how and when complex life evolved on Earth, and how likely it is that it could evolve on other planets.
Lead author Dr Tanai Cardona, from the Department of Life Sciences at Imperial, said: “We know cyanobacteria are very ancient, but we don’t know exactly how ancient. If cyanobacteria are, for example, 2.5 billion years old that would mean oxygenic photosynthesis could have started as early as 3.5 billion years ago. This suggests that it might not take billions of years for a process like oxygenic photosynthesis to start after the origin of life.”
During the study, scientists 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 Photosystem II.
Photosystem II is comprised of two proteins called D1 and D2. Initially, the two proteins were the equivalent, yet despite the fact that they have fundamentally the same as structures, their basic genetic sequences are now different.
This demonstrates D1 and D2 have been advancing independently – in cyanobacteria and plants they just offer 30 percent of their genetic sequence. Indeed, even in their original form, D1 and D2 would have possessed the capacity to perform oxygenic photosynthesis, so realizing to what extent prior they were indistinguishable could uncover when this capacity originally developed.
To find out the difference in time between D1 and D2 is 100 percent identical, and them being only 30 percent the same in cyanobacteria and plants, the team determined how fast the proteins were changing – their rate of evolution. Using powerful statistical methods and known events in the evolution of photosynthesis, they determined that the D1 and D2 proteins in Photosystem II evolved extremely slowly – even slower than some of the oldest proteins in biology that are believed to be found in the earliest forms of life.
From this, they calculated that the time between the identical D1 and D2 proteins and the 30 percent similar versions in cyanobacteria and plants is at least a billion years, and could be more than that.
Dr Cardona said: “Usually, the appearance of oxygenic photosynthesis and cyanobacteria are considered to be the same thing. So, to find out when oxygen was being produced for the first time researchers have tried to find when cyanobacteria first evolved.”
“Our study instead shows that oxygenic photosynthesis likely got started long before the most recent ancestor of cyanobacteria arose. This is in agreement with current geological data that suggests that whiffs of oxygen or localized accumulations of oxygen were possible before three billion years ago.”
Scientists are now trying to recreate what the photosystem looked like before D1 and D2 evolved in the first place. Using the known variation in photosystem genetic codes across all species alive today, they are trying to piece together the ancestral photosystem genetic code.
They have published their story in the journal Geobiology.