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The microbes could have performed photosynthesis producing oxygen at least a billion years ago in the history of the Earth than previously thought.
This discovery could change ideas about how and when complex life evolved on Earth, and about the likelihood that it will evolve on other planets.
Oxygen in the Earth's atmosphere is necessary for complex life forms, which use it during aerobic respiration to produce energy.
Oxygen levels have increased dramatically in the atmosphere about 2.4 billion years ago, but the reason it happened then was debated. Some scientists believe that the appearance of organisms called cyanobacteria occurred 2.4 billion years ago and that photosynthesis producing oxygen (oxygen) could be performed.
Other scientists believe that cyanobacteria have evolved well before 2.4 billion years ago, but something has prevented oxygen from accumulating in the air.
Cyanobacteria perform a relatively sophisticated form of oxygen photosynthesis – the same type of photosynthesis that all plants do today. It has therefore been suggested that simpler forms of oxygen photosynthesis may have existed earlier, before cyanobacteria, which would have resulted in low oxygen availability for life.
A research team led by Imperial College London discovered that oxygen photosynthesis had originated at least a billion years before cyanobacteria developed. Their results, published in the journal geobiology, show that oxygen photosynthesis may have evolved very early in the 4.5 billion Earth's history.
Dr. Tanai Cardona, lead author of the Imperial Department of Life Sciences, said, "We know that cyanobacteria are very old, but we do not know exactly how old they are. For example, if cyanobacteria are 2.5 billion years old, it means that photosynthesis by oxygen could have started as early as 3.5 billion years ago. This suggests that it may not be billions of years before a process such as oxygen photosynthesis starts after the origin of life. "
If oxygen photosynthesis has evolved early, it could mean that the process is evolving relatively easily. The probability that complex lives appear in a distant exoplanet can then be quite high.
Scientists have trouble knowing when the first oxygen producers evolved using the rock disk on Earth. The older the rocks, the rarer they are and the more difficult it is to conclusively prove that the fossil microbes found in these ancient rocks use or produce any amount of oxygen.
Instead, the team studied the evolution of two of the major proteins involved in oxygen photosynthesis.
In the first stage of photosynthesis, cyanobacteria use light energy to split water into protons, electrons and oxygen using a protein complex called Photosystem II.
Photosystem II consists of two proteins called D1 and D2. Originally, the two proteins were identical, but although they have very similar structures, their underlying genetic sequences are now different.
This shows that D1 and D2 have evolved separately – cyanobacteria and plants share only 30% of their genetic sequence. Even in their original form, D1 and D2 could have performed oxygenic photosynthesis. Knowing how long they had been the same could indicate when this ability has changed.
To determine whether the time difference between D1 and D2 was 100% identical and 30% identical in cyanobacteria and plants, the team determined how fast proteins evolved – their rate of evolution.
Using powerful statistical methods and events known in the evolution of photosynthesis, they determined that the Photosystem II proteins D1 and D2 evolved extremely slowly – even more slowly than some of the oldest proteins in biology that the It is thought to be found in the oldest forms of life. .
From there, they calculated that the time between identical D1 and D2 proteins and similar versions at 30% of cyanobacteria and plants is at least a billion years old and could be longer.
Dr. Cardona said: "Usually, the appearance of oxygen photosynthesis and cyanobacteria is considered the same thing. Thus, to find out when oxygen was produced for the first time, researchers tried to determine when the cyanobacterium had evolved.
"Our study shows rather that oxygenic photosynthesis probably began well before the ancestor of the most recent ancestor of cyanobacteria. This is consistent with current geological data that suggest that oxygen flushes or localized accumulations of oxygen were possible before three billion years ago.
"Therefore, the origin of oxygen photosynthesis and the ancestor of cyanobacteria do not represent the same thing. There could be a very large time interval between one and the other. It's a huge change of perspective. "
Now, the team is trying to recreate what the photo system looked like before D1 and D2 evolved. Using the known variation of the genetic codes of the photosystem in all living species today, they are trying to reconstruct the genetic code of the ancestral photosystem.
LONDON IMPERIAL COLLEGE
Header Image – Cyanobacteria – Credit: Matthewjparker
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