Today, photosynthesis is a vital process for all life on earth. Almost all primary producers in food chains around the world use photosynthesis to synthesize food. However, scientists are still not sure how this process evolved or the factors that lead to its evolution.
We do know that the earth formed nearly 4.6 billion years ago, but it did not have oxygen in its atmosphere for another 2.3 billion years. Around 3.4 billion years ago, the first-ever photosynthetic life form was believed to have evolved. It was a precursor to modern-day cyanobacteria – however, how they performed the photosynthesis process was unclear, and it may have been different from the process that we see today. Scientists speculate that the reducing agent would have been hydrogen or other similar elements rather than water. It may also have carried out the process through the oldest (and most common) metabolic pathway – the C3 photosynthesis.
The oxygen that we breathe did not form instantaneously. It was produced by photosynthetic organisms that existed billions of years ago.
Before photosynthesis, the majority of the organisms which existed then was anaerobic, thriving on hydrogen, methane, sulphur and other compounds. Precursors to cyanobacteria evolved photosynthesis and began exhaling oxygen. This gas proved to be lethal to the anaerobes, and they started to die out. This event came to be known as the Great Oxygenation Event, and it caused the extinction of almost all life on earth. Essentially, this is the process responsible for the current levels of oxygen in today’s atmosphere. This event changed the course of evolution of all life on earth, including the ones that rely entirely on photosynthesis.
The event radically changed the earth’s atmosphere from a reducing atmosphere to an oxidizing atmosphere. A reducing atmosphere meant that organisms such as cyanobacteria could thrive and could have been able to evolve other photosynthetic metabolic pathways. When the earth was replaced with an oxidizing atmosphere, more complex life forms were able to form, eventually leading to the formation of multicellular life, such as plants with different types of leaves, complex animals such as mammals and birds.
It would be an interesting thought experiment if organisms such as precursors to cyanobacteria did not evolve. The earth would have been home to fascinating creatures.
All of today’s lifeforms are carbon-based. This is because it is the most stable of the elements. However, the realm of alternate-biochemistries for life is not far off. Scientists theorize that distant planets, millions of light-years from earth may harbour life, unlike anything we have ever seen. For instance, carbon and silicon share many characteristics – each has a valence of four, and each element can bond to oxygen. Each of the element also forms long chains called polymers. These shared characteristics make silicon a feasible candidate for alternate biochemistry necessary for hypothetical lifeforms.
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