Why Did Photosynthesis Evolve?

Why Did Photosynthesis Evolve?

Introduction to Photosynthesis

Life on earth is incredibly complex and diverse, with millions of species living across every corner of the planet. All these organisms require a source of energy for their survival and growth. And one of the most important and prominent sources of energy available on earth is the sun. Granted, except for plants and certain other autotrophs, most organisms cannot use photosynthesis to make their own energy. However, photosynthesis is still a process that is paramount for their survival. 

Why is Photosynthesis Important?

In almost every ecosystem, plants and phototrophic organisms form the producers in a food chain. This means they use sunlight to create their own food and consumers who occupy higher trophic levels depend on the producers for their nutritional needs. We can imagine a hypothetical scenario where the sunlight is blocked off for years together – this will cause the food chain to collapse as producers are unable to create food and die. Consequently, consumers who depend on the producers will also die out due to lack of food. In fact, scientists hypothesize that one of the driving factors of mass extinctions is the collapse of the food chain. Moreover, life on earth would not have evolved if not for photosynthesis. Around 4.6 billion years ago, the earth formed. However, it was vastly different from what we now see. It did not have oxygen nor any kind of life on its surface. But around 3.7 billion years ago, scientists speculate that the first ever organism appeared on earth. These were microscopic organisms and they were most likely autotrophs. But they weren’t photosynthetic, instead, they used other chemicals in the earth’s early atmosphere for their metabolic processes. This meant life was rather simple and the scope to evolve into more complex organisms was limited.

Earliest Phototrophs and The Great Oxygenation Event

Suggestive evidence shows that the first photosynthetic organisms evolved between 3.2 to 3.5 billion years ago. These organisms were quite similar to modern cyanobacteria. From an evolutionary perspective, this was one of the biggest turning points in the history of life on earth. Paleontologists believed that these groups of organisms may have caused the first ever mass extinction on earth. The by-product of the photosynthesis process is oxygen, and during that time period, the earth’s atmosphere was completely devoid of oxygen. The organisms that were not photosynthetic, were poisoned and died out, while the photosynthetic life forms thrived. Over billions of years, these photosynthetic organisms pumped oxygen into the earth’s atmosphere, and this event came to be known as the Great Oxygenation Event. This event was the primary catalyst that enabled the evolution of more complex organisms on earth. The remnants of these organisms can also be found today – structures called Stromatolites can be seen around the world. These are essentially deposits formed by sediment trapping and binding activities of the microbial communities. 

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