History of the Universe

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Photosynthesis 3.5 bya

Bacteria began to trap sunlight and use that energy to make food, such as sugar. This process is called photosynthesis, meaning "constructing with light". This was a great leap forward. Sunlight-using bacteria appeared soon after the first cells did, about 3.5 billion years ago, and sunlight has been the main source of energy for all life ever since. These bacteria are called blue-green bacteria or cyanobacteria (and sometimes wrongly called blue-green algae).

Many blue-green bacteria can also "fix" atmospheric nitrogen. In Southeast Asia, nitrogen-fixing blue-greens often are grown in rice paddies, removing the need for nitrogen fertilizers.

Blue-greens are important in our story for several reasons:

    they were probably the first bacteria which could perform photosynthesis, so leading to the great oxygen poisoning of 3 billion years ago

    they formed a symbiosis with fungi to create lichen, which was probably the first living thing to live on land


Structures called stromatolites have been found from this date, and are believed to have been made by these bacteria. These still exist in Australia.

Stromatolites in Shark Bay, Australia, courtesy of Paul Harrison

How photosynthesis works

A cell uses sunlight to create sugar

As they captured sunlight energy to make sugar so carbon dioxide and water were taken in, and oxygen was given out. This oxygen had a huge effect upon the whole world.

Today we obtain almost all of our food from plants, or from animals which have eaten plants, and so it might be interesting to know a bit about how photosynthesis works. It is a fascinating story of molecular changes. We give more details in the reference section of this website.

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History of the Universe eBook
History of the Universe eBook
Only £5.99

Written by Wyken Seagrave
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