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Is it true the ancestors of plants created oxygen?

After it was first formed, the Earth’s atmosphere didn’t have any oxygen like it does today. So when and how did oxygen come about? The clue to answering this question is related to the origin of living things.

Types of bacteria have been found in old fossils about 3.8 billion years old. The bacteria seem to have used photosynthesis just like modern plants. Photosynthesis is a chemical process that takes carbon dioxide and water and uses the sun’s light to make energy. In this process, photosynthesis produces oxygen. Over a long period of time, these organisms produced lots of oxygen. So gradually the amount of oxygen in the atmosphere increased.

Oxygen was a poisonous gas for living things back then. But as the amount of oxygen in the atmosphere increased, some living things adapted and started to make use of oxygen. These were the ancestors of today’s animals.