Where Did the Oxygen Go?
New clues help explain how oxygen built up in Earth's air
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| photo courtesy of NASA |
| Plant-like bacteria added oxygen to the air |
Take a deep breath. As you breathe in, your body takes in oxygen gas from the air. Like all animals, your body needs oxygen to get energy from the food you eat. Earth's air has plenty of oxygen for animals to use. But the air was very different long ago.
The oxygen in the air comes from plants. Before plants lived on Earth, the air had almost no oxygen. Between about 2.7 and 3.5 billion years ago, plant-like bacteria spread around Earth. Like plants, these bacteria made food by photosynthesis. During photosynthesis, plants make sugars using sunlight, water, and carbon dioxide gas in the air. Living things that use photosynthesis give off oxygen as waste. The plant-like bacteria living on Earth gave off oxygen gas into the air. But oxygen gas did not build up in the air until about 2.5 billion years ago. At least 200 million years passed after plant-like bacteria spread around Earth! Where was the oxygen during all this time?
This week, scientists reported that they found clues to explain the missing oxygen. They found that Earth had many underwater volcanoes before 2.5 billion years ago. Around 2.5 billion years ago, the continents were forming and growing larger. After 2.5 billion years ago, more volcanoes could be found on land than under the water. Underwater volcanoes give off different gases than volcanoes on land. The gases given off by underwater volcanoes quickly react with oxygen to form water and other substances. In other words, these gases take oxygen out of the air. Gases given off by volcanoes on land react with oxygen, too, but not as much as the gases from underwater volcanoes. So, more oxygen was able to build up in the air as volcanic activity moved from the oceans to the land.
Scientists still have work to do to find out if these ideas are correct. Scientists want to understand how living things and Earth processes act together to change the air. By learning how oxygen built up in Earth's air long ago, we can learn about how Earth became a home to life as we know it.