Lesson 11

11

Oxygen Changes Everything

How did something as tiny as a single-celled organism change the entire atmosphere of a planet?

Cyanobacteria released oxygen through photosynthesis, gradually changing Earth's oceans and atmosphere. The lesson covers the Great Oxidation Event and its consequences for organisms that could use oxygen and those that could not tolerate it.

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Key Ideas

• Tiny organisms called cyanobacteria used sunlight to make food through photosynthesis.
• As a byproduct, they released oxygen into the water and air.
• The Great Oxidation Event (~2.4 billion years ago) dramatically increased oxygen in Earth's atmosphere.
• Many organisms that couldn't tolerate oxygen went extinct, but new oxygen-breathing life evolved.
• This oxygen also formed the ozone layer, eventually making land life possible.

Spines

• DK's Science as You've Never Seen it Before: Photosynthesis pg. 148-149
• Mammoth Science: Plants pg. 42
• Visual Timelines: Life on Earth pg. 14-15
• Alternative
  • Kiddle: Cyanobacteria facts for kids (https://kids.kiddle.co/Cyanobacteria)
  • Britannica Student Photosynthesis (https://kids.britannica.com/students/article/photosynthesis/276411)
  • Kiddle: Great Oxygenation Event (https://kids.kiddle.co/Great_Oxygenation_Event)

Vocabulary

• Photosynthesis: The process by which plants and cyanobacteria use sunlight, water, and CO2 to make food, releasing oxygen.
• Great Oxidation Event: A period ~2.4 billion years ago when oxygen from cyanobacteria dramatically built up in Earth's atmosphere.
• Aerobic: Describes organisms that require oxygen; aerobic life only evolved after the Great Oxidation Event.
• Anaerobic: Describes organisms that do not need oxygen; the first life on Earth was anaerobic.
• Ozone Layer: A layer of ozone in the upper atmosphere that absorbs harmful ultraviolet radiation, making land life possible.
• Mass Extinction: An event in which a large proportion of species dies out in a geologically short period of time.

Discussion Questions

1. Why did cyanobacteria become so important to Earth?
   Sample Answer: They were the first organisms to perform oxygenic photosynthesis, slowly pumping oxygen into Earth's atmosphere over billions of years. Without them, there would be no breathable air and complex life could never have evolved.
2. What is photosynthesis, and why does it matter?
   Sample Answer: Photosynthesis is the process by which organisms use sunlight, water, and carbon dioxide to make food, releasing oxygen as a byproduct. It matters because it produced the oxygen that fills our atmosphere and makes complex life possible.
3. How did rising oxygen change life on Earth?
   Sample Answer: Many organisms that could not tolerate oxygen died out, while new oxygen-breathing life forms evolved and spread across the planet. Oxygen also helped build the ozone layer, which shielded life from harmful UV radiation.
4. Why did the rise of oxygen cause some life to go extinct?
   Sample Answer: Early life forms had evolved in a world without oxygen and were harmed or killed by it. When cyanobacteria released large amounts of oxygen, many of these anaerobic organisms died because oxygen was toxic to them.

Digging Deeper

1. What would Earth be like if photosynthesis had never evolved?
   Sample Answer: Earth's atmosphere would still be mostly carbon dioxide and methane with no breathable oxygen. Complex life as we know it could never have developed, and Earth would likely still be home only to simple anaerobic microbes.