11
Oxygen Changes Everything
Imagine a world where there was no oxygen in the air to breathe. That was Earth for the first billion years of life. Then, tiny cyanobacteria started using sunlight to make food, and they released oxygen as waste. Over millions of years, oxygen built up in the atmosphere in an event called the Great Oxidation Event. This lesson explores one of the most dramatic transformations in Earth's history and how it made life as we know it possible.
- Tiny living things called cyanobacteria used sunlight to make food (photosynthesis).
- As they made food, they released oxygen into the water and air.
- The Great Oxidation Event (~2.4 billion years ago) dramatically increased oxygen in Earth's atmosphere.
- This event changed life on Earth forever—many organisms died, but new oxygen-breathing life evolved.
- Many organisms that couldn't tolerate oxygen went extinct; new oxygen-breathing life evolved in their place.
- 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
| ✏️ Notebooking Activity Label a diagram of photosynthesis with the terms sunlight, oxygen, carbon dioxide, water, and sugar (glucose). |
We move into October for the first time. Read the script below before the lesson.
Read aloud: We’ve made it to October on our Cosmic Calendar. It took a while to get here from September, and that’s the point. Life has been on Earth since September 21st, but for more than a month on our calendar, nothing looks very different from the outside. Life was there, but it was microscopic. And then, around October 29th, something happened that completely changed the planet. Cyanobacteria, a type of ancient microbe, had been pumping oxygen into the atmosphere for a long time. And around 2.4 billion years ago, the oxygen levels got high enough to transform Earth’s air. This is called the Great Oxidation Event. For most life at the time, this was a crisis. Oxygen was toxic to many early organisms. But in the long run, this oxygen-rich atmosphere made it possible for complex life to eventually evolve. Everything we’ll study from here on out owes something to what happened on October 29th.
- 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. - 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. - How did the rise of oxygen change Earth for other living things?
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. - How did cyanobacteria eventually change the entire planet?
Sample answer: They invented oxygenic photosynthesis, slowly releasing oxygen as a byproduct over billions of years until Earth's atmosphere became oxygen-rich—making complex life possible. - 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. - What types of organisms benefited from the new oxygen-rich environment?
Sample answer: Aerobic organisms—those that use oxygen to release energy—thrived and diversified. This eventually led to the evolution of all complex life, including animals, plants, and fungi, all of which need oxygen to survive.
- Why do scientists say cyanobacteria changed the world?
Sample answer: Cyanobacteria triggered the Great Oxidation Event by releasing oxygen for billions of years. This single change transformed Earth's atmosphere, allowed aerobic life to evolve, and set the stage for all complex life, including us. - 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.
- Photosynthesis — The process by which plants, algae, and cyanobacteria use sunlight, water, and CO2 to make food, releasing oxygen as a byproduct.
- Great Oxidation Event — A period about 2.4 billion years ago when oxygen produced by cyanobacteria built up dramatically in Earth's atmosphere.
- Oxygen — A gas released as a byproduct of photosynthesis; its build-up in Earth's atmosphere allowed aerobic life to evolve.
- Aerobic — Describes organisms or processes that require oxygen; aerobic life could only evolve after the Great Oxidation Event.
- Anaerobic — Describes organisms or processes 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 on Earth dies out in a geologically short period of time.
- Cyanobacteria (Blue-Green Algae)— The oxygen producers; responsible for the Great Oxidation Event 2.4 billion years ago.
- Grypania spiralis— One of the earliest known eukaryotic algae; a spiral-shaped organism found in 2.1-billion-year-old rock.
- Banded Iron Formations— Not a species, but a key piece of evidence. This was iron-rich rock layers that formed as oxygen first appeared and rusted the oceans.
- Acritarchs— Tiny, hard-walled microfossils; among the earliest evidence of eukaryotic life in the fossil record.