The Great Oxidation Event: Coevolution of Earth and Life

Part 1: Engage (Anchoring Phenomenon)

The Changing Sky and Rusty Oceans: If you could travel back in time 2.5 billion years to the Archaean Eon, you would not be able to breathe. The Earth’s atmosphere was a thick methane and CO₂ haze, and the sky was not blue. The oceans were entirely devoid of free oxygen and were filled with dissolved iron (Fe²⁺). Yet, today, our atmosphere contains ~21% oxygen (O₂), and the sky is a clear blue. This massive shift, known as the Great Oxidation Event, paved the way for all complex aerobic life—including us.

• Brainstorming Question: How could early single-celled life forms radically alter the chemistry of the entire planet’s oceans and atmosphere? What evidence would such a planetary-scale change leave behind in the rock record?

Part 2: Explore (Simulation Investigation)

Materials: The Great Oxidation Event interactive simulation. Estimated Time: 20-30 minutes.

Procedure:

1. Open the simulation and familiarize yourself with the initial Archaean Eon Environment (Time: 2.50 Billion Yrs Ago). Note the starting values for “Ocean Dissolved Iron (Fe²⁺)” and “Atmospheric Oxygen (O₂)”.
2. Before starting time, make a prediction about what will happen to the dissolved iron when photosynthetic life is introduced.
3. Use the Cyanobacteria Population slider to introduce early photosynthetic life to the anoxic ocean (set to ~20).
4. Click Start Time. Observe the changes in the ocean, the atmosphere, and the graph.
5. Pause the simulation periodically to collect data for your table. Pay close attention to what forms on the ocean floor before oxygen begins to build up in the atmosphere.
6. Continue the simulation until the Ocean Dissolved Iron is exhausted and Atmospheric Oxygen begins outgassing to at least 15%. Note the visual changes in the sky (the “oxic-sky”) and the emergence of new life forms (eukaryotes).

Data Collection Table

Part 3: Explain (Sensemaking)

1. The Sink: When cyanobacteria first started producing oxygen, did it immediately build up in the atmosphere? Use your data to explain what happened to the first oxygen produced and why Banded Iron Formations (BIFs) appeared on the ocean floor.
2. The Outgassing: At what point did atmospheric oxygen finally begin to rise? Explain the relationship between the depletion of ocean dissolved iron and the outgassing of O₂ into the atmosphere.
3. Feedback and Extinction: How did the increase in atmospheric oxygen (a waste product of cyanobacteria) act as a toxic stressor for early anaerobic microbes, while simultaneously paving the way for the evolution of new, aerobic (oxygen-using) life forms like eukaryotes?

Part 4: Elaborate/Evaluate (Argumentation & Modeling)

Constructing a Scientific Argument Prompt: Using your data and observations from the simulation, write a scientific argument (Claim, Evidence, Reasoning) addressing the following question:

How does the Great Oxidation Event demonstrate the simultaneous coevolution of Earth’s systems (geosphere, hydrosphere, atmosphere) and life on Earth (biosphere)?

• Claim: State clearly that life and Earth’s physical systems coevolved, with biological processes driving major geological and atmospheric changes.
• Evidence: Provide at least two specific pieces of evidence from your simulation data. You must reference:
  • The initial state of the atmosphere and oceans.
  • The emergence of photosynthetic cyanobacteria.
  • The chemical reaction between produced O₂ and dissolved Fe²⁺ forming Banded Iron Formations.
  • The subsequent rise in atmospheric O₂ once the iron sink was exhausted.
• Reasoning: Explain why the evidence supports your claim. Detail the causal links: How did biology alter chemistry? How did the altered chemistry (new geology and atmosphere) create new environmental pressures that influenced the future evolution of life?

Extension Option: Research the “Boring Billion,” a period of Earth’s history that followed the Great Oxidation Event. How did the initial rise in oxygen lead to an extended period of relative environmental and evolutionary stability?

Teacher Notes & NGSS Alignment

Performance Expectation: HS-ESS2-7. Construct an argument based on evidence about the simultaneous coevolution of Earth’s systems and life on Earth.

Task Alignment:

• Science and Engineering Practices (SEPs): Engaging in Argument from Evidence. Students construct a written argument based on data from the simulation, defending the claim of coevolution using evidence of atmospheric and geological changes.
• Disciplinary Core Ideas (DCIs): ESS2.D: Weather and Climate (Gradual atmospheric changes were due to plants and other organisms that captured carbon dioxide and released oxygen) and ESS2.E: Biogeology (The many dynamic and delicate feedbacks between the biosphere and other Earth systems cause a continual coevolution of Earth’s surface and the life that exists on it).
• Crosscutting Concepts (CCCs): Stability and Change. Students construct explanations of how things change (methane haze to oxic sky) and how delicate balances are upset by biological processes.

Evidence Statements Addressed:

1. Developing the claim: Students develop a claim that there is simultaneous coevolution of Earth’s systems and life on Earth. (Demonstrated in the Elaborate/Evaluate section).
2. Identifying scientific evidence: Students identify evidence including: the composition of Earth’s early atmosphere, evidence for the emergence of photosynthetic organisms, and the effect of free oxygen on processes in other Earth systems. (Demonstrated in the Data Collection and Explain sections).
3. Reasoning and synthesis: Students use examples to construct a logical argument, including that the evolution of photosynthetic organisms led to worldwide deposition of iron oxide formations (BIFs) and an oxidizing atmosphere. (Demonstrated in the final CER Argument).