Earth's oxygen through time - a case study on photo/resp

Earth's oxygen through time

A photosynthesis and respiration case study

Earth's oxygen

Earth's atmosphere has undergone remarkable transformations since the planet's formation over 4.5 billion years ago. Among the most significant changes is the rise of oxygen, a gas essential for most forms of life today. However, this oxygen-rich environment did not always exist.

This assignment explores the fascinating story of how oxygen levels in Earth's atmosphere have changed over time, from the Great Oxidation Event to modern-day levels, and examines the profound impacts these changes have had on Earth's climate, ecosystems, and the evolution of life.

Earth's oxygen

Read through this historical example carefully. Pay close attention to the details, especially any data or graphs. These will be critical for understanding the scenario & making predictions.

You will work through this with your partner. Decide right now who will be designated as Partner A and who will be Partner B.

Open Ended

Partner A name:

Open Ended

Partner B name:

Early Earth

When Earth first formed approximately 4.5 billion years ago, its atmosphere was vastly different from what we experience today. It was dominated by gases such as hydrogen, methane, and water vapor, with little to no free oxygen present. This anoxic (no O₂) environment was inhospitable to most modern life forms and resembled the conditions found on some other planets and moons in our solar system today. Oxygen only began to accumulate in significant amounts billions of years later.

Drag and Drop

Earth formed about ​

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years ago and its atmosphere was mostly gases like ​

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and methane. There was almost no ​

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present.

Drag these tiles and drop them in the correct blank above

4.5 billion

hydrogen

oxygen

4.5 million

nitrogen

Audio Response

Partner A: What would happen to you if you time-traveled back to early Earth? Why?

Open Audio Recorder

Audio Response

Partner B: How would the absence of oxygen have affected early life forms? What adaptations (traits) would they have needed to survive?

Open Audio Recorder

Great Oxidation Event (GOE)

Earth’s oxygen levels started to rise about 2.4 billion years ago during an event called the Great Oxidation Event (GOE). This happened because tiny organisms called cyanobacteria began to grow and spread. Cyanobacteria could do photosynthesis, which means they used sunlight to make food and released oxygen as waste. At first, most of the oxygen reacted with other substances like iron in the oceans, creating rocks called banded iron formations.

Eventually though, oxygen started building up in the atmosphere, changing the planet and making it possible for new types of life to survive.

Hotspot

What "First" from the graph caused the Great Oxidation Event?

Click on the image to select.

Multiple Choice

What chemical reaction were the cyanobacteria performing that caused the Great Oxidation Event?

• CO₂ + H₂O --> C₆H₁₂O₆ + O₂

• CO₂ + O₂ --> C₆H₁₂O₆ + H₂O

• C₆H₁₂O₆ + O₂ --> CO₂ + H₂O

• C₆H₁₂O₆ + CO₂ --> O₂ + H₂O

Dropdown

Photosynthesis caused the amount of oxygen in the atmosphere to ​

Select answer

. This oxygen originally came from molecules of ​

Select answer

.

Aerobic life appears

As oxygen levels slowly increased in Earth’s atmosphere and oceans, the first aerobic organisms began to appear. These organisms used oxygen to break down food more efficiently, giving them an advantage over those that relied on less efficient anaerobic processes. This shift paved the way for more complex life forms.

Around 1.8 billion years ago, eukaryotes—the first cells with a nucleus and other specialized structures—emerged. Eukaryotes had developed mitochondria, which use oxygen to produce energy, marking a major step toward the development of multicellular organisms.

Multiple Choice

Why did aerobic life appear during this time period?

• There was now enough oxygen to support aerobic respiration, which is more efficient than anaerobic.

• Anaerobic organisms had overpopulated and aerobic life was able to get energy by eating them.

• Aerobic organisms could survive without oxygen.

Multicellular life appears

As oxygen levels continued to rise, they created conditions suitable for the evolution of multicellular life. Around 700 million years ago, the first multicellular organisms began to appear.

Higher oxygen levels allowed cells to grow larger and specialize, enabling them to work together in complex ways. This specialization led to the development of tissues and organs, which gave multicellular organisms significant advantages over single-celled life.

Reorder

Put these events in the correct chronological order.

• Cyanobacteria begin performing photosynthesis

• The Great Oxidation Event occurs

• Aerobic life appears on Earth

• Multicellular life appears on Earth

1

2

3

4

• 1
• 2
• 3
• 4

Animals and plants evolve

Animals first evolved in the oceans around 600-700 million years ago, starting with simple organisms like sponges. Plants, derived from green algae, appeared on land about 470 million years ago, transforming the environment by stabilizing soils and producing oxygen. This paved the way for animals to move onto land, with the first terrestrial animals, like arthropods, emerging around 400 million years ago. Together, the evolution of land plants and animals created complex terrestrial ecosystems that reshaped Earth's surface.

Audio Response

Partner B: Before the first true, multicellular animals appeared in the oceans around 650 million years ago, what form of life would have been essential to produce the food and oxygen needed for those animals to evolve and survive? Explain.

Open Audio Recorder

Audio Response

Partner A: Following up on Partner B's answer, why do you think that land animals only appeared after plants had colonized the land?

Open Audio Recorder

Drag and Drop

Plants tend to ​

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the amount of ​

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in the atmosphere while animals will ​

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it.

Drag these tiles and drop them in the correct blank above

increase

decrease

oxygen

glucose

Recent history

Over the past 400 million years, life on Earth has continued to evolve. The rise of plants, followed by the evolution of animals, led to the development of diverse ecosystems on land and in the oceans. During the Mesozoic Era, dinosaurs dominated the land, while flowering plants began to spread. The Cenozoic Era saw the rise of mammals, including humans, and the further expansion of plant life.

Today, Earth’s atmosphere contains about 21% oxygen, supporting an incredible diversity of life forms. This balance of oxygen, along with other elements, continues to shape the planet's climate, ecosystems, and the future of life on Earth.

Multiple Choice

Why do you think flying insects reached their largest size around 275 million years ago?

• Higher oxygen levels allowed insects to grow larger since they take O₂ in without lungs.

• Lower oxygen levels allowed insects to grow larger since they take O₂ in without lungs.

• The higher levels of carbon dioxide in the atmosphere helped insects grow larger bodies.

• The Earth’s temperature was cooler, which slowed down insect metabolism and allowed them to grow to larger sizes.

The largest insect to have ever lived (that we know of) was Meganeuropsis permiana, a dragonfly-like insect with a wingspan of 28 inches!




This image shows a scale model of Meganeuropsis permiana.

Meganeuropsis permiana

Open Ended

Explain how photosynthesis changed Earth from a planet with almost no oxygen into one capable of supporting large multicellular organisms.

Poll

How interesting was this case study to you?

1 = not interesting, 5 = extremely interesting

• 1

• 2

• 3

• 4

• 5