Hands On: Oxygen Experiment
Big Idea:
During the Carboniferous period, oxygen levels in Earth's atmosphere reached about 35%, far higher than today's 21%. This had dramatic effects on life, particularly insects, which grew to sizes we would find unrecognizable today. This activity explores how available oxygen affects combustion, and by extension, how it affected life.

Materials:

• A candle
• Matches (adult supervision required)
• A small glass jar
• A large glass jar or bowl
• A stopwatch or timer
• The lab sheet

What to Do:

Step 1: Set the Scene
"During the Carboniferous period, massive swamp forests were pumping enormous amounts of oxygen into the atmosphere. Oxygen levels may have reached 35%, compared to 21% today. This had extraordinary effects on life, particularly on insects, which grew to sizes that would stop you in your tracks."

Step 2: Make Predictions
Before testing, ask your learner: if you cover a candle with a small jar, what will happen to the flame? With a large jar? Record predictions on the lab sheet.

Step 3: Test the Small Jar
Light the candle. Cover it with the small jar. Time how long the flame lasts before going out.

Step 4: Test the Large Jar
Relight the candle. Cover it with the large jar. Time how long the flame lasts. Is the difference what you predicted?

Step 5: Test Open Air
Relight the candle and let it burn freely for 30 seconds to compare.

Step 6: Connect to the Carboniferous
"A candle needs oxygen to burn. When the jar empties of oxygen, the flame goes out. Insects breathe through tubes called tracheae, oxygen diffuses directly into their bodies. In higher-oxygen air, more oxygen reaches deeper into those tubes, allowing insects to grow much larger than they can today." Meganeura, a dragonfly-like insect, had a wingspan of up to 70 cm during the Carboniferous. Arthropleura, a millipede-like creature, could be nearly 2.5 meters long.

Step 7: Discuss

• What eventually happened to Carboniferous oxygen levels? (They dropped at the end of the period as the forests were buried and the climate shifted.)
• Giant insects disappeared. What does that tell you about the relationship between atmosphere and life?

What's Really Happening (Caregiver Explanation):
Insects breathe through a passive system of tubes called tracheae. Oxygen diffuses directly through these tubes to cells throughout the body without a heart or lungs pumping it. This system works well at today's oxygen levels for small insects, but becomes less efficient at delivering oxygen to the deeper tissues of a very large body. In Carboniferous air with roughly 35% oxygen, the same tracheal system could supply enough oxygen to support a much larger body. When oxygen levels dropped at the end of the Carboniferous, giant insects disappeared. This is one of the clearest known examples of atmospheric composition directly controlling the maximum possible body size of a group of organisms.

Digging Deeper:
Research the oxygen levels in Earth's atmosphere during the Carboniferous period, they reached as high as 35%, compared to 21% today. Look up Meganeura, the giant dragonfly-like insect with a wingspan up to 70 cm. Insects breathe through tubes called tracheae rather than lungs. Research how insect breathing works and explain why higher oxygen levels would allow insects to grow much larger than they can today.