Before there were dinosaurs, plants, or animals, Earth was home to the tiniest, most incredible living things: bacteria and archaea. In this lesson, your learner will discover that these microscopic pioneers were the first to call Earth home over 3.5 billion years ago. They'll learn that these ancient microbes didn't just survive but that they transformed the entire planet in ways that made all other life possible.

Key Ideas

• The first living things on Earth were tiny microbes called bacteria, appearing about 3.5 billion years ago.
• The first life had no eyes or legs—just simple, tiny cells!
• The first life forms were prokaryotes: single-celled organisms without a nucleus. Bacteria and archaea are the two groups of prokaryotic life.
• These ancient microbes lived in the oceans and were the foundation for all life that came after.

Spines

• DK's Science as You've Never Seen it Before:

• 7 Kingdoms of Life pg. 135
• Miniature Life pg. 140-141
• Visual Timelines: Life on Earth pg. 12-13
• Alternatives
  • Britannica Student Bacteria
    
    

✏️  Notebooking Activity Draw a simple bacteria cell and label its basic parts. Then write 2-3 sentences explaining why bacteria were so important, even though they were invisible to the naked eye.

Timeline EntriesLabel the next page in your timeline "The First Life (Bacteria and Archaea): 3.5 Billion Years Ago" and draw tiny single-celled organisms or narrate what the first life was like with words or sentences.
Discussion Questions

1. How long ago did the first life appear on Earth? 
   Sample answer: About 3.5 billion years ago.
2. Why can't we see bacteria without a microscope? 
   Sample answer: They are incredibly tiny—millions of bacteria could fit in a single drop of water.
3. What is the difference between bacteria and archaea? 
   Sample answer: Both are prokaryotes without a nucleus, but archaea often live in extreme environments like hot springs or very salty lakes, and they have different chemical structures in their cell walls.

Digging Deeper

• Why are prokaryotes still essential to life on Earth today? 
  Sample answer: Bacteria and archaea break down dead matter, fix nitrogen in soil, and help animals digest food, they are foundational to almost every ecosystem on Earth.

Vocabulary

• Bacteria — Single-celled microorganisms without a nucleus; among the oldest life forms on Earth, appearing about 3.5 billion years ago.
• Archaea — Ancient single-celled microorganisms similar to bacteria but with different chemistry; often found in extreme environments.
• Prokaryote — A cell that lacks a membrane-bound nucleus; bacteria and archaea are prokaryotes — the first life on Earth.
• Stromatolite — A layered rock structure built up by colonies of cyanobacteria; some of the oldest fossils on Earth.
• Cyanobacteria — Microscopic photosynthetic bacteria that were the first organisms to release oxygen into Earth's atmosphere.
• Fossil — The preserved remains, imprints, or chemical traces of ancient organisms found in rock.
• Microbe — A microscopic living thing, too small to see with the naked eye; includes bacteria, archaea, and other single-celled organisms.