Lesson 10

10

The First Life

What was the very first living thing, and how did it survive in a world that looks nothing like ours today?

The earliest evidence of life comes from microscopic organisms that lived billions of years ago. Bacteria, archaea, and stromatolites introduce the early history of cells and the evidence they left behind.

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Key Ideas

• The first living things on Earth were tiny microbes called bacteria, appearing about 3.5 billion years ago.
• These were prokaryotes: simple cells with no nucleus, no eyes, no legs.
• There are two groups: bacteria and archaea, which have different chemistry and often live in extreme places.
• These ancient microbes lived in the oceans and were the foundation for all life that came after.
• We know they existed because of stromatolites: layered rock structures built by ancient cyanobacteria, some of the oldest fossils on Earth.

Spines

• DK's Science as You've Never Seen it Before:
• Alternatives
  • Britannica Student Bacteria (https://kids.britannica.com/students/article/bacteria/273051)

Vocabulary

• Bacteria: Single-celled microorganisms without a nucleus; among the oldest life forms on Earth.
• 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.
• 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.
• Microbe: A microscopic living thing too small to see with the naked eye.

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.
4. 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.

Digging Deeper

1. Research one archaeon and the extreme environment it lives in. What does its existence tell us about the possibility of life beyond Earth?
   Sample Answer: Answers will vary.