Lesson 13

13

Multicellular Life Begins

How did life go from a single cell floating alone to millions of cells working together as one body?

Multicellular organisms have many cells that work together and perform different jobs. Early soft-bodied animals introduce cell specialization, simple body plans, and the challenges of finding evidence of ancient life.

Click here to hear the lesson video

Key Ideas

• After single-celled life came multicellular animals, made of many cells working together.
• Multicellular organization allowed for specialization: different cells could do different jobs.
• Early animals like jellyfish ancestors had simple body plans and lived in the oceans.
• Some early animals developed radial symmetry: body parts arranged around a center, like a wheel.
• These first animals were soft-bodied with no bones or shells, making them hard to fossilize.

Spines

• DK's Science as You've Never Seen it Before: Cell Variety pg. 143
• Mammoth Science: Types of Cells pg. 40
• Visual Timelines: Life on Earth pg. 16-19
• Alternative: Oak National Academy: Multicellular and unicellular organisms
• Alternative: Britannica Student: Mitosis
• Alternative: BBC Bitesize: Cell Specialization

Vocabulary

• Multicellular: Made of many cells working together; multicellular organisms can develop specialized cells for different jobs.
• Ediacaran: A period (~635–541 million years ago) when the first multicellular animals appeared.
• Cnidarian: A group of early animals (including jellyfish and corals) with radial symmetry; among the oldest animal groups.
• Radial Symmetry: A body plan where parts are arranged equally around a central point, like a wheel, useful for floating animals.

Discussion Questions

1. Why are soft-bodied animals hard to find in the fossil record?
   Sample Answer: They had no bones or shells, so they rarely left fossils behind.
2. Why might being multicellular have been helpful in the ancient oceans?
   Sample Answer: Many cells could work together, and different cells could specialize in different jobs.
3. How were the early animals different from animals today?
   Sample Answer: Early animals had simple body plans and soft bodies with no bones or shells, and they lived in the oceans. Answers will vary.
4. How might jellyfish be similar to their ancient ancestors?
   Sample Answer: Jellyfish are cnidarians, one of the oldest animal groups, and they have radial symmetry, like the early animals.

Digging Deeper

1. What does "specialization" mean for cells in a body?
   Sample Answer: Different cells do different jobs, so each kind of cell develops the shape and abilities it needs for its job.