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14
The Cambrian Explosion
Why did so many different kinds of animals appear on Earth in such a short period of time?

Imagine looking at an ocean 540 million years ago and seeing it suddenly fill with an incredible variety of life. In what seems like a blink of geological time, hundreds of new animal body types appeared: creatures with shells, spines, claws, and armor. This is the Cambrian Explosion, one of the most dramatic events in Earth's history. Your learner will explore what caused this burst of life and meet some of the weirdest, most wonderful animals that ever lived.

S2S_Lesson14_THE_CAMBRIAN_EXPLOSION by Selene

Key Ideas

  • The Cambrian Explosion was a time when many new kinds of animals suddenly appeared in the oceans.
  • Exoskeletons and endoskeletons evolved, allowing for larger and more complex body plans.
  • Increased oxygen levels and competition for food drove rapid evolution and diversification.
  • Fossils from this time are incredibly detailed, giving us a clear picture of ancient ocean life.
  • This was the beginning of most major animal groups we know today.
  • The Burgess Shale is a famous fossil site that preserves soft-bodied Cambrian creatures in remarkable detail.

Spines
  • DK's Science as You've Never Seen it Before:
✏️  Notebooking Activity
There are two notebooking pages for this lesson. Option 1: Make a list of creatures that would fossilize well and those that wouldn’t, with an explanation of why or why not. Option 2: Write some key words or facts about what made each of the featured Cambrian species unique (Opabinia, Anomalocaris, Redlichia trilobite).

Cosmic CalendarWhere we are: December 14
Nine cosmic days have passed since multicellular life began. Read the script below before the lesson.

Read aloud:December 14th on our Cosmic Calendar. We jumped forward nine days from the first multicellular life, and in those nine cosmic days, something extraordinary was building. The Cambrian Explosion, about 550 million years ago in real time, is when animal life as we know it burst onto the scene. In a geologically short period, almost every major body plan we see in animals today appeared for the first time. Eyes. Legs. Shells. Mouths. Things that could hunt and things that could hide. The ocean went from being quiet and soft-bodied to being full of weird, complex, active creatures. Nine days on our calendar. In real time, that’s about 20 to 25 million years. Fast enough that geologists call it an explosion. December 14th is the moment the animal world truly woke up.
Timeline PageLabel the next page in your timeline “The Cambrian Explosion: 541 Million Years Ago”. The workbook prompt asks learners to draw a crowded Cambrian seafloor bursting with life: trilobites, the alien-looking Anomalocaris, five-eyed Opabinia, and Hallucigenia with its legs and spines.

Discussion Questions
  1. Why was the Cambrian Explosion such an important moment in history? 
    Sample answer: Almost every major animal body plan that exists today first appeared during the Cambrian Explosion. In just a few million years, life went from simple soft-bodied organisms to an enormous variety of complex animals with eyes, shells, legs, and specialized body parts.
  1. How were Cambrian animals different from earlier life? 
    Sample answer: Earlier life was mostly soft-bodied and simple, flat mats of cells or drifting blobs. Cambrian animals had hard shells, compound eyes, claws, jointed legs, and much more complex body structures suited for hunting and defense.
  2. Why do you think hard shells and armor became so important? 
    Sample answer: Hard shells provided protection from predators that also evolved during the Cambrian. As predators became more effective hunters, animals with protective coverings survived better and passed those traits to their offspring.

Vocabulary
  • Cambrian Explosion — A period about 541 million years ago when an enormous variety of complex animal life suddenly appeared in the fossil record.
  • Trilobite — An ancient, armored sea creature that lived during the Paleozoic Era; one of the first animals with eyes.
  • Exoskeleton — A hard outer skeleton on the outside of an animal's body that provides support and protection.
  • Predator — An animal that hunts and eats other animals; the first true predators appeared during the Cambrian Explosion.
  • Biodiversity — The variety of life in a given area or on Earth as a whole.
  • Phylum — A major grouping of animals sharing a basic body plan; most of today's animal phyla appeared during the Cambrian Explosion.
  • Natural Selection — The process by which individuals with traits better suited to their environment survive and reproduce more successfully.

Species to ResearchThe Cambrian Explosion produced an astonishing variety of body plans. Here are some key species to research:
  • Anomalocaris— A top Cambrian predator with grasping claws and a circular mouth; one of the largest animals of its time.
  • Opabinia— A five-eyed Cambrian creature with a long, flexible frontal appendage; so bizarre it caused laughter when first presented.
  • Hallucigenia— A spiny, worm-like creature that was originally reconstructed upside down; named for its dreamlike weirdness.
  • Trilobites— The iconic hard-shelled Cambrian arthropods; incredibly diverse and among the best-preserved fossils we have.
  • Wiwaxia— A soft-bodied Cambrian creature covered in scales and spines; its position on the tree of life is still debated.
  • Pikaia— A small, soft-bodied swimmer from the Burgess Shale; one of the earliest known chordates, a very distant ancestor of vertebrates.

Sources

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