Search /
On this page

L02 — The Body’s Memory

Module: M08 — Secrets in the Rocks Periods: 1 (53 minutes) NGSS: HS-LS4-1, HS-LS4-3 Lesson Type: Explore / Explain

Learning Objective

  • By the end of this lesson, students will be able to distinguish between homologous, analogous, and vestigial structures by comparing the anatomy of different vertebrates, and explain how comparative anatomy provides evidence for common ancestry.

Phenomenon / Hook

Cold Case #2: Display an image of a mammoth skeleton alongside an elephant skeleton. “Lyuba’s bones look remarkably similar to an elephant’s. But why? Elephants and mammoths aren’t alive at the same time. How can we explain these structural similarities across species that never coexisted?”

Pose the question: “When body parts look similar, what does that tell us about their origin? Coincidence? Design? Or something else?”

Materials

  • Projector for mammoth/elephant skeleton comparison
  • Printed or digital: [[resources/handouts/M08-L02-anatomy-comparison-images]] (forelimbs: human, whale, bat, cat, horse, mammoth/elephant)
  • [[resources/handouts/M08-L02-homology-vs-analogy-chart]]
  • Colored pencils or markers
  • [[resources/handouts/M08-L02-vestigial-structures-examples]]
  • [[assessments/exit-tickets/L02-exit-ticket]]

Agenda

Opening (7 min)

  • Show mammoth/elephant skeleton comparison (2 min)
  • Quick Think-Pair-Share: “What similarities do you notice? What differences? What might explain why they look so similar?” (3 min)
  • Introduce the question: “When body parts look similar, what does that tell us?” (2 min)

Explore (20 min)

  • Part A: Forelimb Comparison (10 min)

    • Distribute anatomy comparison images — students work in pairs
    • Students outline and color-code the forelimbs: same bone = same color (humerus, radius, ulna, carpals, metacarpals, phalanges)
    • Guiding questions:
      1. How many bones are in each forelimb? Are they the same number?
      2. Which bones are present in ALL the forelimbs? (Should find: humerus, radius, ulna, some carpals/metacarpals)
      3. The mammoth’s forelimb supports massive body weight. The bat’s forelimb is wings. The whale’s forelimb is flippers. What do these different functions tell you about how structures can be modified from a common blueprint?
    • Groups share findings — teacher facilitates identification of the “common blueprint” concept

  • Part B: Homology vs. Analogy Sorting (10 min)

    • Distribute sorting cards: pairs of structures (bird wing + butterfly wing, human arm + octopus arm, etc.)
    • Students categorize each pair as homologous (shared ancestry) or analogous (convergent evolution)
    • Reveal the three categories:
      • Homologous structures: Shared ancestry — same structure, different function (mammal forelimbs)
      • Analogous structures: Shared function, different ancestry — evolved independently (bird wing + insect wing)
      • Vestigial structures: Remnants of structures that had a function in ancestors — no longer functional or reduced (human appendix, whale pelvis, kiwi wings)
    • Discussion: “Why do vestigial structures exist? If they don’t do anything, why are they still there?”

Explain (12 min)

  • Class synthesis on board:
    • Homology = common ancestry evidence: Same underlying structure = inherited from a common ancestor
    • Mammoth/elephant connection: Both have similar forelimb structure because they inherited it from a common mammalian ancestor
    • Vestigial structures: Evidence that ancestors had different features — structures persist because evolution works by modification, not replacement
    • Connection to Cold Case #2: “The similar bones in mammoth and elephant are homologous structures. They tell us these species share a common ancestor. The bones are the ‘memory’ of that ancestry.”
  • Students complete the homology/analogy chart with definitions and examples

Elaborate (9 min)

  • Application Challenge:
    • Groups receive a “mystery structure” card: dolphin flipper, snake skeleton, bat wing
    • Groups must identify: Is this homologous or analogous to a human arm? What does this structure tell us about the organism’s evolutionary history?
    • Share out one per group — emphasize that structure + function + ancestry together tell the story

Closing (5 min)

  • Exit Ticket: [[assessments/exit-tickets/L02-exit-ticket]]
    1. Explain the difference between homologous and analogous structures. Provide one example of each.
    2. How does comparative anatomy provide evidence that organisms share common ancestry? Use the mammoth/elephant example in your answer.

Differentiation

SupportExtension
Provide a pre-labeled forelimb diagram with bones named — students focus on coloring and comparisonResearch project: Find an example of a vestigial structure in humans. What was its function in ancestors? Why did it disappear? Present 2-minute explanation to class.
Give a word bank for the exit ticket: “homologous,” “analogous,” “common ancestor,” “vestigial”Challenge: What would a structure that is BOTH homologous and analogous look like? Design an example.

Assessment

  • Formative: Check sorting activity — are students correctly distinguishing homology from analogy?
  • Exit Ticket: Definitions and application to mammoth/elephant evidence

Teacher Notes

  • Key concept: Homology is about shared ancestry (structure), analogy is about shared function (adaptation). Students often confuse these — reinforce with the bat wing / bird wing / insect wing example.
  • Vestigial structures are powerful: They prove that evolution modifies existing structures rather than creating new ones from scratch. The whale pelvis is a classic example — it used to be legs.
  • Mammoth thread: Connect back to Lyuba: “Lyuba’s forelimbs look like elephant forelimbs. Today we know why — homology. Shared ancestry.”
  • Prep: If you have actual specimens (sheep hearts, chicken wings, etc.), this lesson works great as a hands-on dissection alternative to images.