Evidence for Evolution

Activities to try

1. 1

   Overlay the shared bone plan across four limbs

   Why it works: A bat, a whale, a horse and a human are built on the same underlying limb skeleton reshaped for different functions — a pattern best explained by descent from a shared ancestor.

   1. Examine four forelimbs side by side — a human arm, a bat's wing, a whale's flipper and a horse's front leg — and first note how differently they look and what different jobs they do (gripping, flying, swimming, running).
   2. Select one bone in the human arm (e.g. the single upper-arm bone) and identify the corresponding bone in each of the other three limbs; repeat for the forearm bones, wrist and finger bones.
   3. Colour-match or label corresponding bones across all four limbs so the shared limb plan (upper-arm bone, paired forearm bones, wrist, digits) becomes visible beneath a wing, a flipper and a leg, noting where bones are reduced or fused (e.g. the horse's ulna is reduced and fused to the radius, and it has one main digit).
   4. State in your own words why one inherited bone plan reshaped for different lives points to a common ancestor.
2. 2

   Homologous versus analogous: shared structure versus shared function

   Why it works: Structures can look or work alike for different reasons: homologous structures share an inherited plan (evidence of common ancestry), while analogous structures share only a function or appearance arrived at independently (convergence), so 'looks/works similar' does not mean 'closely related'.

   1. Compare a bat's wing with an insect's wing — both used for flying.
   2. Check whether the insect wing contains the same bones as the bat wing (it does not; it shares none of the pentadactyl bones).
   3. Compare a shark and a dolphin: both have streamlined bodies and fins and look very alike, yet the dolphin is a mammal whose flipper contains the pentadactyl limb skeleton (the same cetacean/mammalian plan seen in the whale flipper earlier) while the shark is a fish — their resemblance comes from independently adapting to fast swimming, not from a recent shared ancestor.
   4. Classify the bat-wing/human-arm and dolphin-flipper/human-arm pairings as homologous (same structure and origin, different function) and the bat-wing/insect-wing and shark/dolphin resemblances as analogous (same function or appearance, different origin).
   5. Explain why similar function or surface resemblance alone is not evidence of close relatedness.
3. 3

   Read the rock layers: fossils as evidence of change over time

   Why it works: Different life forms in older versus younger strata document change over time, and transitional forms link groups; because fossilisation is rare, the record is expected to be incomplete, and gaps do not disprove evolution.

   1. Examine a stratigraphic column and identify which life forms appear in deeper versus shallower rock layers.
   2. Locate a transitional form (e.g. Archaeopteryx) and describe which of its features it shares with earlier reptile/dinosaur groups (teeth, clawed fingers, bony tail) and which with later birds (feathers, wings).
   3. Explain in your own words how a fossil forms and why fossilisation is rare.
   4. Discuss why the fossil record is necessarily patchy and why gaps are expected rather than a disproof of evolution.