Four Independent Records of Common Ancestry

Lab Brief #3

🎯 Your mission

Learn the rule. Then catch nature using it.

⚡ The twist

Nature loves to surprise the careful watcher.

What you'll learn

Four separate lines of evidence point to the same branching tree: homologous skeletons built from the same set of bones in different proportions, embryos that pass through shared developmental stages, vestigial structures with no current function, and DNA sequences whose differences grow steadily with the time since two lineages split. Any one line could be argued about individually. The force of the case is that four records gathered by different methods produce the same tree.

Key words

homologous structure
A body part with the same underlying bone plan in different species, inherited from a shared ancestor.
vestigial structure
A reduced structure with no current function, retained from an ancestor in which it was useful.

Make a hypothesis first

Before you start: write down what you think will happen, and why.

Say your guess out loud — then find out if you were right.

Step 1 of 4 · Read

Think about what you just learned: say what "homologous structure" and "vestigial structure" mean in your own words before you start.

Step 2 of 4 · Do

Compare labelled limb skeletons of a human, whale, bat and horse, colouring the humerus, radius, ulna and digits the same in each. Then build a branching tree from a DNA similarity table and check whether the bone evidence and the DNA evidence give the same tree.

Step 3 of 4 · Observe

Record what actually happened, including anything that did not match what you expected.

Step 4 of 4 · Record

Write a conclusion that answers the question this lesson opened with, and state your uncertainty.

Question 1 of 3

A whale's flipper and a bat's wing share the same bone plan, making them:

How sure are you?😐 maybe

Question 2 of 3

The strength of the evidence for common ancestry comes mainly from:

How sure are you?😐 maybe

Question 3 of 3

DNA differences between two species generally grow with:

How sure are you?😐 maybe

Mind = blown

🔬 The more you look, the more nature shows you.

Nice work!

You got 0 of 3 right.

Next: Selective Breeding Uses the Same Mechanism, and Has a Cost All lessons

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🌍 Where you see this in real life: This shows up in your everyday life more than you'd think.

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👨‍👩‍👧 For the dinner table: “What's the most surprising thing you learned today?”

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Four Independent Records of Common Ancestry

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