Common Ancestry, Evidence & Branching Trees — Print · Grade 7 Science
Chapter: Evolution, Evidence & Life HistoryCommon Ancestry, Evidence & Branching Trees
Learning goals
- I can explain common ancestry, evidence & branching trees.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
A human arm, a bat's wing, a whale's flipper and a horse's front leg do completely different jobs, and inside each one are the same bones in the same arrangement: one long bone, then two, then a cluster, then five sets of digits. Nothing about flying requires that pattern — an insect wing manages without it — and nothing about swimming does either. The shared arrangement is not explained by the shared function, because there is no shared function. It is explained by shared inheritance from an ancestor that had that limb, modified in each lineage for a different use. Structures related in this way are homologous structures, and they are among the strongest evidence for a common ancestor.
The contrast makes the argument sharper. A bird's wing and an insect's wing do the same job and are built on entirely different plans — bone and feather against a membrane with no bones at all. Similar function has produced similar shape independently in lineages that were never closely related, which is why function is a poor guide to relationship and internal structure is a good one. The question is always whether a similarity was inherited or arrived at separately.
Two further lines of evidence matter. Vestigial structures are reduced features that are clearly the remnants of something larger in ancestors — the small hip bones embedded in a whale, the reduced wings of a flightless bird. Calling them useless is a step too far, since several have turned out to retain functions, and the evidence they provide is about history rather than uselessness. And the strongest line is molecular: DNA sequences can be compared directly, and more closely related organisms share more sequence. A phylogenetic tree summarises the conclusions, with each node representing a common ancestor from which two lineages diverged. The real weight of the argument comes from independent lines agreeing — anatomy, fossils and molecules producing the same tree.
Build your understanding
Where this fits
Selection explains how a population changes. Run that over long enough, with populations separated, and lineages diverge — which is where the branching tree from the first chapter comes from. This unit is the evidence for shared ancestry and how to read a tree without misreading it, which is most of what people get wrong about this topic.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · Inherited structure against shared function
Homologous structures share an underlying plan and may do quite different things. The forelimb pattern across mammals is the standard case: one bone, two bones, wrist bones, digits, whether the limb is used for flying, swimming, running or writing. The shared plan is what needs explaining, and inheritance from a common ancestor explains it.
The opposite case is similarity produced independently. A bird's wing and an insect's wing, a shark's body and a dolphin's, a cactus's spines and a thorny shrub's — in each pair, similar demands have produced similar solutions in lineages that are not close relatives. Looking inside separates the cases: the dolphin has the mammal forelimb inside its flipper and the shark does not.
The practical rule is to compare structure rather than function or outward form. Two organisms that look alike may be distantly related; two that look nothing alike may be close. A dolphin is more closely related to a cow than to a shark, which is counter-intuitive from the outside and obvious from the bones.
Pause and explain
Why is a dolphin more closely related to a cow than to a shark?
Check your reasoning after answering
Because the similarities between dolphin and shark are in outward form, produced independently by similar demands of swimming, while the similarities between dolphin and cow are structural and inherited — the mammal forelimb inside the flipper, lungs, warm-bloodedness, milk. Shared inheritance shows in internal structure; shared function shows in outward shape.
Part 2 · Reduced structures, used carefully
A vestigial structure is a reduced version of something that was larger and functional in ancestors. Whales have small hip and sometimes leg bones with no connection to a limb; some flightless birds have wings far too small to fly; some cave-dwelling animals have eyes that cannot see. Each is hard to explain except as a remnant.
What they are not is necessarily useless. Whale pelvic bones anchor muscles; the human appendix, long cited as a classic example, appears to have a role in the gut's bacterial community. The evidence vestigial structures provide is that the ancestor had something larger and the lineage reduced it, which is a claim about history and not about current function.
That distinction keeps the argument honest. If the case rested on uselessness, finding a function would defeat it. Resting it on reduction from an ancestral form keeps it intact, and makes it testable against fossils, which can show the intermediate stages — and in whales they do, with a sequence of fossil forms showing hind limbs becoming smaller.
Pause and explain
Why is "vestigial means useless" a poor way to define the term?
Check your reasoning after answering
Because several vestigial structures turn out to have functions — whale pelvic bones anchor muscles, and the human appendix appears to have a role in the gut's bacteria. Vestigial means reduced from a larger ancestral form, which is a claim about history. Resting the argument on uselessness would make it collapse the moment a function was found.
Part 3 · Independent lines of evidence agreeing
Any single line of evidence can be questioned. Anatomical similarity could in principle be coincidence; the fossil record is patchy, as the first unit of this chapter stressed; a molecular comparison depends on the method used. What makes the conclusion strong is that these independent lines produce the same answer.
Molecular evidence is the most recent and in some ways the most decisive, because DNA sequences can be compared directly and quantitatively. Organisms judged close relatives from anatomy share more sequence; organisms judged distant share less. That agreement was not guaranteed — the molecular evidence could have contradicted the anatomy, and in a few cases it has, prompting revisions that are themselves evidence the method is working.
When independent methods with different assumptions and different failure modes converge, the conclusion is far stronger than any one of them. That is a general principle of evidence, not a fact about biology, and it is the same logic as corroborating a historical claim from unrelated records.
Pause and explain
Why does agreement between anatomical and molecular evidence strengthen a conclusion more than either alone?
Check your reasoning after answering
Because the two methods have different assumptions and different ways of going wrong. An error in reading anatomy would not produce the same error in a DNA comparison, so if both point to the same relationships, the chance that both are wrong in the same direction is small. Independent methods converging is stronger than one method repeated.
Part 4 · Reading a tree without misreading it
A tree shows branching order. A node is a point where a lineage split, representing a common ancestor of everything above it. Relative closeness is read by where lineages share nodes: two that share a recent node are more closely related than either is to one that branched off earlier.
Take a tree in which A splits from a lineage before B and C split from each other. B and C are the closest pair, because they share the more recent node. A is equally related to both B and C — it branched off before they separated, so neither is closer to it than the other, however the tree happens to be drawn on the page.
Two misreadings are worth naming. First, living species do not descend from other living species: C did not come from B, and both descend from a shared ancestor at their node that is not shown and no longer exists. Second, unless a tree is explicitly drawn to a time scale, branch lengths carry no information about when splits happened or how much change occurred. A tree without a stated scale gives order and relationship, and nothing about timing.
Pause and explain
In a tree where A splits before B and C split from each other, which pair is closest, and is A closer to B or to C?
Check your reasoning after answering
B and C are the closest pair, because they share the most recent node. A is equally related to B and to C: it branched off before they separated, so it shares the same older ancestor with both. How near A is drawn to one of them on the page carries no information about relationship.
Apply the whole unit · Interpret a fictional tree
Illustrative tree, not real species: lineage A splits from the line before B and C split from each other. No times, no branch-length scale and no species identities are supplied.
Your task
Identify the closest pair, explain why C need not descend from living B, and state what the tree cannot say about timing.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Find the most recent node: B and C share it, so they are the closest pair in the model.
Place A: it branched before that node, so it is equally related to B and to C.
Explain the descent point: B and C both descend from the ancestor at their shared node, which is not either of them and is not alive.
State the timing limit: with no scale, branch lengths carry no information about when splits occurred or how much change happened.
B and C are the closest pair in this tree, because they share the most recent node — the point representing their common ancestor. A branched from the line before that split, which means A is equally related to B and to C; it shares the same older ancestor with both, and nothing about how near A is drawn to one of them on the page changes that. The second point is the one most often got wrong. C does not descend from living B, and B does not descend from living C. Both descend from the ancestor represented by the node they share, a population that is not identical to either of them and is no longer alive. Living species are the tips of a tree, never the branching points, which is the same error as thinking humans descended from chimpanzees rather than both descending from a shared ancestor that was neither. On timing, the tree supports nothing at all. Unless branch lengths are explicitly drawn to a stated scale, they are a drawing convention: they do not say when A diverged, how long ago B and C split, or how much change occurred along any branch. A tree gives branching order and relative relationship. Adding timing requires dated fossils, or molecular estimates with stated assumptions, and either should be shown as a scale on the diagram so a reader knows the lengths mean something.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
Common ancestry explains inherited similarities among organisms through shared predecessors. Homologous structures share underlying inherited relationships even when functions differ, while similar functions can arise through different histories. A butterfly wing and a bird wing both support flight but do not have the same underlying wing construction.
Worked examples
Example 1 · Separate inherited similarity from independent similarity
- Look past function to internal structure: bat and whale both have the mammal forelimb plan — one bone, two bones, wrist, digits.
- Compare bat and bird: both are wings, and the bird's is built on a different arrangement, with feathers and a reduced, fused hand.
- Apply the rule: shared plan despite different function indicates inheritance; shared function with different plans indicates independent solutions.
Ask what needs explaining. Shared plan with different jobs is the signature of inheritance.
Example 2 · Correct two tree misreadings at once
- Address descent: both are living species at the tips of the tree, so neither descends from the other; they descend from the ancestor at their shared node.
- Address branch length: unless the tree states a time scale, lengths are a drawing convention and carry no temporal information.
- Say what the tree does support: the branching order and which species share more recent ancestors.
Tips are living species; nodes are ancestors. Length means nothing without a scale.
Avoid these mistakes
- Mistake: similar function means close relationship. Bird and insect wings are built on entirely different plans; a dolphin's flipper contains the mammal forelimb and a shark's fin does not.
- Mistake: vestigial means useless. It means reduced from a larger ancestral form — whale pelvic bones anchor muscles, and the claim is about history.
- Mistake: one living species descended from another. Living species are the tips of a tree; two of them descend from a shared ancestor at their node that is neither and is extinct.
- Mistake: branch length shows time. Unless a scale is stated, lengths are a drawing convention and say nothing about when a split happened.
Check your understanding
Does the same function alone prove close ancestry?
Discuss the answer
No. Similar functions can arise through different histories; structural and other evidence must be compared.
Why is “vestigial means no function whatsoever” too strong?
Discuss the answer
Reduced ancestral roles can coexist with remaining or changed functions.
Words to know
- common ancestor
- A population from which two lineages diverged; it is represented by a node on a tree and is not any living species.
- homologous structure
- A feature sharing an underlying plan through inheritance, such as the one-bone, two-bone, digits forelimb across mammals doing quite different jobs.
- vestigial structure
- A feature reduced from a larger ancestral form, such as a whale's hip bones; reduced is the claim, not useless.
- phylogenetic tree
- A diagram of branching order and relationship; without a stated scale, branch lengths say nothing about timing.
- node
- A branching point representing a common ancestor; two lineages sharing a recent node are more closely related than either is to one branching earlier.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Does rotating B and C around their node change their relationship?
Check your explanation after answering
No. The shared node and connections remain the same; visual left-right order is not the ancestry criterion.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Problem 1
Answer and explanation
No. Similar functions can arise through different histories; structural and other evidence must be compared.
Problem 2
Answer and explanation
Reduced ancestral roles can coexist with remaining or changed functions.
Problem 3
Answer and explanation
It may not represent the whole history and can be affected by sampling and different rates; additional regions and other evidence help.
Problem 4
Answer and explanation
No. The shared node and connections remain the same; visual left-right order is not the ancestry criterion.
Problem 5
Answer and explanation
B and C share the more recent common ancestor in the stated topology. C is not represented as descending from living B; both connect through an ancestral lineage. Exact dates or elapsed times cannot be inferred because no time scale is supplied. The diagram is a hypothesis whose supporting evidence would need examination.
Find the shared B–C node.
Treat living B and C as tips, not parent and child.
Respect the missing time scale.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 6
Answer and explanation
Shared branching nodes determine the modeled relationship, not distance between printed tip labels.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 7
- Shared branching nodes
- Alphabetical tip order
- The prettiest color
- Every tip’s horizontal spacing
Answer and explanation
Shared branching nodes
Topology and shared nodes represent relationships; display choices need a stated meaning.
Problem 8
Answer and explanation
Different observations can test related aspects of a relationship hypothesis. Agreement strengthens it while limitations or contradictions guide further investigation.