Common Ancestry
All of life traces back to shared ancestors, and Topic 7.7 is where you learn to read the evidence for it. When two species share a characteristic — the same bones in a limb, the same genetic code, the same core proteins — the most parsimonious explanation is that they inherited it from a common ancestor rather than arriving at it independently. The deeper the shared features run, especially at the molecular level, the further back that common ancestor sits. Common ancestry is not a story about one living species turning into another; it is a branching pattern in which today's species are the tips, and the shared traits are the record of the branch points behind them.
The molecular clock sharpens this into a timeline. Because certain stretches of DNA and protein accumulate changes at a roughly steady rate over time, the number of differences between two species estimates how long ago their lineages diverged from a shared ancestor. This is where two errors surface. First, relatedness is symmetric: humans and living monkeys share a common ancestor — humans did not evolve from the monkeys you see today, which are themselves the modern tips of their own lineage. Second, similarity only counts as evidence for common descent when the structures are homologous — inherited from that shared ancestor, like the forelimb bones of a bat, a whale, and a human. Structures that merely look or work alike because of independent adaptation to a similar problem are analogous, like the wings of a bird and an insect, and they say nothing about how recently the lineages branched.
Interactive · Molecular Clock
Pick a pair of species, let differences accumulate at a steady rate, and read the divergence time off the clock. Watch how more molecular differences push the shared ancestor further into the past — and see why two living species branch from a common ancestor rather than one descending from the other.
Molecular Clock · Open the full sandbox →The mistakes here cluster around a few failure modes. Students read common ancestry as goal-directed or progressive — a ladder climbing toward "higher" forms — when it is a branching pattern with no direction (U7-BIO1); they say humans evolved from modern monkeys rather than that the two share a common ancestor and each lineage kept changing (U7-BIO14); and they treat analogous structures as evidence of common descent when only homologous structures — inherited from a shared ancestor — carry that signal (U7-BIO12). Every scenario in this topic asks you to keep the tree branching, not laddered, and to sort shared traits into inherited versus independently evolved before drawing any conclusion about relatedness.
The work
3 ways in · any order
Lesson
Common Ancestry
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Shared characteristics across species are evidence of descent from a common ancestor, and a molecular clock uses the steady rate of molecular change to estimate how long ago two lineages diverged. The lesson walks the ways students misread that — treating ancestry as a ladder toward "higher" forms, saying humans evolved from modern monkeys, and counting analogous look-alike structures as evidence of common descent. It closes with a ten-scenario applet that asks you to keep the tree branching and to sort homologous from analogous before judging relatedness.
Diagnostic
10-item topic check
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Ten items on the evidence for common ancestry — that it is a branching pattern, not a goal-directed climb toward "higher" forms (U7-BIO1); that humans and living monkeys share a common ancestor rather than one evolving from the other (U7-BIO14); and that only homologous structures, not analogous ones, are evidence of common descent (U7-BIO12). Take it cold to surface which of these are still tangled, or after the lesson to confirm they hold.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.