Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Fossils show extinct species and changes in lineages over time.
- Relative dating: in undisturbed sedimentary rock, lower layers are older.
- Radiometric dating: isotopes decay at a constant rate. Carbon-14 (half-life about 5,700 years) dates once-living remains up to about 50,000 years. Older fossils are bracketed by dating volcanic layers above and below them with longer-lived isotopes.
- Geographical data: matching layers and fossils between sites gives ages; the same fossil species on now-separate continents shows they were once joined.
- Homologous structures: same basic parts, different functions (mammal forelimbs) → common ancestry.
- Vestigial structures: reduced remnants (whale pelvis, flightless-bird wings, cave-fish eyes) → descent from ancestors that used them.
- Molecular evidence: compare DNA or amino acid sequences; differences build up after lineages split.
- Biogeography: island species resemble the nearest mainland species.
Key relationships
| Idea | Rule |
|---|---|
| fraction left | (1/2)ⁿ after n half-lives |
| age | number of half-lives × half-life |
| 50%, 25%, 12.5%, 6.25% | 1, 2, 3, 4 half-lives |
| sequence differences | fewer differences → more recent common ancestor |
| percent identity | (matching positions ÷ total positions) × 100 |
| Structure type | Same build? | Same function? | Evidence of common ancestry? |
|---|---|---|---|
| homologous | yes | often no | yes |
| vestigial | reduced version | lost main function | yes |
| analogous | no | yes | no |
Assumptions behind the numbers
- Layers have not been overturned or disturbed.
- Decay rate is constant; nothing added or removed from the sample after death.
- The starting amount of carbon-14 matched the atmosphere at the time.
- Sequence comparisons use the same gene, aligned position by position.
Mistakes to avoid
- Using carbon-14 for fossils millions of years old.
- Saying nothing is left after two half-lives.
- Calling analogous structures evidence of common ancestry.
- Saying vestigial means “useless”.
- Describing data without quoting values.
- Saying one living species evolved from another living species.
Quick self-check
- A bone has 25% of its original carbon-14. Age, with half-life 5,700 years? (2 half-lives: 11,400 years)
- Why do a bat wing and a whale flipper count as evidence of common ancestry? (Same set of bones, different functions: inherited from a shared ancestor.)
- Species A and B differ at 3 amino acids; A and C at 12. Which pair shares the more recent common ancestor? (A and B.)
Next: practice questions.