Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Common ancestry: two lineages trace back to the same ancestral population. Living species are relatives, not ancestors of each other.
- All life shares DNA, a nearly universal genetic code, ribosomes, a phospholipid membrane and ATP. These link all three domains.
- All eukaryotes also share three features that most bacteria and archaea lack. These point to a later common ancestor of eukaryotes.
- A feature counts as evidence only if it is homologous: inherited, and built from related molecules, not just similar in job.
Key relationships
| Shared eukaryote feature | What it involves | Typical prokaryote |
|---|---|---|
| Membrane-bound organelles | nucleus with double envelope and pores; endomembrane system; mitochondria (nearly all), chloroplasts (plants, algae) | no nucleus; DNA in a nucleoid; no membrane-bound organelles |
| Linear chromosomes | several chromosomes with ends capped by telomeres; separated by mitosis | usually one circular chromosome, no telomeres |
| Genes with introns | exons interrupted by introns; spliceosome removes introns from pre-mRNA | introns rare; no spliceosome |
| Line of reasoning | Why it supports one ancestor |
|---|---|
| Features match in fine detail | separate origins would not produce the same molecules for the same jobs |
| Features occur together | unrelated features shared as a package were inherited together |
| Same intron positions in distant species | independent insertion at the same codon is very unlikely |
| Fits sequence data (Topic 7.6) | independent lines of evidence agree |
Assumptions and limits
- Introns can be lost or gained, so a missing intron does not rule out common ancestry. Compare many genes.
- A few eukaryotes have lost mitochondria or kept only remnants. Loss is change within a lineage.
- Percent-of-control calculations assume each treated culture started at the same density as its own control and differs only in the treatment.
Mistakes to avoid
- Saying humans evolved from yeast (or any living species).
- Using features shared by all life as evidence for the eukaryote ancestor only.
- Counting analogous features, such as cell walls built from different molecules, as homology.
- Forgetting telomeres when describing linear chromosomes.
- Predicting that a spliceosome-blocking drug would harm bacteria.
- Comparing treated groups without their own controls.
Quick self-check
- Name the three shared eukaryote features. (Membrane-bound organelles, linear chromosomes, genes containing introns.)
- Why are shared intron positions strong evidence? (An intron is unlikely to be inserted at exactly the same codon in separate lineages; inheritance from a shared ancestor explains it.)
- A culture reaches 6.0 million cells per mL as a control and 1.5 million with a drug. Growth as a percentage of control? (1.5 ÷ 6.0 × 100 = 25%)
Next: practice questions.