AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/7/7-7)
Unit 7 · Topic 7.7
7.7 Common Ancestry
All eukaryotes, from yeast to oak trees to you, share membrane-bound organelles, linear chromosomes and genes that contain introns. Features this specific and this widespread are best explained by inheritance from a single common ancestor of all eukaryotes.
Key terms
- common ancestry
- eukaryote
- membrane-bound organelles
- linear chromosomes
- introns
The argument for common ancestry
If many unrelated features are shared by a whole group, the simplest explanation is that the group inherited them from one ancestor that already had them, rather than each lineage inventing the same set independently. Eukaryotes (animals, plants, fungi and protists) share several structural and functional features that prokaryotes (bacteria and archaea) lack. Together they point to a single common ancestor of all eukaryotes.
Three key shared features
- Membrane-bound organelles: every eukaryotic lineage has a nucleus and an internal membrane system (endoplasmic reticulum and Golgi), and nearly all have mitochondria. Many of these organelles work the same way across groups. Mitochondria, for example, make ATP by the same basic process in a mushroom and in a mouse.
- Linear chromosomes: eukaryotes store DNA in multiple linear chromosomes wound around histone proteins, while prokaryotes typically have a single circular chromosome.
- Genes with introns: eukaryotic genes are interrupted by noncoding introns that must be spliced out of the RNA before translation, using similar splicing machinery across eukaryotes. Bacterial protein-coding genes generally lack them.
How this connects to the rest of the course
These features tie together earlier units. The nucleus and endomembrane system come from Unit 2, and endosymbiotic theory (2.10) explains that mitochondria and chloroplasts came from bacteria taken in by an ancestral cell. The fact that all eukaryotes' mitochondria trace back to that kind of event is itself evidence of shared ancestry. Linear chromosomes with histones (6.1) and introns removed during RNA processing (6.3) are the molecular features in this list.
There's also evidence for the common ancestry of all life, not just eukaryotes: all organisms use DNA as genetic material, nearly the same genetic code (6.4), ribosomes to make proteins, ATP as an energy currency and very similar core pathways such as glycolysis. Those features place eukaryotes, bacteria and archaea on one tree of life.
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Membrane-bound organelles (nucleus, ER, mitochondria) | No | Yes |
| Chromosome structure | Usually one circular chromosome | Multiple linear chromosomes with histones |
| Introns in protein-coding genes | Generally absent | Common |
| DNA, genetic code, ribosomes, ATP | Yes | Yes (shared by all life) |
Making predictions from common ancestry
Common ancestry lets you predict features of organisms you haven't studied. If a newly discovered single-celled organism has a nucleus, you can predict it will also have linear chromosomes and genes with introns, because all known eukaryotes inherited those together. A finding that broke this pattern would be surprising and would lead scientists to revise their hypotheses, which is how evolutionary models are tested.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Using shared features as evidence
Researchers find a new microorganism in deep-sea mud. It has a nucleus and mitochondria. Predict two other features they will likely find and explain why, using common ancestry.
Show the solutionHide the solution
- Step 1: A nucleus and mitochondria are membrane-bound organelles, which places the organism among the eukaryotes.
- Step 2: All eukaryotes descend from a common ancestor that had linear chromosomes and genes with introns, and these features were passed to all its descendants.
- Step 3: Prediction 1: its DNA will be organized in multiple linear chromosomes wrapped around histones.
- Step 4: Prediction 2: many of its protein-coding genes will contain introns that are spliced out of the mRNA.
Answer: Linear chromosomes (with histones) and genes containing introns, because these features were inherited from the common ancestor of all eukaryotes along with membrane-bound organelles.
Common mistakes
- Listing features shared by all life (like ribosomes or DNA) as evidence that eukaryotes specifically share an ancestor separate from prokaryotes. Use features unique to eukaryotes for that argument.
- Saying prokaryotes have membrane-bound organelles or a nucleus. They have ribosomes but no membrane-bound organelles.
- Claiming shared features prove one modern species evolved from another. They show both descend from a common ancestor.
On the exam
- Expect a question asking for structural or molecular evidence that eukaryotes share a common ancestor; name the three features and explain why shared features imply shared ancestry.
- Questions may ask you to predict what happens to a cell if one of these shared systems is disrupted, such as a mutation that blocks intron removal.
Connected topics
Videos
Check yourself
4 questions on 7.7 Common Ancestry. Pick an answer to see if you got it, and why.
Which observation provides the best evidence that all eukaryotes, from yeasts to oak trees to humans, share a single common ancestor?
| Organism | Nucleus | Mitochondria | Linear chromosomes | Genes commonly split by introns | Ribosomes |
|---|---|---|---|---|---|
| Yeast | Yes | Yes | Yes | Yes | Yes |
| Amoeba | Yes | Yes | Yes | Yes | Yes |
| Oak tree | Yes | Yes | Yes | Yes | Yes |
| E. coli (bacterium) | No | No | No | No | Yes |
Features of three eukaryotes and one prokaryote
Which feature in the table provides no evidence that eukaryotes share a common ancestor that prokaryotes do not share?
Which of the following best explains why these shared features support a single common ancestor for yeast, amoebas and oak trees?
A single-celled parasite lives in places with little oxygen and lacks typical mitochondria. However, it has tiny organelles surrounded by two membranes, and its nucleus contains genes very similar to genes that other eukaryotes use to build mitochondria. Which conclusion is best supported?
0 of 4 answered