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Common
Ancestry

Explore the evidence for universal common ancestry, how phylogenetic trees map evolutionary relationships, and why shared traits reveal the deep history of life on Earth.

AP Biology

What Is Common Ancestry?

Common ancestry is the idea that all living organisms descended from a shared ancestor through a branching process of evolution. Over billions of years, populations diverged, accumulated genetic changes, and gave rise to the vast diversity of life we see today.

Lines of Evidence

Molecular
Universal genetic code: all life uses DNA/RNA and the same 20 amino acids.
Anatomical
Homologous structures share an underlying skeletal plan despite different functions.
Fossil
Transitional fossils show intermediate forms linking major groups over time.
Embryological
Similar early developmental stages across vertebrates reveal shared genetic programs.

Concordance of fossil, anatomy and molecular evidence

A reconstructed lobe-finned fish sits above an illustrative fossil slab and a separate reconstructed limb specimen.Compare fossils, corresponding anatomical structures and molecular sequences with a branching hypothesis. Tiktaalik is a transitional relative, without requiring it to be a direct ancestor of a particular living lineage.

Evidence should fit a branching history

Evidence should fit a branching history. Compare fossils, corresponding anatomical structures and molecular sequences with a branching hypothesis. Tiktaalik is a transitional relative, without requiring it to be a direct ancestor of a particular living lineage.Compare fossils, corresponding anatomical structures and molecular sequences with a branching hypothesis. Tiktaalik is a transitional relative, without requiring it to be a direct ancestor of a particular living lineage.

Shared bones do not make a direct ancestor

Shared bones do not make a direct ancestor. Tiktaalik lived about 375 million years ago and shares corresponding limb-bone regions with tetrapods. Fin rays differ from digits, and correspondence does not establish direct ancestry.Tiktaalik lived about 375 million years ago and shares corresponding limb-bone regions with tetrapods. Fin rays differ from digits, and correspondence does not establish direct ancestry.

A molecular comparison adds another test

A molecular comparison adds another test. Compare the listed cytochrome c amino-acid differences from the human sequence across the same 104-residue protein.Compare the listed cytochrome c amino-acid differences from the human sequence across the same 104-residue protein.

The Universal Genetic Code

From bacteria to blue whales, nearly every organism translates the same codons into the same amino acids. This universality is powerful evidence that all life shares a single origin.

DNA → RNA AUG → Met Protein Bacteria Plants Fungi Animals

Reading a Phylogenetic Tree

A phylogenetic tree is a branching diagram that represents hypothesized evolutionary relationships. Each node represents a common ancestor, and each branch tip represents an extant or extinct taxon.

Root Node A Fish Node B Amphibians Node C Reptiles Mammals Reptiles and Mammals share Node C Time

Key Tree Vocabulary

Node
A branching point representing a hypothetical common ancestor of the lineages that descend from it.
Clade
A group consisting of an ancestor and all of its descendants: a complete branch cut from the tree.
Outgroup
A taxon outside the clade of interest, used to root the tree and determine which traits are ancestral.

Homology vs Analogy

Homologous Structures Analogous Structures
Origin Shared common ancestor Independent evolution
Structure Similar underlying anatomy Different underlying anatomy
Function May differ (arm vs wing vs flipper) Similar (bird wing vs insect wing)
Evidence for Common ancestry (divergence) Convergent evolution
Example Human arm, whale flipper Bird wing, butterfly wing

Molecular Clocks

Because DNA mutations accumulate at roughly constant rates in neutral regions, scientists can estimate when two lineages diverged by counting the number of sequence differences between them.

Time Since Divergence (millions of years) Sequence Differences (%) 100 200 300 400 10 20 30 40 Neutral mutation rate

How to Determine Relatedness

1
Identify shared derived traits (synapomorphies) that unite groups but are absent in the outgroup.
2
Compare DNA or protein sequences between species: fewer differences means more recent common ancestor.
3
Build a phylogenetic tree using parsimony (fewest evolutionary changes) or maximum likelihood models.
4
Root the tree using the outgroup to determine which traits are ancestral versus derived.
5
Identify the most recent common ancestor (MRCA) of any two taxa by tracing their branches back to their shared node.

Example: Which Are Most Closely Related?

Lizard Mouse Cat Human MRCA of Cat & Human

Example: DNA Sequence Comparison

Consider a short gene region from four species. Positions where nucleotides differ from Species A are highlighted.

Species Sequence Differences from A
Species A ATGCCTGA 0
Species B ATGCATGA 1
Species C ACGCATAA 3
Species D GTACATAT 5

Example: Homologous Forelimbs

Human H R/U C Whale H Bat H Humerus Radius/Ulna Carpals Digits
Corresponding skeletal parts
Corresponding skeletal parts
Shared origin schematic
Shared origin schematic
Human, bat and cat forelimb illustrations are compared. A separate schematic marks corresponding humerus, radius, ulna, wrist and five digits, with elongated bat digits and short thumb. An unresolved shared-ancestry companion has three living tips.
These independently enlarged forelimb topology schematics identify corresponding humerus, radius, ulna, wrist and five digits. They compare homology rather than exact bone shapes or proportions, and do not make living species ancestors of one another.

Homologous forelimbs versus analogous wings

Human arm, bat wing, whale flipper and cat foreleg specimens accompany an authored proximal-to-distal skeletal correspondence diagram.Corresponding forelimb bones reflect shared ancestry despite different functions. Bird and bat forelimb bones are homologous, while bird and insect flight surfaces are analogous.

Same skeletal plan, different uses

Same skeletal plan, different uses. Corresponding forelimb bones reflect shared ancestry despite different functions. Bird and bat forelimb bones are homologous, while bird and insect flight surfaces are analogous.Corresponding forelimb bones reflect shared ancestry despite different functions. Bird and bat forelimb bones are homologous, while bird and insect flight surfaces are analogous.

Vestigial Structures & Biogeography

Vestigial Structures
Reduced or functionless features inherited from ancestors where they were functional: whale hip bones, human appendix, snake leg buds.
Biogeography
Geographic distribution of species matches evolutionary predictions: island species resemble nearby mainland relatives, not ecologically similar distant species.

Endosymbiosis & Deep Ancestry

Mitochondria and chloroplasts have their own circular DNA, double membranes, and reproduce by binary fission: traits shared with free-living bacteria. This is strong evidence that eukaryotic cells arose from an ancient symbiotic relationship between prokaryotes.

Ancestral Host Cell Engulfs Bacterium Endosymbiosis Mito N Eukaryote

Common Mistakes

✕ Humans Evolved from Chimps
Humans and chimps share a common ancestor: neither descended from the other. Both lineages have evolved independently since diverging.
✕ Analogy = Homology
Analogous structures (convergent evolution) look similar but have different origins. Only homologous structures indicate shared ancestry.
✕ Reading Trees Left-to-Right
The order of branch tips does not indicate relatedness. Only the branching pattern (nodes) determines which taxa share a more recent ancestor.

Summary of Evidence

Evidence Type What It Shows Example
Molecular Shared DNA/proteins across species Universal genetic code
Anatomical Homologous structures from common plan Vertebrate forelimbs
Fossil Transitional forms in rock record Tiktaalik (fish → tetrapod)
Embryological Similar developmental stages Pharyngeal pouches in vertebrates
Biogeographic Distribution matches ancestry Darwin's finches
Vestigial Reduced ancestral features persist Whale pelvic bones

Question 1 of 5

Which of the following is the strongest evidence that all life shares a common ancestor?

Question 2 of 5

On a phylogenetic tree, two species are most closely related if they:

Question 3 of 5

The forelimbs of a human, whale, and bat have different functions but share the same set of bones. These structures are best described as:

Question 4 of 5

Scientists compare cytochrome c protein sequences across species. Species X and Y differ by 2 amino acids, while Species X and Z differ by 15 amino acids. What can you conclude?

Question 5 of 5

Which of the following correctly explains why mitochondria are considered evidence for endosymbiosis and common ancestry?