What features shared by all living things point to a common ancestor?
Topic 7.7 Common Ancestry: describe the structural and molecular features shared by all organisms that indicate common ancestry.
A focused answer to AP Biology Topic 7.7, covering the shared features of all life (DNA, the genetic code, ribosomes, core metabolism, membranes) that indicate common ancestry, and how conserved features reveal deep relationships, with a worked example.
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What this topic is asking
The College Board (Topic 7.7) wants you to describe the structural and molecular features shared by all organisms that indicate common ancestry, and explain why these conserved, fundamental features point to descent from a single common ancestor.
The shared features of all life
Why this indicates common ancestry
Conserved features reveal deep relationships
Features tend to be conserved when they are essential and any change would be harmful, so natural selection removes variants that alter them. The genetic code is a good example: a mutation that changed which amino acid a codon specified would disrupt every protein at once, so the code has barely changed in billions of years. This is why the most fundamental, life-supporting features are the most widely shared, and the most recently evolved, less essential features differ between closely related species.
Common ancestry is the unifying theme of all the evidence in this unit. The shared features here, the homologous structures and molecular similarities of the evidence topic, and the branching pattern of phylogenetic trees all describe the same history: life diversifying from common ancestors. This is why the four big ideas place evolution at the center of biology.
Try this
Q1. List two features shared by all living organisms. [2 points]
- Cue. Any two of: DNA as the genetic material; the same near-universal genetic code; ribosomes for protein synthesis; ATP as an energy currency; phospholipid cell membranes.
Q2. Explain why a shared genetic code is evidence of common ancestry. [2 points]
- Cue. The same code in all organisms is most simply explained by inheritance from one common ancestor, because independent evolution of an identical code in unrelated lineages is extremely unlikely.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2020 (style)3 marksSection II (short FRQ). (a) Identify two features shared by all living organisms that provide evidence of common ancestry. (b) Explain why these shared features support the idea that all life shares a common ancestor.Show worked answer →
A 3-point identify-and-explain short FRQ on common ancestry.
(a) Identify (2 points): any two of (1 point each): all organisms store information in DNA; all use the same (near-universal) genetic code; all use ribosomes to make proteins; all use ATP as an energy currency; all have cell membranes made of phospholipids.
(b) Explain (1 point): these fundamental features are shared because they were inherited from a single common ancestor; it is highly unlikely that unrelated lineages would independently arrive at the same code and machinery, so the simplest explanation is common descent.
Markers reward two valid shared features and the inference that shared fundamental features point to common descent.
AP 2017 (style)1 marksSection I (multiple choice). The fact that nearly all organisms use the same genetic code (the same codons for the same amino acids) is strong evidence that: (A) all organisms evolved independently. (B) all organisms share a common ancestor. (C) the genetic code is the only possible code. (D) mutations never occur.Show worked answer →
The correct answer is (B).
A shared, near-universal genetic code is most simply explained by inheritance from a common ancestor; it would be very unlikely to arise independently in unrelated lineages. It does not imply independent evolution (A), that no other code is possible (C), or that mutations do not occur (D).
Related dot points
- Topic 7.6 Evidence of Evolution: describe the lines of evidence (fossil, anatomical, molecular, biogeographical) that support evolution.
A focused answer to AP Biology Topic 7.6, covering fossil, anatomical (homologous and vestigial structures), embryological, molecular and biogeographical evidence for evolution, with a worked interpretation of molecular data.
- Topic 7.9 Phylogeny: interpret and construct phylogenetic trees and cladograms from shared characters and molecular data.
A focused answer to AP Biology Topic 7.9, covering phylogenetic trees and cladograms, shared derived characters, nodes and common ancestors, out-groups, and reading relatedness from a tree, with a worked tree interpretation.
- Topic 7.13 Origin of Life on Earth: describe the scientific models for the origin of life, including the RNA world and the evidence supporting them.
A focused answer to AP Biology Topic 7.13, covering models for the origin of life, the formation of organic monomers, the RNA world hypothesis, protocells, the geological timeline, and the evidence behind these models, with a worked example.
- Topic 7.1 Introduction to Natural Selection: explain the conditions required for natural selection and how it leads to changes in a population.
A focused answer to AP Biology Topic 7.1, covering Darwin's reasoning, the conditions for natural selection (variation, heritability, overproduction, differential reproduction), fitness, and how selection changes allele frequencies, with a worked example.
- Topic 2.11 Origins of Cell Compartmentalization: describe the similarities and differences in compartmentalization between prokaryotic and eukaryotic cells, and the evidence for the endosymbiotic origin of mitochondria and chloroplasts.
A focused answer to AP Biology Topic 2.11, covering the endosymbiotic theory, the evidence that mitochondria and chloroplasts descend from free-living prokaryotes, and the origin of the endomembrane system.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)