7.7 Common Ancestry

Common ancestry means different species share a single ancestral lineage—descendants inherited the same basic cellular and molecular traits. Scientists think all eukaryotes are related because they share conserved structural and functional features that are unlikely to evolve independently (LO 7.7.A; EK 7.7.A.1). Key evidence: membrane-bound organelles (nucleus, mitochondria, endomembrane system), linear chromosomes, and genes with spliceosomal introns (and the spliceosome machinery). The endosymbiotic theory explains mitochondria (from alphaproteobacteria) and chloroplasts (from cyanobacteria), which supports a common last eukaryotic common ancestor (LECA). Conserved processes like mitosis/meiosis and similar intron–exon architecture across diverse eukaryotes also point to shared ancestry. For AP review, focus on those EK examples and the endosymbiotic origin of organelles (see the Topic 7.7 study guide: https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK). More unit review and practice questions are at https://library.fiveable.me/ap-biology/unit-7 and https://library.fiveable.me/practice/ap-biology.

Membrane-bound organelles support common ancestry of eukaryotes because their shared presence and similar structure/function across diverse eukaryotic lineages imply they were inherited from a single last eukaryotic common ancestor (LECA). For example, all eukaryotes have an endomembrane system (nuclear envelope, ER, Golgi) and mitochondria (or mitochondrial remnants), which fits the endosymbiotic model: mitochondria descended from an alphaproteobacterium and chloroplasts from cyanobacteria. Those organelles have their own genomes and bacterial-like features, showing a common origin. Along with linear chromosomes and spliceosomal introns found broadly in eukaryotes, membrane-bound organelles are structural/molecular evidence that eukaryotes share ancestry rather than evolving those features independently. This aligns with EK 7.7.A.1 in the CED and is the kind of explanation you should give on the AP exam. For a quick review, see the Topic 7.7 study guide (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK) and try practice questions (https://library.fiveable.me/practice/ap-biology).

Prokaryotic chromosomes are usually single, circular DNA molecules located in the cytoplasm (no nucleus) and lack spliceosomal introns. Eukaryotic chromosomes are multiple, linear DNA molecules inside a membrane-bound nucleus, packaged with histones, and their genes often contain introns that are removed by the spliceosome. Those differences matter for evolution because linear chromosomes, introns, and a nucleus enable new gene architectures (like exon shuffling and alternative splicing) and complex regulation. That increases heritable variation and allows multicellularity, mitosis/meiosis, and the rise of diverse eukaryotic lineages from the last eukaryotic common ancestor (LECA). Also, the endosymbiotic origin of mitochondria/chloroplasts links prokaryotes to eukaryotes, supporting common ancestry. For AP Bio, remember EK 7.7.A.1: membrane-bound organelles, linear chromosomes, and introns are evidence for eukaryote common ancestry—useful for free-response explanations. Review this topic on the Fiveable study guide (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK) and practice questions (https://library.fiveable.me/practice/ap-biology).

Think of linear chromosomes as a shared “design choice” that all eukaryotes inherited from their last eukaryotic common ancestor (LECA). Most prokaryotes have circular chromosomes; eukaryotes have linear ones with telomeres, packaged with histones, and replicated by mitosis/meiosis. Because linear chromosomes (plus associated machinery like telomerase, nucleosomes, and spindle-based segregation) appear across diverse eukaryotic lineages, they’re structural/molecular evidence of common ancestry (EK 7.7.A.1: linear chromosomes). In short: the fact that widely different eukaryotes all use linear chromosomes—not different, incompatible systems—is best explained by descent from a single ancestor that already had that system. That’s exactly the kind of cellular/molecular evidence the AP CED expects you to recognize (LO 7.7.A). For a quick refresher, check the Topic 7.7 study guide (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK) and more Unit 7 review (https://library.fiveable.me/ap-biology/unit-7). Practice questions are at (https://library.fiveable.me/practice/ap-biology).

Genes that contain introns support a single eukaryotic ancestor because both the intron–exon architecture and the complex machinery that removes introns (the spliceosome) are shared across all eukaryotes. Spliceosomal introns and the spliceosome are unlikely to have evolved independently many times because the spliceosome is a large, conserved molecular machine—so the simplest explanation is inheritance from a common ancestor (the last eukaryotic common ancestor, LECA). That fits the CED’s EK 7.7.A.1 (iii): intron-containing genes are molecular-level structural evidence for common ancestry. In short: shared introns + shared spliceosome = homologous trait inherited from LECA, not repeated convergence. For a quick review of this idea tied to the AP framework, see the Topic 7.7 study guide (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK). Want practice Qs on this? Check the unit page (https://library.fiveable.me/ap-biology/unit-7) and 1000+ practice problems (https://library.fiveable.me/practice/ap-biology).

Three big lines of evidence the CED lists that all eukaryotes share a common ancestor: 1) Membrane-bound organelles—All eukaryotes have an endomembrane system (nucleus, ER, Golgi) and organelles like mitochondria (and chloroplasts in photosynthetic lineages). Their shared presence—and mitochondrial/chloroplast similarities to alphaproteobacteria and cyanobacteria—supports a common eukaryotic ancestor and endosymbiotic events (endosymbiotic theory). 2) Linear chromosomes—Eukaryotes package DNA into linear chromosomes with telomeres and use mitosis/meiosis for segregation. That conserved chromosome architecture and cell-division machinery points to a last eukaryotic common ancestor (LECA). 3) Genes with spliceosomal introns—Most eukaryotic genes have introns removed by the spliceosome. The widespread intron–exon architecture and conserved splicing machinery imply inheritance from a common ancestor. These are the exact CED essentials (EK 7.7.A.1 i–iii). For a focused review, see the Topic 7.7 study guide on Fiveable (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK) and practice problems at (https://library.fiveable.me/practice/ap-biology).

Scientists trace evolutionary relationships by comparing cell-level features that are shared because of common ancestry, especially across eukaryotes (LO 7.7.A). Key structural/functional evidence includes: presence of membrane-bound organelles (nucleus, ER), linear chromosomes and mitosis/meiosis, and genes with spliceosomal introns—these point to a Last Eukaryotic Common Ancestor (LECA). Endosymbiotic evidence ties mitochondria to alphaproteobacteria and chloroplasts to cyanobacteria: similar double membranes, reduced mitochondrial genomes, and homologous genes support a shared origin (endosymbiotic theory). On the AP exam, be ready to name these EKs (EK 7.7.A.1 i–iii) and link structures to evolutionary processes (e.g., how intron–exon architecture implies conserved molecular machinery like the spliceosome). For a focused review, see the Topic 7.7 study guide (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK), Unit 7 overview (https://library.fiveable.me/ap-biology/unit-7), and tons of practice Qs (https://library.fiveable.me/practice/ap-biology).

Introns are noncoding sequences inside eukaryotic genes that get transcribed into pre-mRNA but are removed before translation by the spliceosome (these are called spliceosomal introns). Exons are the coding pieces that stay in the mature mRNA. Introns matter for evolution because (1) many eukaryotes share similar intron–exon architectures and conserved intron positions, which is strong molecular evidence that the last eukaryotic common ancestor (LECA) already had spliceosomal introns—supporting common ancestry of eukaryotes (EK 7.7.A.1, genes that contain introns). (2) Alternative splicing of introns/exons can create protein diversity without new genes, affecting phenotype and selection. (3) Bacteria lack spliceosomal introns, so the presence and patterns of introns help trace deep branching and genome evolution. For AP prep, know terms like spliceosome, spliceosomal introns, intron–exon architecture, and how shared molecular features support common ancestry (see the Topic 7.7 study guide: https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK). For more practice, check the AP problem sets (https://library.fiveable.me/practice/ap-biology).

Structural evidence that plant and animal cells share a common eukaryotic ancestor includes: (1) membrane-bound organelles—both have a nucleus (nuclear envelope) and an endomembrane system (ER, Golgi), which points to a shared eukaryotic cell plan; (2) linear chromosomes—eukaryotes package DNA on linear chromosomes with histones and undergo mitosis/meiosis, unlike most prokaryotes; and (3) genes with spliceosomal introns—both lineages use intron–exon architecture and the spliceosome to remove introns, a molecular/structural trait inherited from the last eukaryotic common ancestor (LECA). Also note mitochondria (in both) and chloroplasts (plants) fit the endosymbiotic story (alphaproteobacteria and cyanobacteria origins) but the shared nucleus/endomembrane, linear chromosomes, and intron-exon systems are core CED EK 7.7.A.1 evidence for common ancestry (LO 7.7.A). For a quick topic review, see the Fiveable study guide (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK). For broader Unit 7 review and practice, check (https://library.fiveable.me/ap-biology/unit-7) and (https://library.fiveable.me/practice/ap-biology).

Because the CED (LO 7.7.A) is asking for evidence that all eukaryotes share a common ancestor, the “evidence” it lists are eukaryote-specific shared derived characters—things prokaryotes simply don’t have. Eukaryotes share membrane-bound organelles (nucleus, mitochondria, chloroplasts via endosymbiosis), linear chromosomes, and spliceosomal introns/ spliceosome machinery. Those features point to a last eukaryotic common ancestor (LECA). Prokaryotes are much more diverse in cell plan (no nucleus or true membrane-bound organelles), usually have circular chromosomes, and generally lack spliceosomal introns, so you won’t find the same suite of shared eukaryotic characters across all bacteria/archaea. In short: the AP evidence for “common ancestry of all eukaryotes” is based on shared eukaryotic innovations that prokaryotes don’t possess (CED EK 7.7.A.1). For a quick review, see the Topic 7.7 study guide (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK) and more unit resources (https://library.fiveable.me/ap-biology/unit-7).

Having a nucleus is evidence for common ancestry because it’s a complex, shared feature that likely evolved once in the last eukaryotic common ancestor (LECA) and was passed to all eukaryotes. The nuclear envelope and other membrane-bound organelles (endomembrane system) plus linked traits—linear chromosomes, spliceosomal introns, and the spliceosome—form a suite of cellular and molecular characters (EK 7.7.A.1) that are rare to evolve independently many times. That shared complexity is stronger evidence for common descent than a simple trait that can arise by convergence. Also, eukaryotes share similar processes (mitosis/meiosis) and intron-exon gene architecture; these molecular details match across diverse eukaryotic lineages, so they point to a single ancestral origin. For AP exam focus, be ready to cite membrane-bound organelles, linear chromosomes, and introns as structural/functional evidence (LO 7.7.A). For a focused review, see the Topic 7.7 study guide (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK) and unit overview (https://library.fiveable.me/ap-biology/unit-7). Practice questions are at (https://library.fiveable.me/practice/ap-biology).

Membrane-bound organelles—especially mitochondria and chloroplasts—support common ancestry by matching predictions of the endosymbiotic theory (an EK in LO 7.7.A). They have their own genomes (often circular like bacteria), bacterial-type ribosomes, and double membranes consistent with a once-independent cell being engulfed (mitochondria from alphaproteobacteria; chloroplasts from cyanobacteria). Those shared cellular features (membrane-bound organelles, linear nuclear chromosomes, and intron-containing genes) are structural and molecular evidence that eukaryotes descended from a last eukaryotic common ancestor (LECA) that integrated these endosymbionts. On the AP exam, you should be able to name these specific lines of evidence and connect them to endosymbiosis (EK 7.7.A.1). For a concise recap tied to the CED, see the Topic 7.7 study guide (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK). For extra practice, check the unit overview (https://library.fiveable.me/ap-biology/unit-7) and practice problems (https://library.fiveable.me/practice/ap-biology).

Structural evidence = shared parts or features you can point to in cells and genomes. For AP Topic 7.7 that means things like membrane-bound organelles (nucleus, mitochondria, chloroplasts), linear chromosomes, and genes with spliceosomal introns—these physical similarities imply a common ancestor (LECA) and support endosymbiotic origins for mitochondria/chloroplasts (alphaproteobacteria, cyanobacteria). Functional evidence = shared processes or molecular machines that work the same way across eukaryotes. Examples: the spliceosome that removes introns, mitosis/meiosis machinery, conserved metabolic pathways in mitochondria/chloroplasts, and similar intron-exon architecture. Function shows the same systems were inherited, not just similar-looking parts. Both kinds of evidence together (structure + function) make a stronger case for common ancestry (LO 7.7.A). For review, see the topic study guide (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK) and practice questions (https://library.fiveable.me/practice/ap-biology).

Short answer: because all eukaryotes share the same complex cellular and molecular machinery that’s unlikely to have evolved independently. Why that matters: every eukaryote has membrane-bound organelles (nuclei, mitochondria; chloroplasts only in photosynthetic lineages), linear chromosomes, spliceosomal introns and the spliceosome, and the same basic processes (mitosis, meiosis, endomembrane system). These are complex, interdependent features that phylogenetic and molecular data trace back to a single last eukaryotic common ancestor (LECA). Mitochondria carry related alphaproteobacterial genomes across eukaryotes, supporting a single endosymbiotic origin rather than many independent origins. Sequence comparisons and conserved intron/exon architectures also show common ancestry. That collection of shared, detailed traits (not just superficial similarity) is why scientists conclude eukaryotes didn’t evolve multiple times independently. For AP review, this maps to LO 7.7.A—see the Topic 7.7 study guide (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK) and the Unit 7 overview (https://library.fiveable.me/ap-biology/unit-7) for practice and examples.

Because all eukaryotes have linear chromosomes (unlike the circular chromosomes of most bacteria), it points to a shared cellular plan and common ancestry for eukaryotes—a key piece of EK 7.7.A.1 in the CED. Linear chromosomes require a nucleus/nuclear envelope and special processes: mitosis and meiosis to segregate multiple linear pieces, and telomeres plus telomerase to prevent loss of ends during replication. Those linked features (nuclear envelope, spliceosomal introns, linear chromosomes, and the spliceosome) likely existed in the last eukaryotic common ancestor (LECA), so their presence across eukaryotes is strong structural/molecular evidence of relatedness. On the exam, be ready to connect linear chromosomes to mechanisms (telomeres/replication, mitosis/meiosis) as evidence for common ancestry (LO 7.7.A). For a concise review, check the Topic 7.7 study guide (https://library.fiveable.me/ap-biology/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK) and more unit resources (https://library.fiveable.me/ap-biology/unit-7); practice questions are at (https://library.fiveable.me/practice/ap-biology).