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7.7 Common Ancestry

Keywords

English Term 中文翻译 Definition & Explanation
Eukaryote 真核生物 Organisms whose cells contain a nucleus and other membrane-bound organelles (includes animals, plants, fungi, and protists).
Membrane-bound Organelles 膜结合细胞器 Intracellular compartments (like mitochondria, ER, Golgi) that allow for spatial separation of biochemical processes.
Linear Chromosomes 线状染色体 DNA structures with distinct ends, characteristic of eukaryotic genomes.
Introns 内含子 Non-coding segments of nucleic acid that lie between coding sequences (exons) in eukaryotic genes and are spliced out during RNA processing.

1. The Eukaryotic Family Tree

While all life on Earth shares a universal common ancestor (evidenced by DNA and glycolysis), the domain Eukarya branches off into a highly distinct lineage.

Even though a mushroom (Fungi), an oak tree (Plant), and a human (Animal) look incredibly different on a macroscopic level, their microscopic structure provides undeniable evidence that they all evolved from a recent shared common eukaryotic ancestor.

2. Structural Evidence for Eukaryotic Ancestry

The AP Exam requires you to identify three specific structural features that are universally conserved across all eukaryotes, but are generally absent in prokaryotes (bacteria and archaea):

  1. Membrane-Bound Organelles: All eukaryotes possess internal compartments (like the nucleus, endoplasmic reticulum, Golgi apparatus, and mitochondria) that separate metabolic processes.
  2. Linear Chromosomes: While prokaryotes typically have a single, circular chromosome floating in the cytoplasm, eukaryotes organize their massive genomes into multiple linear chromosomes tightly wrapped around histone proteins inside a nucleus.
  3. Genes That Contain Introns: In eukaryotes, genes are interrupted by long stretches of non-coding DNA called introns. Before mRNA can be translated into a protein, the cell must process the RNA by cutting out these introns and splicing the coding segments (exons) together. Prokaryotes rarely have introns.

Because all modern eukaryotic organisms share these highly complex features, we can confidently conclude they inherited them from a common ancestor that had already evolved these exact traits.

Quiz