Eukaryotic Origins

Core Concepts

The eukaryotic cell in brief

A eukaryotic cell differs from a prokaryotic cell in more than having a nucleus. Eukaryotes are typically larger, their DNA is organized into multiple linear chromosomes, and their cytoplasm holds membrane-bound compartments (endoplasmic reticulum, Golgi apparatus, lysosomes) that divide labor. A cytoskeleton of protein filaments gives the cell shape and enables internal transport. Mitochondria appear in nearly all eukaryotes; chloroplasts appear only in photosynthetic lineages such as plants and algae.

The endosymbiotic theory

The theory proposes a two-step story for many eukaryotic lineages:

1. Mitochondria first. An ancestral host cell engulfed an aerobic (oxygen-using) bacterium, which took up residence instead of being digested; the host gained a reliable way to harvest energy from oxygen. Mitochondria are thought to descend from alpha-proteobacteria, the same group that includes the bacterium causing Rocky Mountain spotted fever.
2. Chloroplasts later. In lineages that became photosynthetic, an early eukaryotic cell engulfed a cyanobacterium, and its descendants became chloroplasts. Because chloroplasts came after mitochondria, every chloroplast-bearing cell also carries mitochondria.

Evidence that supports the theory

• Double membranes. Mitochondria and chloroplasts have two membranes, consistent with an engulfed cell (inner membrane = the original prokaryote; outer = the host's engulfing membrane).
• Own DNA. Both organelles contain small, circular DNA molecules, like bacterial chromosomes, separate from nuclear DNA.
• Bacterial-style ribosomes. The organelles translate proteins with 70S ribosomes, like prokaryotes, not the larger ribosomes of eukaryotic cytoplasm.
• Independent division. Mitochondria and chloroplasts split in two, like binary fission, on their own schedule inside the cell.
• Molecular phylogenetics. Gene sequences place mitochondrial genes with alpha-proteobacteria and chloroplast genes with cyanobacteria on evolutionary trees — evidence extremely unlikely if the organelles had evolved from scratch inside the host.

How the nucleus and other structures may have formed

Not every eukaryotic feature came from engulfment. The nucleus and the endomembrane system are widely thought to have arisen when the host cell's plasma membrane folded inward, creating internal sacs. This is why the nuclear envelope is continuous with the endoplasmic reticulum in living cells. This part of the story is a hypothesis; the exact sequence of events remains an active research area.

Secondary endosymbiosis: chloroplasts can be "re-engulfed"

Some protists carry chloroplasts that did not come directly from a cyanobacterium. In secondary endosymbiosis, a eukaryotic cell engulfs another eukaryote that already has chloroplasts (often a red or green alga) and keeps them. This explains why some algae have chloroplasts with four membranes instead of two, and why photosynthetic lineages are scattered across the eukaryotic family tree.

How It Works / Step-by-Step Process

1. A large host cell engulfs a smaller aerobic bacterium by folding its membrane around it; the bacterium escapes digestion and reproduces inside the host by splitting.
2. Its descendants pass to daughter cells when the host divides; over generations the host supplies shelter and raw materials while the bacterium supplies energy.
3. The engulfed cell's genes shrink (some move to the nucleus), and it becomes a permanent organelle — a mitochondrion.
4. Later, in a photosynthetic lineage, the same process repeats with a cyanobacterium, producing the chloroplast.
5. In some protists, a further round (a eukaryote swallowing a chloroplast-bearing eukaryote) produces secondary plastids with extra membranes.

Common Confusions