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By about 2.1 billion years agoPeer-reviewed · 2 sourcesWell documented

Eukaryotes appear: a cell swallows a cell

On the timeline · around By about 2.1 billion years ago · The Great OxidationThe Great OxidationEukaryotes appear: a cell swallows a cell2.3 Ga2.3 Ga2.2 Ga2.2 Ga2.1 Ga2.1 Ga2 Ga2 Ga1.9 Ga

What happened

The simple bacteria and archaea that had run life for two billion years shared one limitation: no internal compartments, no nucleus, no organelles. That changed with the eukaryotes, cells built from the inside out through endosymbiosis, in which a host cell engulfed a free-living bacterium instead of digesting it and kept it on as a permanent, useful resident. Biologist Lynn Margulis argued in 1967 that mitochondria, chloroplasts, and even the whip-like flagella eukaryotic cells use to swim were each once independent bacteria, a claim rejected by journal after journal before it was finally published and eventually became the accepted explanation. The oldest strong fossil evidence for this new kind of cell is Grypania spiralis, a spiral-coiled, thread-like organism up to half a metre long, found in the 2.1-billion-year-old Negaunee Iron Formation near Marquette, Michigan.

Why it matters

Every plant, animal, fungus, and alga alive is a eukaryote, and none of them would exist without that original act of one cell absorbing another instead of eating it. Mitochondria still carry their own leftover bacterial DNA today, a signature of their origin as a separate organism.

How we know

The Grypania fossils were dated using the surrounding iron formation's geology and reported in Science in 1992. The endosymbiosis mechanism itself is not read from any single fossil. It comes from mitochondria and chloroplasts having their own small genomes, their own double membranes, and their own division cycle, all consistent with bacterial ancestry and inconsistent with an organelle built from scratch by its host cell.

Sources

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Part of a timelineEarly Life on Earth9 events · How a single microbial ancestor became a planet run by oxygen, complex cells, and eventually animals, from LUCA to the doorstep of the Cambrian explosion.View all →
Eukaryotes appear: a cell swallows a cell · Early Life on Earth · SourcedStory