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First complex cells (eukaryotes)
c. 2–1.6 billion years ago· Event· Life & early humans
Mostly agreedGood evidence; some details, dates or numbers uncertain.
Cells with a nucleus and internal compartments appeared: the type that makes up all plants, animals and fungi.
See it on the History map →What happened
In the leading view, eukaryotes arose when one simple cell, an archaeon, took in a bacterium that became the mitochondrion, a built-in power plant. The extra energy let cells grow far larger and more complex, with DNA stored in a nucleus. Later, a eukaryote captured a cyanobacterium the same way, gaining chloroplasts: the start of algae and plants. Lynn Margulis championed this 'endosymbiosis' idea in 1967, against strong skepticism.
Why it matters
Every plant, animal, fungus and person is built from these cells. Without them, life might have stayed microbial.
How we know
Mitochondria still carry their own bacteria-like DNA; microfossils about 1.6–1.8 billion years old; and genetic studies linking the host cell to Asgard archaea, found in seafloor mud in 2015.
Still debated
Exactly when eukaryotes arose, and whether the merger or the complex cell came first, is still argued.
Inside this (1)
Connections (3)
▸Great Oxidation Event paved the way for First complex cells (eukaryotes)Mostly agreed
The connection
Breathing oxygen releases far more energy from food than processes that work without it. Once oxygen was available, cells with oxygen-using mitochondria could power larger, more complex bodies. Complex cells appear in the fossil record a few hundred million years after oxygen first rose.
When and how
In oceans worldwide, between about 2.4 and 1.6 billion years ago.
How sure are we?
The timing fits and mitochondria run on oxygen, but the link is indirect, inferred from the order of events in rocks.
Debates and open questions
Some scientists think early eukaryotes needed little oxygen, so the rise may have helped rather than caused their appearance.
▸First life evolved, by merging cells, into First complex cells (eukaryotes)Mostly agreed
The connection
Two kinds of simple cells merged. An archaeon, a microbe from a branch of life separate from bacteria, engulfed or partnered with a bacterium. Instead of digesting it, the host kept it, and the bacterium became the mitochondrion. The partnership produced a new, more complex kind of cell with a nucleus.
When and how
In ancient seas, about 2–1.6 billion years ago; Lynn Margulis championed the idea in 1967, building on suggestions from the early 1900s.
How sure are we?
Mitochondria have their own bacteria-like DNA and divide like bacteria; Asgard archaea, found in seafloor mud in 2015, share key genes with eukaryotes.
Debates and open questions
Whether the nucleus or the mitochondrion came first, and what the host cell looked like, are still debated.
▸First complex cells (eukaryotes) gave rise to Multicellular lifeMostly agreed
The connection
Eukaryotic cells had the size, energy and genetic tools to stick together and specialize. Some colonies began dividing tasks: some cells fed, others reproduced. Over time such colonies became single organisms, as red algae, green algae, fungi and animals each evolved multicellular bodies separately.
When and how
In the oceans, beginning at least 1.6 billion years ago with red algae; animals by about 600 million years ago.
How sure are we?
All complex multicellular organisms are eukaryotes; fossils of multicellular algae appear after the first eukaryote microfossils.
Debates and open questions
Why complex multicellular life arose only among eukaryotes, and only a handful of times, is still debated.