Life › Complex cells & first multicellular life
Eukaryotes: complex cells with a nucleus
c. 1.8–1.6 billion years ago (first fossils); possibly older· Evolutionary innovation· Microbes & cells
Strong evidenceGood evidence; dates or details still being refined.
Cells with a nucleus and internal compartments: the cell type behind every plant, animal and fungus, and many single-celled organisms.
See it on the Life map →What happened
Eukaryotic cells keep their DNA in a membrane-bound nucleus, have an internal scaffold (the cytoskeleton) and contain mitochondria. They are typically about ten times wider than bacteria. Genes show they arose from a partnership: an Asgard archaeon host plus a bacterium that became the mitochondrion. The oldest widely accepted fossils, ornamented single cells from China and Australia, date to about 1.65 billion years ago; molecular clocks allow somewhat older origins.
Why it matters
Nearly all visible life is eukaryotic. Their extra energy and genetic machinery made large cells, sex and complex multicellular bodies possible.
The evidence (fossils, genes, rocks)
Microfossils with complex walls, chemical fossils (steroid molecules made mainly by eukaryotes), and gene trees comparing archaea, bacteria and eukaryotes.
Still debated
When exactly they arose, the order in which the nucleus and mitochondria appeared, and why this merger seems to have succeeded only once.
Inside this (5)
Connections (5)
▸Asgard archaea: the host cell provided the host cell for Eukaryotes: complex cells with a nucleusStrong evidence
The connection
Gene trees place eukaryotes inside or next to the Asgard archaea, so an Asgard-type cell was probably the host that took in the bacterium that became the mitochondrion. Asgard cells already carried some eukaryote-style genes for membranes and internal scaffolding, raw material for the nucleus and other compartments.
When and how
Before about 1.8 billion years ago, probably in oxygen-poor seafloor sediments; recognized through Thijs Ettema's 2015 DNA study and later analyses.
How sure are we?
Shared 'eukaryotic signature' genes, gene trees (Eme and colleagues, 2023), and a lab-grown Asgard cell whose shape fits a possible precursor.
Debates and open questions
Some scientists question whether eukaryotes branch inside Asgard or beside it, and which Asgard group was the actual ancestor.
▸Mitochondria: endosymbiosis powered Eukaryotes: complex cells with a nucleusStrong evidence
The connection
Mitochondria gave the host cell many internal power plants. With far more energy available per gene, eukaryotes could support large genomes, big cells, internal transport and complex regulation. Biochemists Nick Lane and Bill Martin argue this energy boost explains why complex cells arose only once.
When and how
During the origin of eukaryotes, roughly 2.0–1.6 billion years ago; the energy argument was published by Lane and Martin in Nature, 2010.
How sure are we?
All known eukaryotes have mitochondria or traces of them; the energy calculations are estimates and contested.
Debates and open questions
Some biologists argue the host was already complex and energy was not the bottleneck. 'Mito-early versus mito-late' remains open.
▸Eukaryotes: complex cells with a nucleus evolved Sex & meiosisStrong evidence
The connection
Meiosis appears to be a modified version of ordinary eukaryotic cell division, made possible by eukaryotes' nucleus and linear chromosomes. Because its core genes occur in nearly all eukaryote groups, sex probably evolved once, in their common ancestor, rather than separately in each group.
When and how
In early eukaryotes, likely well before about 1.05 billion years ago; the shared-gene evidence comes from genome surveys since the 2000s.
How sure are we?
Meiosis-specific genes such as SPO11, DMC1 and HOP1 turn up even in single-celled eukaryotes once thought asexual, such as Giardia.
Debates and open questions
Whether meiosis grew out of ordinary cell division, out of DNA-repair systems inherited from microbes, or both, and why early eukaryotes adopted it.
▸Eukaryotes: complex cells with a nucleus made possible Multicellular lifeStrong evidence
The connection
Bacteria can form simple chains and mats, but complex multicellular bodies have evolved only among eukaryotes. Their bigger cells, internal scaffolding, signaling systems and large genomes let cells stick together, communicate and specialize, the basis of seaweeds, fungi, plants and animals.
When and how
From about 1.6 billion years ago in the oceans; complex multicellular eukaryotes then arose separately in several lineages.
How sure are we?
Fossils of many-celled algae appear only after eukaryotic microfossils, and every known complex multicellular group is eukaryotic.
Debates and open questions
Why the jump to complex bodies happened only about six times, and why it took so long after eukaryotes appeared.
▸The 'Boring Billion' saw slow diversification of Eukaryotes: complex cells with a nucleusStrong evidence
The connection
Eukaryotes appeared near the start of the Boring Billion but stayed a minor part of ocean life for hundreds of millions of years, while bacteria dominated. Low oxygen and scarce nutrients may have held them back. Steroid chemicals suggest eukaryotic algae became abundant only around 650 million years ago.
When and how
Oceans worldwide, about 1.8–0.8 billion years ago; Jochen Brocks's team traced the later rise of algae using steroid biomarkers in 2017.
How sure are we?
Eukaryotic microfossils are present but sparse, and eukaryote-specific steroid molecules remain scarce until after the Boring Billion.
Debates and open questions
A 2023 study found primitive steroids suggesting early eukaryote relatives were common but different. Whether the delay was environmental or biological is debated.