Life › Complex cells & first multicellular life › Eukaryotes: complex cells with a nucleus
Mitochondria: endosymbiosis
c. 2.0–1.6 billion years ago (debated)· Evolutionary innovation· Microbes & cells
Strong evidenceGood evidence; dates or details still being refined.
The cell's power plants, descended from free-living bacteria that were taken in by a host cell and never left.
See it on the Life map →What happened
Mitochondria make most of a complex cell's usable energy (ATP) by burning food with oxygen. They have their own small, circular DNA, bacteria-like ribosomes and a double membrane, and they divide like bacteria. Gene trees place them among the alphaproteobacteria. Over time most of their genes moved to the host's nucleus; human mitochondria keep just 37. The merger probably happened between about 2.0 and 1.6 billion years ago.
Why it matters
Energy per gene rose enormously, which may be what let eukaryotes afford large genomes, big cells and complexity. Every known eukaryote has or once had mitochondria.
Think of it like
Like a property that took in a power plant as a permanent tenant; with cheap on-site energy, much bigger buildings became possible.
The evidence (fossils, genes, rocks)
Mitochondrial DNA and ribosomes match bacteria. The idea, first proposed in the early 1900s, was revived by Lynn Margulis in 1967 and confirmed by gene sequencing from the late 1970s.
Still debated
'Mito-early or mito-late': did the bacterial partner trigger complexity, or did an already complex host swallow it? Researchers still disagree.
Connections (4)
▸Bacteria & archaea: the microbial world supplied the partner that became Mitochondria: endosymbiosisWell established
The connection
The mitochondrion began as a free-living bacterium related to today's alphaproteobacteria. It ended up inside an archaeal host, perhaps as a partner trading chemicals, and was never expelled. Over many generations it handed most of its genes to the host and became a permanent part of the cell.
When and how
Roughly 2.0–1.6 billion years ago, location unknown; the idea dates to the early 1900s and was championed by Lynn Margulis from 1967.
The evidence
Mitochondrial DNA, ribosomes and membranes are bacterial; gene sequencing since the late 1970s places mitochondria among or beside the alphaproteobacteria.
Debates and open questions
Which exact bacterial group was the ancestor, and whether the relationship began as partnership, parasitism or the host trying to eat it.
▸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.
▸Mitochondria: endosymbiosis gave complex cells Aerobic respirationStrong evidence
The connection
Aerobic respiration, using oxygen to burn food for energy, evolved first in bacteria. When the mitochondrial ancestor became part of the cell, it brought this efficient process along, so complex cells could get up to about 15 times more energy from sugar than they could without oxygen.
When and how
Inside early eukaryotic cells, roughly 2.0–1.6 billion years ago; the bacterial machinery itself spread after about 2.4 billion years ago.
How sure are we?
The respiratory enzymes in mitochondria are bacterial in origin, and some, such as parts of cytochrome oxidase, are still encoded in mitochondrial DNA.
Debates and open questions
Whether the ancestral mitochondrion breathed oxygen, lived without it, or did both is debated; some eukaryotes' mitochondria still work without oxygen.
▸Lynn Margulis championed the endosymbiotic origin of Mitochondria: endosymbiosisWell established
The connection
Margulis argued in 1967 that mitochondria, the cell's power plants, descend from oxygen-using bacteria swallowed by a host cell that kept them instead of digesting them. She pointed to their own DNA, their bacteria-like ribosomes and their habit of dividing in two like bacteria. Earlier versions of the idea had been dismissed; she made it testable.
When and how
Her 1967 paper in the Journal of Theoretical Biology, published as Lynn Sagan while at Boston University, and her 1970 book Origin of Eukaryotic Cells.
The evidence
Gene sequencing from the late 1970s placed mitochondria among the alphaproteobacteria; their own circular DNA and double membranes fit a swallowed bacterium.
Debates and open questions
Whether the host was already a complex cell or a simple archaeon, and whether the merger came before or after the nucleus evolved.