What Does the Endosymbiotic Theory State? A Clear Guide

The endosymbiotic theory states that mitochondria and chloroplasts — the energy-producing and photosynthesis-related organelles inside eukaryotic cells — were once free-living prokaryotes (simple cells without a nucleus) that were engulfed by a larger host cell and eventually became permanent, integrated parts of it. Two separate organisms merged into one, and that merger gave rise to the complex cells that make up plants, animals, and fungi. It remains the leading scientific explanation for the evolutionary origin of eukaryotes, the cells with a true nucleus.

Core Claims of the Theory

The endosymbiotic theory, also called symbiogenesis or the serial endosymbiotic theory, proposes that certain organelles inside eukaryotic cells descend from formerly independent bacteria. The central claim is straightforward: a host cell took in free-living prokaryotes, and instead of digesting them, it formed a lasting partnership with them.

Over time, those internal partners lost the ability to live on their own and became the organelles we now call mitochondria and chloroplasts. Mitochondria handle respiration and appear related to Alphaproteobacteria, a group that includes Rickettsiales. Chloroplasts carry out photosynthesis and are thought to descend from cyanobacteria. Because both organelles are surrounded by their own membranes in addition to the membranes around other organelles, the theory treats them as former cells nested inside a larger one.

A key detail is that the theory is often described as “serial” because it likely involved more than one event. The ancestor of mitochondria entered the host first, and the ancestor of chloroplasts was acquired later — and only in the lineage that became plants and algae.

Origins and Key Scientists

The idea did not appear overnight. Botanist Andreas Schimper noticed that chloroplast division in green plants closely resembled division in free-living cyanobacteria, and he suggested in a footnote that plants might have arisen from a union of two organisms. The Russian botanist Konstantin Mereschkowski then articulated the theory in his early twentieth-century work on chromatophores, proposing that complex life arose through episodes of symbiogenesis. Other researchers, including Paul Portier and Ivan Wallin, argued for a symbiotic origin of mitochondria in the following years.

These early proposals were largely ignored until better tools arrived. Electron microscopy in the early 1960s, plus the discovery that mitochondria and plastids contain their own DNA, revived the concept. In 1967, Lynn Margulis advanced and substantiated the theory with microbiological evidence in her paper on the origin of mitosing cells, and she continued developing it in her later work. Her evidence-based articulation is why she is most closely associated with the theory today, as summarized in Encyclopaedia Britannica.

Evidence Supporting Endosymbiosis

The theory’s strength comes from multiple independent lines of evidence, all pointing the same way. Each one is consistent with a bacterial ancestry for these organelles.

• Their own DNA: Mitochondria and plastids contain their own chromosomes, separate from the DNA in the cell nucleus.
• Binary fission: They reproduce by dividing on their own, in parallel with — but separate from — the sexual reproduction of the rest of the cell.
• Circular chromosomes: In some mitochondria and plastids, this DNA takes the form of a single circular molecule, resembling the circular chromosomes of bacteria.
• Bacterial-like membranes: They carry a separate biomembrane layer, and specific membrane molecules tie them to bacteria.

Two molecular details strengthen the case. Porins, a class of transport proteins, appear in the outer membranes of mitochondria and chloroplasts as well as in bacterial cell membranes. Cardiolipin, a lipid, is found only in the inner mitochondrial membrane and in bacterial membranes. These shared features are difficult to explain unless the organelles share a bacterial ancestor, as detailed in Wikipedia’s overview of symbiogenesis.

Consider a mitochondrion inside one of your muscle cells. It has its own small circular chromosome, divides by splitting in two, and sits behind a double membrane. Each of those features mirrors bacterial biology rather than the biology of the surrounding cell — which is exactly the pattern the theory predicts.

Evolutionary Impact on Eukaryotes

This theory does more than explain two odd organelles — it accounts for the rise of eukaryotes themselves. Before this merger, life was prokaryotic: small, simple cells. The acquisition of an energy-generating partner gave the host cell a far greater energy budget, and that surplus is thought to have made larger, more complex cells possible. In this view, the origin of eukaryotes from prokaryotic ancestors is fundamentally a story of symbiosis, which is why symbiogenesis is described as the leading scientific theory for that origin.

The consequences differ between the two organelles. Mitochondria became the metabolic engines of nearly all eukaryotes, enabling aerobic respiration and the energy demands of complex life. Chloroplasts, acquired later in a plant and algal lineage, added photosynthesis and turned those lineages into primary producers. Each organelle retains a remnant genome, a lasting signature of its free-living past. Research on how these partnerships shaped genomes and cell biology continues, with ongoing work published in journals such as Nature.

The theory has a defined scope, and knowing its limits prevents overreach. It explains mitochondria and chloroplasts, plus their close relatives such as plastids in algae. It does not claim to explain every organelle. The nucleus, the endoplasmic reticulum, and the Golgi apparatus are not treated as former bacteria. Their membranes and genomes do not match the bacterial pattern, and no remnant chromosome ties them to a free-living ancestor. Lynn Margulis extended her ideas further — proposing that flagella and cilia arose from endosymbiotic spirochaetes — but that particular proposal has not gained wide acceptance, since flagella lack the DNA and membrane signatures the theory requires.

Key Takeaways

• The endosymbiotic theory states that mitochondria and chloroplasts originated from free-living prokaryotes taken inside a host cell and kept as permanent partners.
• It is also known as symbiogenesis or the serial endosymbiotic theory, and it is the leading explanation for the evolutionary origin of eukaryotes.
• Mitochondria are related to Alphaproteobacteria such as Rickettsiales; chloroplasts are related to cyanobacteria.
• Early ideas came from botanists such as Andreas Schimper and were articulated by Konstantin Mereschkowski in the early 1900s, then substantiated with microbiological evidence by Lynn Margulis in 1967.
• Evidence includes organelle DNA, binary fission, circular chromosomes, porins, and cardiolipin — all bacterial-like features.
• The theory explains the rise of complex eukaryotic cells by showing how an energy-producing merger supplied the resources for greater size and complexity.