Which example most directly describes endosymbiosis?

One organism lives within the cells of another. Exactly. The defining relationship is residence within a host, not similarity or proximity.

Why is organelle DNA useful evidence?

Its sequences can reveal ancestry beyond superficial resemblance. Exactly. Genetic relationships provide testable evidence for particular origin claims.

Which account best represents the 1967 paper's legacy?

Mitochondrial and plastid ancestry gained support, while other proposals did not. Exactly. A paper can be influential and partly correct without every hypothesis surviving scrutiny.

From the archive · September 17, 2026

The partnership inside a cell

Can one organism become part of another?

01 · Word

Endosymbiosis

Pronounced en-doh-sim-by-OH-sis

noun

A relationship in which one organism lives inside another; some such relationships gave rise to cell organelles

Examples

Endosymbiosis helps explain the bacterial ancestry of mitochondria.
An organism living inside a host may participate in endosymbiosis without becoming a permanent organelle.

Origin

The word combines endo, meaning within, with symbiosis, living together. The general relationship is broader than the evolutionary theory of organelle origins. Living inside a host and becoming an inherited part of its cell are related possibilities, not identical events.

02 · Idea

A cell can inherit a partnership

A complex cell need not have built every working part from scratch. An organism living inside another can supply a useful capability. Over evolutionary time, a persistent association can become so integrated that the descendants function as parts of one cell. Independence can be lost while the partnership survives.

Mitochondria and plastids retain genetic evidence of bacterial ancestry. Their genomes offer a way to test the proposal beyond a visual resemblance to bacteria. The history is one of integration as well as residence: many functions of the resulting organelles depend on the host cell.

What looks like one living unit can carry the history of several.

Shared life becomes a new level of organization when the partners are inherited together.

Limits

Coexistence alone does not establish an organelle's origin. Specific relationships require evidence, and the success of one endosymbiotic explanation does not validate every proposed cellular merger. Evolution still includes competition and selection; symbiosis does not replace them.

03 · Moment

A disputed ancestry returns to print

Boston University, 1967; Lynn Sagan publishes On the Origin of Mitosing Cells

The biologist later known as Lynn Margulis published a paper reviving the proposal that major cell structures descended from once-independent organisms. Mitochondria and photosynthetic plastids were central examples. The paper brought several claims into a broad account of how complex cells evolved.

Author: Lynn Sagan, later Margulis
Publication: 1967
Subject: Origins of cell structures

The caveat

Earlier scientists had proposed endosymbiotic origins before Margulis. Her influence lay in reviving and developing the argument. This was a theoretical paper drawing evidence together, not a 1967 observation of the ancient merger itself.

Subsequent molecular evidence strongly supported bacterial origins for mitochondria and plastids. But the paper also proposed an endosymbiotic origin for the flagellar apparatus, a claim that did not gain comparable support. The legacy cannot be reduced to a story in which every disputed suggestion was eventually vindicated.

The connection

Some cell structures descend from organisms that lived inside other cells, making biological individuality an evolutionary achievement.