Biology · Earth, Origins & the History of Life

Endosymbiosis, eukaryotic origins and organelle evolution

A cell can contain another cell without forming a new kind of integrated organism. A lasting endosymbiotic relationship involves repeated transmission and functional connections. The histories of mitochondria and plastids show why the ancestry of a cellular component and its present location need not be the same thing.

Energy, Enzymes & Metabolism supplies electron donors and acceptors, photosynthesis and material/energy accounts. Phylogeny, Classification & the Tree of Life supplies ancestry and evidence comparisons; its methods are not repeated here. Species, Speciation & Macroevolution helps with origin/loss counts, while Development, Differentiation & Stem Cells helps with cell specialisation. The chapter introduces its own dating notation and numerical rules. You can follow the qualitative history first and return to the arithmetic models afterward.

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By the end, you can

Explain how an inherited intracellular association can become an integrated organelle, while distinguishing DNA transfer, protein import and later organelle loss.

Retention is only part of the transition

Endosymbiosis is an association in which one organism lives inside another. For a relationship to contribute to an inherited organelle, the partner must persist through descendant cells and its functions must become integrated with the host system. Merely enclosing a bacterium does not supply those conditions: a captured cell could instead be digested, lost or remain a less integrated symbiont.

Mitochondria descend from a bacterial endosymbiotic contribution to the eukaryotic lineage. Plastids, including chloroplasts, have a cyanobacterial endosymbiotic ancestry. Some later plastid histories involve retaining an already plastid-containing eukaryotic cell. Such histories connect formerly distinct cellular lineages; they are more informative than a claim that an organelle simply appeared because a host needed one.

The model shows host and bacterial contributions meeting in an intracellular association, followed by conditional integration. Its vertical arrangement is conceptual. It supplies no ancient date, exact host anatomy or resolved order for acquiring the nucleus and other eukaryotic features. Competing accounts of those events should not be compressed into the assertion that a modern-looking host definitely engulfed a modern-looking bacterium.

Two contributions to an integrated cell history

Intracellular retention alone does not make an organelle. This conceptual model does not resolve the order of other early eukaryotic features.

Read the diagram in words

At the top, host and bacterial partner nodes are separate. Both connect to a persistent intracellular association. That association connects to an integrated descendant cell only under the stated conditions of functional integration and inherited transmission. The links identify different contributions rather than treating both as ordinary cell division.

Two contributions to an integrated cell history
Model identity	Meaning
host	Host lineage
partner	Bacterial partner lineage
association	Persistent intracellular association
integrated	Integrated descendant cell

Contributions to descendant cells
From	To	Relationship
Host lineage	Persistent intracellular association	Host contribution
Bacterial partner lineage	Persistent intracellular association	Retained partner contribution
Persistent intracellular association	Integrated descendant cell	Functional integration and inherited transmission

Generic conceptual integration, not a claim that every engulfed cell becomes an organelle or a dated sequence for eukaryogenesis. A bacterial partner contributes inherited cellular material; retention, functional integration and repeated transmission are additional conditions. The host's exact ancestral state is not assigned.

A gene can move while its product works elsewhere

During organelle evolution, some DNA from an endosymbiotic ancestor became part of the nuclear genome, while other genes remained in the organelle or were lost. Timmis and colleagues describe how gene transfer and the import of protein products can link these compartments. Transfer alone does not make a nuclear DNA copy functional: expression and appropriate delivery of its product are additional issues.

Follow the two arrows separately. The DNA-transfer arrow concerns a change during lineage history: a copy moves from organelle-associated DNA into nuclear DNA. The protein-import arrow concerns the later cell's operation: a protein made through cytosolic translation is delivered to the organelle. Different material travels in different directions on different timescales.

The invented twelve-gene account makes these distinctions visible. Four identities, G01–G04, remain in the organelle. Five, G05–G09, have one functional nuclear copy in the later model and an assigned imported product. G10–G12 are absent, with no replacement supplied. The diagram follows these chosen gene identities; it is not a complete inventory of all genes or proteins in a real cell.

Gene location and protein destination

DNA transfer and protein import concern different materials and timescales. The fractions describe tracked gene identities, not proportions of DNA length or every protein in a real organelle.

Read the diagram in words

The earlier organelle contains G01–G12. Later, G01–G04 remain organelle-encoded, G05–G09 have functional nuclear copies and G10–G12 are absent. A historical DNA arrow runs from organelle to nucleus. A different, later protein arrow runs from cytosolic translation toward the organelle, with import explicitly assigned.

Gene identity, not length
Gene identity	Earlier location	Later location
G01	Earlier model organelle: twelve identities	Later organelle: four identities
G02	Earlier model organelle: twelve identities	Later organelle: four identities
G03	Earlier model organelle: twelve identities	Later organelle: four identities
G04	Earlier model organelle: twelve identities	Later organelle: four identities
G05	Earlier model organelle: twelve identities	Later nucleus: five functional transferred identities
G06	Earlier model organelle: twelve identities	Later nucleus: five functional transferred identities
G07	Earlier model organelle: twelve identities	Later nucleus: five functional transferred identities
G08	Earlier model organelle: twelve identities	Later nucleus: five functional transferred identities
G09	Earlier model organelle: twelve identities	Later nucleus: five functional transferred identities
G10	Earlier model organelle: twelve identities	Absent identities: no replacement assigned
G11	Earlier model organelle: twelve identities	Absent identities: no replacement assigned
G12	Earlier model organelle: twelve identities	Absent identities: no replacement assigned

Gene location and protein destination
From	To	Relationship
Organelle	Nuclear DNA	DNA transfer during lineage history
Cytosolic translation of assigned G05 product	Organelle	Assigned imported protein products

Nine of twelve identities remain functional

Four organelle + five nuclear + three absent = twelve

Twelve original model genes are tracked by identity, not DNA length or a real genome. Four remain in the organelle; five retain one functional copy in the nucleus, with assigned protein return; three are absent with no replacement in this model. Real DNA transfers need not become functional or return their products automatically.

Descendants need not retain every ancestral feature

Organelle genomes and functions vary among living lineages. A small remaining genome does not show that the organelle arose from a tiny bacterium with only that gene set. It can instead reflect transfer, loss and later modification. Likewise, a gene count is not a DNA-length measurement: four short genes and four long genes need not occupy the same fraction of a genome.

Some mitochondrion-related organelles are highly modified and do not carry out the familiar oxygen-linked ATP production taught for typical mitochondria. An organelle can retain a different set of functions. Calling all mitochondrial descendants identical energy factories hides the history that the comparison is meant to explain.

A particularly useful exception comes from Karnkowska and colleagues' 2016 study of Monocercomonoides. Their genomic analysis found no credible set of mitochondrial hallmark proteins and supported complete secondary loss of the mitochondrial organelle, associated with an alternative cellular pathway. 'Secondary' means that the lineage lost an inherited feature. It does not place this organism before mitochondrial ancestry or demonstrate that all early eukaryotes arose without that contribution.

An ancestry claim does not settle every historical step

Evidence for a bacterial contribution can be strong while the detailed ecological circumstances and order of integration remain uncertain. Think of reconstructing a building's history: identifying reused material does not tell you the dates of every renovation. In a cell, comparisons of genes and machinery identify historical connections, but the exact setting still needs its own evidence.

The previous phylogeny chapter explains how ancestry evidence is evaluated. Here, use its conclusion carefully: a component has a history that can include transmission, transfer, integration and loss. Neither a similarity by itself nor the visual appeal of an engulfment drawing should stand in for that combined evidence.

Evolution also supplies no obligation to retain a feature once an alternative arrangement can support descendants. Loss, replacement and specialisation belong to biological history alongside gains. A lineage's present simplicity can be the outcome of change, rather than a surviving photograph of the first cells.

Worked example

Account for G05 after relocation

In the earlier model, G05 belongs to the twelve organelle-associated ancestral gene identities. In the later model, its one functional copy is nuclear. Its protein is made through cytosolic translation and imported into the organelle by an explicitly assigned targeting/import arrangement. The model does not imply that DNA transfer automatically produced that arrangement.

Count the remaining functional identities: four organelle genes plus five transferred nuclear genes equals nine. Nine out of twelve is 75%. Four out of twelve is one third, so one third of the tracked ancestral gene identities remain encoded in the organelle. That fraction cannot be re-labelled 'one third of the original DNA length'.

Now remove the assigned protein-import condition for G05. A nuclear copy could still exist, but the supplied diagram would no longer establish that its product reaches the organelle. This change leaves a specific functional gap. It does not reverse the DNA arrow or move translation into the nucleus.

Check your understanding

Does the presence of a bacterium inside a cell establish that it is an inherited organelle?
Show reasoning
No. Intracellular location establishes an association. Persistence through descendants and functional integration need additional evidence, and a contained bacterium might instead be transient, digested or lost. The model's last arrow is explicitly conditional on these further connections.
How many of the twelve ancestral gene identities remain functional in the model, and what does this tell us about genome length?
Show reasoning
Four remain organelle-encoded and five are functional nuclear copies: nine of twelve, or 75%. Three are absent. No gene lengths, intergenic DNA or other genome components are specified, so the count gives no genome-length percentage.
Why does Monocercomonoides not demonstrate that eukaryotes originally lacked mitochondrial ancestry?
Show reasoning
The study interprets its condition as secondary loss within a lineage with mitochondrial ancestry, rather than a primitively mitochondrion-free origin. Present absence and ancestral absence are different historical claims. The exception corrects a statement about every current eukaryote without erasing the earlier endosymbiotic contribution.

What to carry forward

Track component ancestry, DNA location, protein delivery and later losses separately. An integrated organelle is a changing inherited relationship, rather than an unchanged captured cell.

References

Resources supporting this lesson’s explanations and examples.