CHLOROPLASTS
Plants borrowed the sunlight
Chloroplasts, which capture sunlight in plants and algae, descended from cyanobacteria. Gene studies in the 1970s found that chloroplast DNA was cyanobacterial DNA.
Check the evidenceInside almost every one of your cells are hundreds to thousands of tiny power stations.
They have their own DNA, and it looks like a bacterium's. Biologists now think complex life was built partly by merging with other organisms, and our genome still carries pieces of ancient viruses.
- Power from bacteria
- Sunlight from cyanobacteria
- Genes from viruses
- Three borrowings that shaped life
THE SHORT ANSWER
Endosymbiosis:
cells built from partners.
Three borrowings, from the cell's power supply
to genes that help build the placenta.
MITOCHONDRIA
Our energy came from bacteria
Mitochondria make most of a cell's chemical energy. Their DNA resembles a group of bacteria that includes the kind that causes typhus.
See the numbers
VIRAL GENES
Viruses left genes behind
Ancient viral infections of germ line cells left DNA that is still inherited. Genes captured from them, the syncytins, help form the placenta.
Read more
FOUR PLACES TO SEE IT
Where to see
endosymbiosis today.
You cannot see a mitochondrion without a microscope, but the partnerships it stands for are easy to find once you know where to look.
Any green leaf
Each green plant cell holds chloroplasts, the descendants of cyanobacteria, capturing sunlight to make organic matter.
Ponds and seawater
Cyanobacteria still abound in oceans and fresh water, the free-living relatives of the chloroplast.
Your own muscles
Muscle, liver and kidney cells are rich in mitochondria because they need a lot of energy.
Your family tree
Mitochondrial DNA passes down the maternal line, so it can trace ancestry far back through mothers.
TWO NUMBERS WORTH KNOWING
Viral and mitochondrial DNA
in numbers.
One figure on the viral share of our genome,
one on the small genome inside mitochondria.
≈ 8%
Share of the human genome from endogenous retroviruses
Human endogenous retroviruses make up about 8% of the human genome. They came from retroviral infections of germ line cells millions of years ago and are inherited like other genes.
VIRAL DNA · 2021 REVIEW
37 genes
Genes in human mitochondrial DNA
Human mitochondrial DNA spans about 16,500 base pairs and holds 37 genes, all essential for normal mitochondrial function. Thirteen help make the enzymes that produce the cell's energy.
MITOCHONDRIAL DNA · MEDLINEPLUS
NHGRI notes that mitochondria, and so mitochondrial DNA, are generally inherited only from the mother, which is why it can be used to trace ancestry back through the maternal line.
A QUICK EVIDENCE CHECK
What did life
borrow?
Pick a borrowed part to see where it is thought to come from. The bar shows how strong the evidence is, according to the sources below.
Based on the University of California Museum of Paleontology's Understanding Evolution site, NHGRI and MedlinePlus Genetics, and a 2021 review of human endogenous retroviruses.
THE IDEA THAT WON THROUGH
From odd theory
to textbook.
Scientists had noticed that mitochondria looked like bacteria ever since they were discovered at the end of the 1800s. In the late 1960s Lynn Margulis built the case that complex cells formed by merging lineages through symbiosis, and in 1970 she published the argument in The Origin of Eukaryotic Cells.
Genetic tools from the 1970s backed her up for mitochondria and chloroplasts. As the Understanding Evolution site puts it, nucleated cells are more like tightly knit communities than single individuals.
In conversation
Questions people ask
What is endosymbiosis?
It is the idea that one organism came to live permanently inside another and became part of it. Mitochondria and chloroplasts are thought to have formed this way from bacteria.
Do mitochondria really have their own DNA?
Yes. MedlinePlus Genetics says human mitochondrial DNA spans about 16,500 base pairs and contains 37 genes, a small fraction of the total DNA in a cell.
Why is mitochondrial DNA inherited from the mother?
Offspring inherit their mitochondria, and so their mitochondrial DNA, from their mother. NHGRI notes this is why it can be used to trace ancestry through the maternal line.
How much of our DNA came from viruses?
A 2021 review says human endogenous retroviruses account for about 8% of the human genome, left by infections of germ line cells millions of years ago.
Are borrowed viral genes useful?
Some are. Syncytin-1 and syncytin-2, encoded by endogenous retroviruses, are expressed in the placenta and help form the layer where mother and fetus meet.
How was the chloroplast link shown?
With gene comparisons in the 1970s. Understanding Evolution describes two teams of microbiologists, one led by Carl Woese and the other by W. Ford Doolittle at Dalhousie University in Nova Scotia, who studied genes inside the chloroplasts of some algae. Those genes bore little resemblance to the genes in the algae's own nuclei: chloroplast DNA turned out to be cyanobacterial DNA.
Is all of Lynn Margulis's theory accepted?
Not all of it. Her case for mitochondria and chloroplasts is supported by phylogenetic analyses of genetic sequences, but her further claim that spiral-shaped bacteria called spirochetes were incorporated into cells and gave rise to cell tails, such as those on sperm, leaves most researchers sceptical, Understanding Evolution notes.
Sources
Each figure on this page comes from the sources below.
- Understanding Evolution (UC Museum of Paleontology): Endosymbiosis, Lynn Margulis
- MedlinePlus Genetics: Mitochondrial DNA
- NHGRI Talking Glossary: Mitochondria
- NHGRI Talking Glossary: Mitochondrial DNA
- Mao et al. (2021), Human endogenous retroviruses in development and disease, Comput Struct Biotechnol J (PMC)