Single-Celled Organisms

A single-celled organism is exactly what the name says: a living thing made of only one cell. Single-celled organisms were the only life on Earth for most of the planet's history (roughly 2 billion years), and they are still by far the most numerous. There are more bacteria living in a teaspoon of soil than there are humans on Earth. Single-celled organisms include bacteria, archaea and many protists (a group that includes amoebas and the things that make pond water green).

• First appearedapprox. 3.7 billion years agoLong before plants or animals
• SmallestMycoplasmaapprox. 0.2 micrometres across
• Largest single cellCaulerpa taxifoliaA green algae up to 3 m long
• Main groupsBacteria, archaea, protistsPlus single-celled fungi (yeast)
• Number on Earthapprox. 5 nonillion5 followed by 30 zeros
• Where they liveAlmost everywhereFrom boiling springs to Antarctic ice

The first life on Earth

For the first 2 billion years of Earth's history, the only life was single-celled. The oldest fossils ever found are tiny mineral mats called stromatolites: layered structures built by colonies of bacteria living in shallow seas around 3.5 billion years ago. The chemistry of some even older rocks suggests bacteria-like cells may have been around as far back as 3.7 billion years ago, only a few hundred million years after Earth itself formed.

Multicellular life (the kind made of many cells working together) did not appear until much later, around 600 million years ago. Before then, the entire history of life was a single-celled story.

The main types

Single-celled organisms fall into a few big groups:

• Bacteria: simple prokaryotes, the most familiar single-celled life. Most are harmless or helpful; a few cause disease.
• Archaea: another group of prokaryotes that look similar to bacteria but are actually very different. Many archaea live in extreme places like boiling water, salt lakes or deep under the ocean floor.
• Protists: a "leftovers" group of single-celled eukaryotes. Includes amoebas, paramecia, single-celled algae and many things that float in pond water. Some can do photosynthesis like plants; others hunt other cells like tiny animals.
• Single-celled fungi: like yeast (which makes bread rise and beer ferment).

Where they live

Single-celled organisms have colonised every corner of the planet, including many that would kill humans instantly. Some examples of extremophiles (extreme-lovers):

• Bacteria living in volcanic hot springs at over 100 °C.
• Archaea in the Dead Sea, where the water is 10 times saltier than the ocean.
• Bacteria deep inside Antarctic ice that has been frozen for hundreds of thousands of years.
• Microbes 3 km underground in solid rock, surviving on energy from radioactive decay.
• Bacteria around deep-sea hydrothermal vents, where boiling acidic water meets freezing ocean.
• Microbes in clouds, on the underside of polar bear hair, and even inside other living cells.

Why bacteria matter

People sometimes think of bacteria as "germs". A few do cause disease, but the overwhelming majority are harmless or actively helpful. Bacteria:

• Recycle nutrients in the soil so plants can grow.
• Help cows, sheep and termites digest grass and wood.
• Live in your gut and digest food you could not otherwise use, plus produce vitamins.
• Make food possible: yoghurt, cheese, bread, vinegar, soy sauce, sauerkraut and many others all depend on bacteria or yeasts.
• Clean up pollution by breaking down oil spills and other chemicals.
• Produce most of the oxygen released by Earth's oceans (via single-celled photosynthetic bacteria called cyanobacteria).

Fact About 50 to 80% of all the oxygen in the atmosphere came not from plants or trees but from cyanobacteria, tiny single-celled organisms that have lived in the oceans for billions of years. Around 2.4 billion years ago, cyanobacteria pumped out so much oxygen that they accidentally caused a planet-wide change called the Great Oxygenation Event, which killed off most of the existing life but eventually made the air breathable for everything that came after.

How they reproduce

Single-celled organisms can reproduce extraordinarily fast. Most bacteria simply split into two identical copies in a process called binary fission. In ideal conditions, a single bacterium can divide every 20 minutes. If you started with one bacterium and let it divide every 20 minutes without limit, you would have 1,000 in less than 4 hours and a million in less than 7. After 24 hours of perfect conditions you would have a colony with about 4 sextillion bacteria, weighing more than the planet Earth.

This rarely happens in real life because bacteria run out of food, run out of space, or get killed by predators (or each other). But the potential for explosive growth is one of the main reasons bacteria are so successful: even if 99.9% of them die, the survivors can rebuild a huge population in hours.

Did you know? You are not really "you" alone. Your body carries roughly 38 trillion single-celled bacteria, mostly in your gut, which is about the same as the number of your own human cells. Together they are called your microbiome, and they help digest your food, train your immune system and produce a few vitamins your body cannot make on its own. Different people have very different microbiomes, and the differences may affect everything from mood to weight to how easily you catch certain infections.

Deeper dive: how cyanobacteria changed the planet

About 2.4 billion years ago, the Earth went through one of the biggest changes in its history. Up until that point, the atmosphere had almost no oxygen: it was mostly methane, carbon dioxide and nitrogen. Almost all life on Earth was bacteria and archaea that lived without oxygen.

Then a single group of bacteria called cyanobacteria (also called blue-green algae) discovered how to do photosynthesis using water as a raw material. Photosynthesis makes oxygen as a waste product. For hundreds of millions of years cyanobacteria pumped out oxygen, which dissolved in the ocean, reacted with iron in the rocks and eventually started filling the atmosphere too. Oxygen levels rose from almost zero to a few percent of the air.

This change is called the Great Oxygenation Event or, more dramatically, the Oxygen Catastrophe. Oxygen was a deadly poison for almost all the bacteria living on Earth at the time. Most of them died off in what was possibly the largest mass extinction in the planet's history. The survivors either retreated to oxygen-free habitats (some deep-sea bacteria still live this way today) or evolved new chemistry that turned oxygen from a poison into a fuel. Those oxygen-using cells eventually became the ancestors of every plant, animal and fungus alive today.

So in a very real sense, the lush oxygen-breathing planet we live on was built by tiny single-celled cyanobacteria. Every breath you take comes from a chain of biochemistry that started 2.4 billion years ago in bacteria so simple that today they would barely count as life.

For multi-celled life, see animal cells vs plant cells. For how single cells divide, see mitosis.