A group of living things
Bacteria
Bacteria
Earth's ancient crowd — trillions live in you too
Its members' last common ancestor lived about 3.4 billion years ago.
You and bacteria last shared an ancestor about 4.2 billion years ago (on 1 January, if life's history were one year).
You are carrying trillions of these tiny neighbours right now, in your gut and on your skin. They are among the oldest and most numerous forms of life on Earth. The oldest fossils that look like tiny cells are about 3,500 million years old — far older than the dinosaurs. There are about 5 nonillion of them on the planet — a 5 with 30 noughts after it — with about 40 million in a single gram of soil. Most kinds do not make people sick; many are helpful.
Each one is a single cell that is almost always far too small to see, with hundreds of thousands fitting on a pinhead. There are no limbs or eyes, and no nucleus inside to hold the DNA, which sits in loose loops. They come as rods, balls and spirals, sometimes joined into chains, clusters or long threads. They live almost everywhere, from mountain tops and deep oceans to Antarctic ice.
Early kinds that used sunlight released the oxygen in our air over about 2,000 million years, so living things with many cells could later appear. Today others help rot down dead leaves and animals to make soil. Helpful gut kinds aid digestion and help the body take up vitamins. People also use them to make yoghurt, cheese and pickles.
Living rocks in Shark Bay
Mats of cyanobacteria trap sand and minerals to build a layered mound called a stromatolite. You can still see these mounds growing in shallow water at Shark Bay in Western Australia.
Green leaves hold old partners
The green parts inside plant cells that catch sunlight began as free-living cyanobacteria. An ancient ancestor of plants took them in as partners, and they stayed.
Seen on teeth
In 1683, Leeuwenhoek looked at scrapings from his own teeth under an early microscope. His drawings of the tiny shapes he saw were published soon after.