A group of species
Cyanobacteria
Cyanobacteria
Pond scum that filled the air with oxygen and lives on in leaves.
You and cyanobacteria last shared an ancestor about 4.2 billion years ago (on 1 January, if life's history were one year).
That green scum on a pond is a surprise relative. These tiny bacteria are cousins of the E. coli bacterium, not true algae at all. Long ago their photosynthesis filled the air with oxygen we now breathe. Later, one was taken inside an early cell that led to plants and stayed there as the chloroplast that makes food for the plant.
Single cells are far too small to see. Many join into chains or hairs, sometimes wrapped in clear jelly. Blooms colour the water like pea soup or spilt paint. Though often blue-green, they can also be bright green, olive, purple, red or pink.
They live almost anywhere wet and sunlit, from lakes and rivers to the open sea and hot springs. They need light, warmth and rich nutrients to bloom. Some kinds make natural poisons that can sicken people and animals.
Earth's first reefs
A layered mound called a stromatolite grows as a microbe mat traps sand and mud and slowly turns to rock. These mounds were Earth's first reefs and were once widespread.
Eaten by Aztecs
One kind, Spirulina, is high in protein and easy to grow in ponds. It was eaten regularly by the Aztecs and is still used in some tropical diets and Asian dishes.
Living fertiliser
Some grow special thick-walled cells that turn nitrogen into plant food. A floating fern that houses one kind is spread through rice paddies as natural fertiliser.