A group of species
Diatoms
Bacillariophyta
Tiny glass houses that help you breathe.
You and diatoms last shared an ancestor about 1.8 billion years ago (on 28 July, if life's history were one year).
Each diatom is a single cell living in a glass house. It builds a tiny see-through box in two halves, like the lid and base of a pillbox, often patterned with fine pores and lace-like dots. Many look round from above, while others are long like boats or needles. Both look box-like from the side.
Surprisingly, these tiny cells are distant cousins of giant kelp, not of land plants. Like kelp, they look golden-brown as they catch sunlight to make sugars and oxygen. Diatoms alone make about one breath in five that you take, yet almost every cell is far smaller than a millimetre, like a speck of dust.
Many drift in sunlit surface waters of oceans, lakes and rivers, while others live on rocks, seaweeds and mud. As a group they help form the base of water food webs, feeding tiny water life and baby animals.
Shrinking families
In most diatoms, when a cell splits, each new wall forms inside an old one, so the family gets smaller and smaller until sexual reproduction builds big new houses.
Glass to toothpaste
Fallen shells pile up over time into thick white deposits that people dig up for toothpaste, paints, cleansers and filters.
Chains and gliders
Many link into chains, stars or zigzags, while some glide slowly across surfaces.