A group of living things
Arthropods
Arthropoda
Jointed legs and outer armour: Earth's most successful design
Its members' last common ancestor lived about 540 million years ago.
You and arthropods last shared an ancestor about 580 million years ago (on 11 November, if life's history were one year).
Meet the jointed-leg gang. The name arthropod means 'jointed feet', and every member wears its armour on the outside. Hard plates cover a body built from joined sections, with soft, bendy skin at the joints. Muscles pull on the inside of the shell, so it works like a suit of armour that also slows water loss.
The family surprises are inside. A fly is a close cousin of a prawn or crab: insects sit inside the same wider branch as crabs and prawns. Spiders belong to the other big branch. Adult insects have six legs and three body parts, while spiders have eight walking legs and two main body parts.
It is the most successful animal design on Earth. Arthropods make up about three in every four known animal species, with more than a million kinds described. Between them, they range from the deep sea to mountain peaks, working as pollinators, recyclers of nutrients, scavengers and food for other animals.
Grow by shedding
A hard shell cannot stretch, so to get bigger each arthropod must shed its old shell and grow a larger one.
Only fliers without backbones
Insects are the only animals without backbones that have wings, and flying helped them spread into new homes.
Long-lived trilobites
Trilobites were sea arthropods that are now extinct. They survived shifting continents, ice ages and times when many living things died out for more than 250 million years, until the last ones died out about 250 million years ago.