A group of living things
Rodents
Rodentia
Teeth that never stop growing
Its members' last common ancestor lived about 65 million years ago.
You and rodents last shared an ancestor about 90 million years ago (on 24 December, if life's history were one year).
Compare with something elseAbout four in ten mammal species are rodents, and all share ever-growing front teeth. They are distant cousins of primates, the group that includes us, which is a surprise when you watch a mouse nibble. Each one has the same toolkit: two chisel teeth on top and two below that grow all the time.
The front of each tooth has hard enamel, but the back is softer. Gnawing wears away the softer back first, so the edge stays sharp by itself. That sharp edge lets a beaver bite through a sapling as thick as your finger in one go.
Rodents live wild almost all over the world, from deserts and forests to rivers and burrows, and mice and rats also live with people in houses, sheds and farms. The biggest, the capybara, stands knee-high by the water, about the size of a small pig, and chats with barks and purrs.
A gap in the smile
Rodents have front gnawing teeth and back grinding teeth. A toothless gap sits between them, with no pointed teeth in the middle.
Busy builders
Beavers build dams that make slow ponds. The ponds cut down erosion and give homes and food to dozens of other species.
Not a rodent
Rabbits, hares and pikas are not rodents. They have an extra pair of top front teeth and belong to a different group.