A group of living things
Four-limbed animals
Tetrapoda
Fish that learned to walk
Its members' last common ancestor lived about 350 million years ago.
You are part of this group. See how.
Four-limbed animals grew from fish whose fins became limbs. Their ancestors were lobe-finned fish with fleshy fins strengthened by bones and muscles. That shared past is why frogs, lizards, birds, possums and people share one arm and leg plan. It is one long bone from the shoulder or hip, then two long bones, then small wrist or ankle bones with fingers or toes. Bird wings, bat wings and whale flippers are that same plan bent into new shapes.
Not all keep four working limbs. Snakes have no legs for walking, and caecilians, legless amphibians, have none either. Whales keep two flippers with tiny back-leg bones buried inside. They still belong here by ancestry. Living members form two branches. Amphibians, like frogs and salamanders, mostly stay tied to damp places and fresh water for at least part of their lives. Amniotes, like lizards, birds and mammals, have fluid-filled sacs that protect the young so they can breed on land. That second branch lives from dry deserts to the open ocean and the sky.
First hands had extra fingers
The oldest four-limbed animals had seven or eight fingers or toes on each limb. The five-per-limb pattern seen in people and many living members came later.
A fish that could prop itself up
About 375 million years ago, Tiktaalik grew up to about 2.7 metres long, longer than a tall person. It had gills, scales and fins, but also a mobile neck, a sturdy rib cage and early lungs. Its big front fins had shoulders, elbows and partial wrists, and it hunted in shallow fresh water.
The biggest animal ever known
The blue whale is the largest animal known to have lived, heavier than the largest dinosaurs. It can weigh up to about 180 tonnes and reach up to about 29 metres long. Newborns are already about seven to eight metres long.