The first tetrapods
Acanthostega and Ichthyostega have limbs with digits — but they are still animals of the water.
- When
- Known to
- ± 3 million years
- Era
- Devonian, Paleozoic
By around 365 million years ago tetrapods with recognisable arms, legs and digits appear. Acanthostega has eight fingers on each hand, internal gills and a fish-like tail; its limbs were probably for moving through weed-choked shallows rather than walking on land. Limbs evolved in water and were only later co-opted for land — one of the clearest cases of a structure acquiring its familiar function long after it arose.