Event 15

360 million years ago

Acanthostega and Ichthyostega
Proto-amphibians (tetrapods) from the late Devonian period, about 365 million years ago.
Credit: Illustration by John Sibbick, courtesy of John Sibbick Illustration. This illustration is used with permission. All rights reserved.

We are land-living animals with four limbs – so are most of the animals we know and love. But go back in time 400 million years or so, you would not see any such animals. Life was primarily in the sea, which teemed with invertebrates like trilobites (extinct relatives of spiders and crabs) and brachiopods as well as early fish. On land there were some arthropods (early spiders and insects, including a scorpion the size of a man!) starting a life on a landscape studded with early land plants. But starting 360 million years ago this began to change, when creatures called ‘tetrapods’ (meaning ‘four-legged) began to move out of the seas. Descendants of early fish – in fact they looked like fish with thick, muscular fins – they were already adapted to water. But living in shallow water swamps, they were also beginning to adapt to land. These tetrapods were the ancestors of amphibians, whose very name means “double life,” as most of them spend some on the land (as adults) but tend to lay their eggs in the water. Amphibians today include frogs, toads, newts, and salamanders.

Ichthyostega and Acanthostega, Devonian period forms, may have been among the first tetrapods.
Credit: Image by Dave Souza/ Wikipedia.

The early tetrapods evolved from a lobe-finned fish similar to the modern coelocanth and lungfish, whose fin bones evolved into limb bones. In the world of early vertebrates, these early amphibian ancestors were true pioneers – able to move from water onto land at least some of the time and using limbs to help them crawl through shallow water. Their amphibian descendants were the reigning predators on the Earth for over 100 million years, from about 360 to 250 million years ago. The early amphibians were the ancestors of the later vertebrates – the reptiles, the birds, and the mammals.


Many fossils of early tetrapod amphibian ancestors have been found. The majority are from Europe and North America, which were joined to form a huge continent at the time. These include the two-foot long form, Acanthostega, and the four-foot long Icthyostega, both showing feet adapted to walking on land or paddling through shallow water, but with some fish-like features also retained.


These early ancestors of the amphibians (and of later tetrapods, including us) were pioneers – the first animals to start to adapt to life out of the water. Life – and the land – was never the same after this. They developed basic adaptations, including limbs with digits (eventually settling down to five per limb, the five fingers of our hands and five toes of our feet), a new form of locomotion (walking), and new ways of living (including preying on insects and eventually other land animals). Overall, they set the stage for later evolution of reptiles and mammals from this foundation.





A YouTube video of a BBC documentary about the first tetrapod, Part 1.

A YouTube video of a BBC documentary about the first tetrapod, Part 2.

A YouTube video of a BBC documentary about the first tetrapod, Part 3.

A YouTube video of a BBC documentary about the first tetrapod, Part 4.

A YouTube video of a BBC documentary about the first tetrapod, Part 5.

This is an article from the New York Times which details the transition of sea creatures onto land.

Fossils suggest a murky land-water transition.



Comments are closed.