Although it's been long known that the first backboned land animals or "tetrapods" -- the ancestors of amphibians, reptiles, birds and mammals, including humans -- evolved from a group of fishes about 370 million years ago during the Devonian period. But scientists had not obtained a complete picture of the intermediate steps in that transition.

But now new fossils of the extremely primitive tetrapod Ventastega, from the Devonian of Latvia, cast light on that key phase of the transition.

"Ventastega was first described from fragmentary material in 1994; since then, excavations have produced lots of new superbly preserved fossils, allowing us to reconstruct the whole head, shoulder girdle and part of the pelvis", says Professor Per Ahlberg.

Reconstructions made by Ahlberg and Assistant Professor Henning Blom, together with British and Latvian colleagues, show Ventastega was more fish-like than any of its contemporaries.

The researchers say it appears different parts of the body evolved at different speeds during the transition from water to land.

The study is detailed in the journal Nature.