The discovery of a fossilised crocodile-like creature gives us one of the missing links between fish and land-living animals.
The new species, Tiktaalik roseae, has features found in fish, such as fins and gills, and also features that are only found in land-living animals, such as a wrist, elbow and neck.
The crocodile-like creature probably lived in shallow water but had limb-like lobe-fins that enabled it to carry itself up onto land.
'This is the fossil of the year and a most significant addition to our knowledge,' said Natural History Museum palaeontologist Dr Andrew Milner.
'It is a stepping-stone in the water-land transition showing us a permutation of features not seen before, notably the combination of lobe-fins with the beginnings of a neck.'
Tiktaalik roseae lived 375 million years ago in the late Devonian period - the time where some types of fish left the water and adapted to land living - a process that took millions of years.
Edward Daeschler of the Academy of Natural Sciences in Philadelphia, Pennsylvania, Neil Shubin of the University of Chicago in Illinois, Farish Jenkins of Harvard University in Cambridge, Massachusetts and colleagues discovered the specimens on Ellesmere Island in Arctic Canada. They were looking for a missing link like Tiktaalik and had decided to investigate the exposed Devonian rock in the area.
They found three near-complete specimens, the largest measuring almost 3m (9 ft) in length.
'Tiktaalik is particularly valuable because it is based on three articulated skeletons, which means that its anatomy can be described and interpreted with a great deal of confidence,' said Dr Milner.
Fossils of creatures with both fish and land-living features have been found before. But there has been a gap of 20 million years between the last fish specimen showing early land-living features and the earliest known tetrapod, (four-limbed animal).
Dr Milner explains, 'Tiktaalik sits between Devonian fish such as Panderichthys, which have just a few tetrapod-like features, and early tetrapods like Acanthostega that retain a few residual fishy characteristics.'
Dr Milner describes how Tiktaalik shows the beginnings of a neck, 'In most fish, the head, gill-covers and shoulder girdle are a single dense unit which 'wags' the body and tail during swimming. Here the gill-covers are missing, the head and shoulders are decoupled and it appears that head mobility was more important than swimming.'
Tiktaalik roseae helps to fill the gap in our understanding of how fish adapted to live on land, paving the way for all other land-living animals including humans.
The research is reported in the journal Nature .