Scientists have made one of the most important fossil finds in history: a missing link between fish and land animals, showing how creatures first walked out of the water and on to dry land more than 375m years ago.
Palaeontologists have said that the find, a crocodile-like animal called the Tiktaalik roseae and described today in the journal Nature, could become an icon of evolution in action - like Archaeopteryx, the famous fossil that bridged the gap between reptiles and birds.
As such, it will be a blow to proponents of intelligent design, who claim that the many gaps in the fossil record show evidence of some higher power.
Richard Dawkins, the evolutionary biologist, said: "Our emergence on to the land is one of the more significant rites of passage in our evolutionary history, and Tiktaalik is an important link in the story."
Tiktaalik - the name means "a large, shallow-water fish" in the Inuit language Inuktikuk - shows that the evolution of animals from living in water to living on land happened gradually, with fish first living in shallow water.
The animal lived in the Devonian era lasting from 417m to 354m years ago, and had a skull, neck, and ribs similar to early limbed animals (known as tetrapods), as well as a more primitive jaw, fins, and scales akin to fish.
The scientists who discovered it say the animal was a predator with sharp teeth, a crocodile-like head, and a body that grew up to 2.75 metres (9ft) long.
"It's very important for a number of reasons, one of which is simply the fact that it's so well-preserved and complete," said Jennifer Clack, a paleontologist at Cambridge University and author of an accompanying article in Nature.
Scientists have previously been able to trace the transition of fish into limbed animals only crudely over the millions of years they anticipate the process took place. They suspected that an animal which bridged the gap between fish and land-based tetrapods must have existed - but, until now, there had been scant evidence of one.
"Tiktaalik blurs the boundary between fish and land-living animal both in terms of its anatomy and its way of life," said Neil Shubin, a biologist at the University of Chicago, and a leader of the expedition which found Tiktaalik.
The near-pristine fossil was found on Ellesmere Island, Canada, which is 600 miles from the north pole in the Arctic Circle.
Scientists from the Academy of Natural Sciences in Philadelphia, the University of Chicago, and Harvard University led several expeditions into the inhospitable icy desert to search for the fossils.
The find is the first complete evidence of an animal that was on the verge of the transition from water to land. "The find is a dream come true," said Ted Daeschler of the Academy of Natural Sciences.
"We knew that the rocks on Ellesmere Island offered a glimpse into the right time period and were formed in the right kinds of environments to provide the potential for finding fossils documenting this important evolutionary transition."
When Tiktaalik lived, the Canadian Arctic region was part of a land mass which straddled the equator. Like the Amazon basin today, it had a subtropical climate and the animal lived in small streams. The skeleton indicates that it could support its body under the force of gravity.
Farish Jenkins, an evolutionary biologist at Harvard University said: "This represents a critical early phase in the evolution of all limbed animals, including humans - albeit a very ancient step." Tiktaalik also gives biologists a new understanding of how fins turned into limbs. Its fin contains bones that compare to the upper arm, forearm and primitive parts of the hand of land-living animals.
"Most of the major joints of the fin are functional in this fish," Professor Shubin said.
"The shoulder, elbow and even parts of the wrist are already there and working in ways similar to the earliest land-living animals."
Dr Clack said that, judging from the fossil, the first evolutionary transition from sea to land probably involved learning how to breathe air. "Tiktaalik has lost a series of bones that, in fishes, covers the gill region and helps to operate the gill-breathing mechanism," she said. "The air-breathing mechanism it had would have been elaborated and having lost the series of bones that lies between the head and the shoulder girdle means it's got a neck, it can raise its head more easily in order to gulp the air.
"The flexible robust limbs appear to be connected with pushing the head out of the water to breathe the air."
H Richard Lane, director of sedimentary geology and palaeobiology at the US National Science Foundation, said: "These exciting discoveries are providing fossil Rosetta stones for a deeper understanding of this evolutionary milestone - fish to land-roaming tetrapods."
A cast of the fossil goes on display at the Science Museum in South Kensington central London today.