Scientists Call Fish Fossil the ‘Missing Link’

AN EVOLUTIONARY missing link that was among the first fish to leave the sea and walk on land has been unearthed in the Canadian Arctic. The fossil discovery illuminates a chapter in the history of life on Earth that was essential to the ultimate emergence of human beings. Tiktaalik roseae, which lived about 375 million years ago, has features that blur the distinction between fish and terrestrial limbed creatures.

The fossils found on Ellesmere Island, 600 miles from the North Pole, are a fine example of evolution in action. They have allowed scientists to freeze-frame a process of adaptation to land that took tens of millions of years, and which made possible the development of all the mammals, birds, reptiles and amphibians that have existed since.

Without creatures such as Tiktaalik there would have been no dinosaurs, no primitive mammals and none of the hominids such as Australopithecus africanus and Homo erectus that started the human family tree.

“This animal represents the transition from water to land — the part of history that includes ourselves,” said Neil Shubin, of the University of Chicago, who led the discovery team. “It’s as much a part of our history as, say, Australopithecus africanus.”

Tiktaalik roseae was a predator with sharp teeth and a head shaped like a crocodile that grew to between 4ft and 9ft (1.2m and 2.7m) long.

It was named after consultation with elders from the Inuktikuk people native to the region, who suggested their word for “large shallow-water fish”. The second part of the name honours an anonymous benefactor of the research team.

It had several remarkable anatomical features that show it was capable not only of wading in shallow water, like slightly earlier fish on the cusp of the move to land, but also of supporting itself outside the water in the manner of four-limbed animals or tetrapods.

“Tiktaalik blurs the boundary between fish and land animals,” Dr Shubin said. “This animal is both fish and tetrapod. We jokingly call it a fishapod.”

Unlike fish, it had a defined neck and a strong ribcage that would have enabled it to stand outside water. Its pectoral fins had a wrist joint which enabled it to crawl on the ground.

Dr Shubin said this wrist is sufficiently similar to that of later animals, including human beings, to suggest that Tiktaalik or something very like it was an ancestor of all subsequent land animals. “Here’s a creature that has a fin that can do push-ups,” he said. “When we talk about the fish’s wrist, we’re talking about the origin of parts of our own wrist.”

Farish Jenkins, of Harvard University, said it was clear from Tiktaalik’s skeleton that it could support itself in shallow water or on land. “This represents a critical early phase in the evolution of all limbed animals, including humans.”

The Tiktaalik fossils were found in 2004 after a five-year search of a rock formation on Ellesmere, chosen because it was laid down during the late Devonian period, between 380 million and 365 million years ago, when the transition to land is known to have taken place. Although the rocks are now within the Arctic Circle, in the late Devonian they lay close to the Equator.

Richard Lane, of the US National Science Foundation, said: “These exciting discoveries are providing fossil ‘Rosetta Stones’ for a deeper understanding of this evolutionary milestone.

Independent experts said the find was spectacular. Andrew Milner, of the Natural History Museum, said: “Previous fossils representing this evolutionary event have really been fish with a few land characteristics, or land vertebrates with a few residual fish characteristics. These fossils show an animal that sits bang in the middle.”

Details are published in Nature and casts of the fossils will be displayed from today at the Science Museum, in London.

Mark Henderson